Circle of Willis

Overview
The circle of Willis (circulus arteriosus cerebri) is an anastomotic system of arteries that sits at the base of the brain. The “circle” was named after Thomas Willis by his student Richard Lower. Willis was the author of Cerebri Anatome, a book that described and depicted this vascular ring. Although such a vascular ring had been described earlier, the name Willis has been eponymously propagated.

The circle of Willis encircles the stalk of the pituitary gland and provides important communications between the blood supply of the forebrain and hindbrain (ie, between the internal carotid and vertebrobasilar systems following obliteration of primitive embryonic connections). A complete circle of Willis is present in most individuals, although a well-developed communication between each of its parts is identified in less than one half of the population.

The circle of Willis is formed when the internal carotid artery enters the cranial cavity, bilaterally, and divides into the anterior cerebral artery (ACA) and middle cerebral artery (MCA). The anterior cerebral arteries are then united by an anterior communicating artery (ACOM). These connections form the anterior half (anterior circulation) of the circle of Willis. Posteriorly, the basilar artery, formed by the left and right vertebral arteries, branches into a left and right posterior cerebral artery (PCA), forming the posterior circulation. The posterior cerebral arteries complete the circle of Willis by joining the internal carotid system anteriorly via the posterior communicating arteries (PCOM).

The images below depict the circle of Willis.

Gross Anatomy

Anterior Cerebral Artery

A1 segment and anterior communicating artery

The A1 segment extends from the internal carotid artery (ICA) bifurcation in a medial and superior direction to its junction with the anterior communicating artery (ACOM) within the longitudinal fissure. Branches include the medial lenticulostriate arteries (A1) that supply the anterior hypothalamus, anterior commissure, fornix, striatum, optic chiasm, and optic nerves. ACOM branches include perforators that supply the hypothalamus and optic chiasm.

A2 segment

This portion of the anterior cerebral artery extends from the ACOM artery to its division into the pericallosal and callosomarginal arteries, at the genu of the corpus callosum. Branches include perforators to the frontal lobe, and the recurrent artery of Heubner, which is a large lenticulostriate vessel. This latter vessel supplies the caudate nucleus, internal capsule, and putamen. Other branches of A2 include the orbitofrontal and frontopolar arteries.

A3 segment

These include all branches of the anterior cerebral artery (ACA) distal to the origin of the pericallosal and callosomarginal arteries, but other subdivisions have been used. Many anastomoses occur with distal branches of the middle cerebral artery (MCA) and posterior cerebral artery (PCA).[2] The pericallosal artery travels posteriorly over the corpus callosum and anastomoses with the splenial artery. The callosomarginal artery courses over the cingulate gyrus. A paracentral artery arises from the pericallosal or callosomarginal arteries and supplies the paracentral lobule. This segment terminates by providing parietal arteries to the corpus callosum and precuneus.

Middle Cerebral Artery

Most classification schemes divide the middle cerebral artery (MCA) into 4 segments, including M1 from the ICA to the bifurcation (or trifurcation), M2 from the MCA bifurcation to the circular sulcus of the insula, M3 from the circular sulcus to the superficial aspect of the Sylvian fissure, and M4, which are cortical branches.

M1 segment

Most anatomic studies define the M1 segment as ending where the MCA branches take a right angled turn within the Sylvian fissure; however, the division point of the MCA trunk is considered by most clinicians to be the M1/M2 junction.[3] The MCA most commonly bifurcates but may also trifurcate or quadfurcate.[4] Branches include lenticulostriate arteries that supply the anterior commissure, internal capsule, caudate nucleus, putamen, globus pallidus, and an anterior temporal artery that supplies the anterior temporal lobe.[5]

M2 segment

The M2 segment extends from the main division point of the M1 segment, over the insula within the Sylvian fissure, and terminates at the margin of the insula.

M3 segment

The M3 segment begins at the circular sulcus of the insula and ends at the surface of the Sylvian fissure. This part travels over the surface of the frontal and temporal operculae to reach the external surface of the Sylvian fissure. The M3 and M2 segments give rise to stem arteries from which cortical branches are derived.

M4 segment

The M4 segment begins at the surface of the Sylvian fissure and extends over the surface of the cerebral hemisphere. Its cortical branches supply the frontal, parietal, temporal, and occipital lobes. These branches include orbitofrontal, prefrontal, precentral, central, anterior, and posterior parietal, angular, temporo-occipital, temporal, and temporopolar branches. The MCA branches that form the so-called "candelabra" are the prefrontal, precentral, and central arteries.
 

Posterior Cerebral Artery

A commonly used subdivision for this vessel includes dividing it into a P1 segment from the basilar artery bifurcation to the junction with the posterior communicating arteries (PCOM) artery, a P2 segment from the PCOM artery to the posterior aspect of the midbrain, a P3 segment from the posterior aspect of the midbrain to the calcarine fissure, and a P4 segment that describes terminal branches of the PCA distal to the anterior aspect of the calcarine fissure.

P1 segment andposterior communicating arteries

The P1 segment supplies perforating branches to the brainstem. These are termed the posterior thalamoperforators to distinguish them from the anterior thalamoperforators, which arise from the PCOM artery. The direct perforators supply the thalamus, brainstem, and internal capsule. Short and long circumflex arteries supply the thalamus and midbrain. A meningeal branch may supply the inferior surface of the tentorium cerebelli.[7]

P2 segment

The P2 segment begins at the PCOM artery junction and travels around the lateral aspect of the midbrain. Direct perforators supply the thalamus, internal capsule, and the optic tract. Branches include the posteromedial choroidal artery that supplies the midbrain, pineal gland, thalamus, and medial geniculate body and the posterolateral choroidal artery that supplies the choroid plexus, thalamus, geniculate bodies, fornix, cerebral peduncle, pineal body, corpus callosum, tegmentum, and temporal occipital cortex. A hippocampal artery may be present. The inferior temporal arteries anastomose with anterior temporal branches of the MCA. The parieto-occipital artery arises as a single trunk from the P2 segment more commonly than from the P3 segment. This artery supplies the posterior parasagittal region, cuneus, precuneus, and lateral occipital gyrus.

P3 segment

The P3 segment extends from the tectum to the anterior aspect of the calcarine fissure. The PCA often divides into its 2 terminal branches, the calcarine and parieto-occipital arteries.

P4 segment

The P4 segment begins at the anterior limit of the calcarine fissure and often includes one of the 2 main terminal branches of the PCA, the calcarine artery. The other main terminal branch of the PCA, the parieto-occipital artery, frequently arises from the P2 or P3 segment. The splenial artery arises from the parieto-occipital artery in most individuals and usually anastomoses with the pericallosal artery.
 

Basilar Artery

The basilar artery originates at the junction between the left and right vertebral arteries and travels anterior to the brainstem. Branches include the superior cerebellar artery and anterior inferior cerebellar artery (AICA).[9] The superior cerebellar artery (SCA) arises from the basilar artery immediately prior to the basilar bifurcation. The SCA often comes into contact with the trigeminal nerve and is usually the target of surgical microvascular decompression for trigeminal neuralgia.

The artery sends branches to the tectum, vermis, and the medial aspect of the cerebellar hemisphere. AICA travels toward the cerebellopontine angle. The posterior inferior cerebellar artery (PICA) is the largest of the cerebellar arteries and arises from the vertebral artery. It supplies the medulla, cerebellar tonsils and vermis, and inferolateral cerebellar hemisphere.

Natural Variants

Anterior circulation

The anterior cerebral arteries may be united in a single trunk, which runs in the longitudinal fissure, giving branches to both hemispheres. The left and right A1 segments are asymmetric in size in most individuals and may be absent or fenestrated. Rarely, this segment may travel inferior to or through the optic nerve. An accessory ACA may be present, and the A1 segment may arise from the cavernous or contralateral ICA.

Either the right and left anterior cerebral arteries may run as one vessel (azygos) to divide distally, or one may be a branch of the contralateral artery. Other variations of the ACOM include aplasia, fenestration, and duplication.[10] This vessel may be curved, kinked, or tortuous. The artery is rarely absent. One A2 segment may be hypoplastic; thus, the contralateral A2 supplies both hemispheres. A2 may be duplicated. In an azygos ACA, both A1 segments join to form a single A2 segment. Branches to the contralateral hemisphere may be found.

Posterior circulation

When a fetal PCOM artery is present, the ipsilateral P1 is typically hypoplastic. Variations of the P1 segment include duplication, fenestration, and a bilateral shared origin of the PCA and SCA. A prominent perforating branch may supply portions of both the ipsilateral and contralateral thalamus and potentially the mid brain. The posterior cerebral may course below, rather than over, the oculomotor nerve, or it may be absent and replaced by an accessory contralateral vessel. The posterior cerebral may give rise to the anterior cerebral artery. The posterior cerebral artery may arise from the internal carotid.

The PCOM may be absent, or the branch representing it may fail to join the posterior cerebral. Fenestration of the basilar artery is found in less than 1% of cases. The basilar artery may exist as 2 longitudinal trunks united across the midline. The SCA may be duplicated or absent. The internal auditory artery is most often a branch of the AICA but may arise from the SCA (in up to 25% of cases) or the basilar artery (in less than 20% of cases).

Pathophysiological Variants

Asymmetry of the circle of Willis results in significant asymmetry of flow and is an important factor in the development of intracranial aneurysms and ischemic stroke.[6] Patients with aneurysms are more likely to have asymmetry or an anomaly of the circle, and the presence of a nonfunctional anterior collateral pathway in the circle of Willis in patients with ICA occlusive disease is strongly associated with ischemic stroke. Uncommonly, persistence of fetal anastomoses is found involving the circle of Willis. These include persistent trigeminal, otic, hypoglossal and proatlantal arteries. These arteries, more or less, unite the internal carotid and vertebrobasilar systems. 

 

 

 

 

Breast Anatomy

Anatomy of the Breast

As with all surgical procedures, understanding the anatomy is crucial prior to performing an operative procedure. Comprehension of breast anatomy enhances the surgeon's ability to perform surgery safely and effectively. For information on various breast surgery procedures, please see the Breast section of eMedicine's Plastic Surgery journal.

Vascular Anatomy

The blood supply to the breast skin depends on the subdermal plexus, which is in communication with underlying deeper vessels supplying the breast parenchyma. The blood supply is derived from (1) the internal mammary perforators (most notably the second to fifth perforators), (2) the thoracoacromial artery, (3) the vessels to serratus anterior, (4) the lateral thoracic artery, and (5) the terminal branches of the third to eighth intercostal perforators. The superomedial perforator supply from the internal mammary vessels is particularly robust and accounts for some 60% of the total breast blood supply. This rich blood supply allows for various reduction techniques, ensuring the viability of the skin flaps after surgery.

Innervation of the Breast

Sensory innervation of the breast is dermatomal in nature. It is mainly derived from the anterolateral and anteromedial branches of thoracic intercostal nerves T3-T5. Supraclavicular nerves from the lower fibers of the cervical plexus also provide innervation to the upper and lateral portions of the breast. Researchers believe sensation to the nipple derives largely from the lateral cutaneous branch of T4.

Breast Parenchyma and Support Structures

The breast is made up of both fatty tissue and glandular milk-producing tissues. The ratio of fatty tissue to glandular tissue varies among individuals. In addition, with the onset of menopause (ie, decrease in estrogen levels), the relative amount of fatty tissue increases as the glandular tissue diminishes.

The base of the breast overlies the pectoralis major muscle between the second and sixth ribs in the nonptotic state. The gland is anchored to the pectoralis major fascia by the suspensory ligaments first described by Astley Cooper in 1840. These ligaments run throughout the breast tissue parenchyma from the deep fascia beneath the breast and attach to the dermis of the skin. Since they are not taut, they allow for the natural motion of the breast. These ligaments relax with age and time, eventually resulting in breast ptosis. The lower pole of the breast is fuller than the upper pole. The tail of Spence extends obliquely up into the medial wall of the axilla.[1]

The breast overlies the pectoralis major muscle as well as the uppermost portion of the rectus abdominis muscle inferomedially. The nipple should lie above the inframammary crease and is usually level with the fourth rib and just lateral to the midclavicular line. The average nipple – to – sternal notch measurement in a youthful, well-developed breast is 21-22 cm; an equilateral triangle formed between the nipples and sternal notch measures an average of 21 cm per side.

Musculature Related to the Breast

The breast lies over the musculature that encases the chest wall. The muscles involved include the pectoralis major, serratus anterior, external oblique, and rectus abdominus fascia. The blood supply that provides circulation to these muscles then perforates through to the breast parenchyma, thus also supplying blood to the breast. By maintaining continuity with the underlying musculature, the breast tissue remains richly perfused, thus preventing complications arising from aesthetic or reconstructive surgery that requires the placement of a breast implant. (For more information, see the Breast section of eMedicine's Plastic Surgery journal.)

Pectoralis major
The pectoralis major muscle is a broad muscle that extends from its origin on the medial clavicle and lateral sternum to its insertion on the humerus. The thoracoacromial artery provides its major blood supply, while the intercostal perforators arising from the internal mammary artery provide a segmental blood supply. The medial and lateral anterior thoracic nerves provide innervation for the muscle, entering posteriorly and laterally. The action of the pectoralis major is to flex, adduct, and rotate the arm medially.

The pectoralis major is extremely important in both aesthetic and reconstructive breast surgery, since it provides muscle coverage for the breast implant. In reconstructive surgery, the pectoralis major muscle covers the implant, providing a decreased risk of exposure of the implant since the skin and underlying subcutaneous tissues are often thin following mastectomy. The muscle also provides additional tissue between implant and skin, thus decreasing palpability of the implant. Often, placement of the implant beneath the muscle causes it to be noticeable when the pectoralis is contracted. In these instances, it may be helpful to release the pectoralis muscle from its inferior and medial attachments to decrease the incidence of noticeable contractions. In addition, with inferior release of the pectoralis muscle, lower positioning of the implant can be achieved, resulting in a more aesthetically pleasing appearance.

Serratus anterior
The serratus anterior muscle is a broad muscle that runs along the anterolateral chest wall. Its origin is the outer surface of the upper borders of the first through eighth ribs, and its insertion is on the deep surface of the scapula. Its vascular supply is derived equally from the lateral thoracic artery and branches from the thoracodorsal artery. The long thoracic nerve serves to innervate the serratus anterior, which acts to rotate the scapula, raising the point of the shoulder and drawing the scapula forward toward the body. Transection of the long thoracic nerve is carefully avoided during an axillary lymph node dissection, since its loss results in "winging" as the scapula is released from the chest wall and moves upward and outward.

Because the serratus anterior underlies the lateral aspect of the breast, in aesthetic surgery, blunt elevation of the pectoralis major laterally inadvertently elevates a small portion of the serratus muscle. To completely cover the implant with muscle in reconstructive surgery, often the serratus anterior must be elevated sharply to obtain a sufficient muscle layer to provide coverage.

Rectus abdominus
The rectus abdominus muscle demarcates the inferior border of the breast. It is an elongated muscle that runs from its origin at the crest of the pubis and interpubic ligament to its insertion at the xiphoid process and cartilages of the fifth through seventh ribs. It acts to compress the abdomen and flex the spine. The 7th through 12th intercostal nerves provide sensation to overlying skin and innervate the muscle. Vascularity of the muscle is maintained through a network between the superior and inferior deep epigastric arteries.

When placing an implant for breast reconstruction, in attempting to achieve complete coverage with muscle, the rectus fascia must often be elevated to place the implant sufficiently caudal. This dense thick fascia is often intimately adherent to the ribs below it. Once it is elevated and released, proper positioning and expansion of the implant can proceed.

External oblique
 The external oblique muscle is a broad muscle that runs along the anterolateral abdomen and chest wall. Its origin is from the lower 8 ribs, and its insertion is along the anterior half of the iliac crest and the aponeurosis of the linea alba from the xiphoid to the pubis. It acts to compress the abdomen, flex and laterally rotate the spine, and depress the ribs. The 7th through 12th intercostal nerves serve to innervate the external oblique. A segmental blood supply is maintained through the inferior 8 posterior intercostal arteries.

The external oblique muscle abuts the breast on the inferior lateral aspect. Elevated along with the rectus abdominus fascia to provide inferior coverage of the breast implant during reconstructive surgery, its fascia, like the fascia of the rectus abdominus muscle, must be released adequately to provide for proper placement and expansion of the implant. In aesthetic surgery, placement of the implant inferiorly is usually not below these fascial attachments. If the implant is placed behind the fascia, the implant often "rides too high" and may result in a "double bubble" effect, wherein the breast parenchyma slides over and off the implant.

Conclusion
Breast shape varies among patients, but knowing and understanding the anatomy of the breast ensures safe surgical planning. When the breasts are carefully examined, significant asymmetries are revealed in most patients. Any preexisting asymmetries, spinal curvature, or chest wall deformities must be recognized and demonstrated to the patient, as these may be difficult to correct and can become noticeable in the postoperative period. Preoperative photographs with multiple views are obtained on all patients and maintained as part of the office record.

Anatomy of Olfactory System

Overview

The olfactory system represents one of the oldest sensory modalities in the phylogenetic history of mammals. Olfaction is less developed in humans than in other mammals such as rodents. As a chemical sensor, the olfactory system detects food and influences social and sexual behavior. The specialized olfactory epithelial cells characterize the only group of neurons capable of regeneration. Activation occurs when odiferous molecules come in contact with specialized processes known as the olfactory vesicles. Within the nasal cavity, the turbinates or nasal conchae serve to direct the inspired air toward the olfactory epithelium in the upper posterior region. This area (only a few centimeters wide) contains more than 100 million olfactory receptor cells. These specialized epithelial cells give rise to the olfactory vesicles containing kinocilia, which serve as sites of stimulus transduction.

The image below depicts olfactory anatomy.

Olfactory Epithelium

The olfactory epithelium consists of 3 cell types, basal, supporting, and olfactory receptor cells. Basal cells are stem cells that give rise to the olfactory receptor cells. The continuous turnover and new supply of these neurons are unique to the olfactory system. In no other location in the mature nervous system do less differentiated stem cells replace neurons. Supporting cells are scattered among the receptor cells and have numerous microvilli and secretory granules, which empty their contents onto the mucosal surface. The receptor cells are actually bipolar neurons, each possessing a thin dendritic rod that contains specialized cilia extending from the olfactory vesicle and a long central process that forms the fila olfactoria. The cilia provide the transduction surface for odorous stimuli.

The vomeronasal organ is a specialized bilateral membranous structure located in the base of the anterior nasal septum, at the junction of the septal cartilage and the bony septum. It is believed to detect external chemical signals called pheromones. These signals, which are not detected consciously as odors by the olfactory system, mediate human autonomic, psychological, and endocrine responses.

The trigeminal nerve innervates the posterior nasal cavity to detect noxious stimuli.

Olfactory Nerve and the Cribriform Plate

The small unmyelinated axons of the olfactory receptor cells form the fine fibers of the first cranial nerve and travel centrally toward the ipsilateral olfactory bulb to make contact with the second-order neurons. Conduction velocities are extremely slow, and support is provided in bundles by a single Schwann cell. As previously mentioned, the trigeminal nerve (cranial nerve V) sends fibers to the olfactory epithelium to detect caustic chemicals such as ammonia. The cribriform plate of the ethmoid bone, separated at the midline by the crista galli, contains multiple small foramina through which the olfactory nerve fibers, or fila olfactoria, traverse. Fracture of the cribriform plate in traumatic settings can disrupt these fine fibers and lead to olfactory dysfunction.

Olfactory Bulb

The olfactory bulb lies inferior to the basal frontal lobe. The olfactory bulb is a highly organized structure composed of several distinct layers and synaptic specializations. The layers (from outside toward the center of the bulb) are differentiated as follows:
Glomerular layer
External plexiform layer
Mitral cell layer
Internal plexiform layer
Granule cell layer

Mitral cells are second-order neurons contacted by the olfactory nerve fibers at the glomerular layer of the bulb. The glomerular layer is the most superficial layer, consisting of mitral cell dendritic arborizations (glomeruli), olfactory nerve fibers, and periglomerular cells. Periglomerular cells contact multiple mitral cell dendrites within the glomeruli and provide lateral inhibition of neighboring glomeruli while allowing excitation of a specific mitral cell dendritic tree. Each mitral cell is contacted by at least 1000 olfactory nerve fibers.

The external plexiform layer contains the passing dendrites of mitral cells and a few tufted cells, which are similar in size to mitral cells. Some of the granule cell dendrites in the plexiform layer contact mitral cell dendrites through a specialized dendrodendritic synapse, which also is termed a reciprocal synapse (vesicles seen within both presynaptic and postsynaptic membranes).

Tufted cells also receive granule cell input through dendrodendritic and dendrosomatic contact. Pyramidal mitral cells are the largest neurons in the bulb and are located in a narrow band between the external and internal plexiform layers. The granule cell layer contains multiple small round neurons that lack axons. Long dendritic processes of the neurons reach the more superficial layers and inhibit mitral cells and tufted cells. Small distal processes make contacts with the exiting mitral cell axons.

Olfactory Tract and Central Pathways

Mitral cell axons project to the olfactory cortex via the olfactory tract. Medial fibers of the tract contact the anterior olfactory nucleus and the septal area. Some fibers project to the contralateral olfactory bulb via the anterior commissure. Lateral fibers contact third-order neurons in the primary olfactory cortex (prepyriform and entorhinal areas) directly. Third-order neurons send projections to the dorsomedial nucleus of the thalamus, the basal forebrain, and the limbic system.

The thalamic connections are thought to serve as a conscious mechanism for odor perception, while the amygdala and the entorhinal area are limbic system components and may be involved in the affective components of olfaction. Investigations of regional cerebral blood flow have demonstrated a significant increase in the amygdaloid nucleus with the introduction of a highly aversive odorant stimulus, and this has been associated with subjective perceived aversiveness.

Central Projections

The pyriform lobe includes the olfactory tract, the uncus, and the anterior part of the parahippocampal gyrus. The prepyriform and the periamygdaloid areas of the temporal lobe represent the primary olfactory cortex. The entorhinal area is known as the secondary olfactory cortex and is included in the pyriform lobe. The olfactory system is the only sensory system that has direct cortical projections without a thalamic relay nucleus. The dorsomedial nucleus of the thalamus receives some olfactory fibers that ultimately reach the orbitofrontal cortex.

The anterior olfactory nucleus receives collateral fibers from the olfactory tract and projects to the contralateral olfactory bulb and anterior olfactory nucleus via the anterior commissure.

The region of anterior perforated substance contains cells that receive direct mitral cell collaterals and input from the anterior olfactory nucleus, amygdaloid nucleus, and temporal cortex. This area ultimately projects to the stria medullaris and the medial forebrain bundle.

Using the uncinate fasciculus, the entorhinal area sends projections to the hippocampal formation, anterior insula, and frontal cortex.

Clinical Correlation

As many as 2 million people in the United States experience some type of olfactory dysfunction, causes of which include head trauma, upper respiratory infections, tumors of the anterior cranial fossa, and exposure to toxic chemicals or infections. The following terms are used to describe the degree of smell aberration:
Anosmia - Absence of smell sensation
Hyposmia - Decreased sensation
Dysosmia - Distortion of smell sensation
Cacosmia - Sensation of a bad or foul smell
Parosmia - Sensation of smell in the absence of appropriate stimulus

Olfactory dysfunction is a hallmark of certain syndromes such as Kallmann syndrome (ie, hypogonadism with anosmia) and Foster Kennedy syndrome (ie, papilledema, unilateral anosmia, and optic atrophy usually associated with an olfactory groove meningioma).

The classic description of partial complex epilepsy with a mesial temporal focus includes an aura of foul-smelling odors (termed uncinate fits) that occur before seizure onset, emphasizing presumed origination at the uncus.

Olfactory dysfunction is associated with early Parkinson disease and with other neurodegenerative disorders such as Alzheimer disease and Huntington chorea.[1] An association also exists between abnormal olfactory identification and obsessive-compulsive disorder.[2]

Head trauma leading to fracture of the cribriform plate may cause cerebrospinal fluid (CSF) rhinorrhea and a potential for meningitis. Paranasal sinus endoscopy may lead to violation of the cribriform plate and potential infectious complications. Olfactory structures also can be injured during craniotomies involving the anterior cranial base or from subarachnoid hemorrhage, which may disrupt the fine fibers of the olfactory nerve.

Clinical Evaluation

Detection of olfactory dysfunction begins with sampling of a series of common odors, which can be performed at the bedside with odiferous substances such as coffee, lemon, and peppermint. Tests, including those developed at the Connecticut Chemosensory Clinical Research Center (CCCRC), have aided examiners in identification of abnormalities in odor detection and discrimination. The University of Pennsylvania Smell Identification Test (UPSIT) is another useful tool; it consists of 40 items for evaluation of olfactory and trigeminal nerve function in the nasal cavity.

Central hyposmia may manifest as abnormalities in odor recognition rather than odor detection. Thorough evaluation of patients who have anosmia includes imaging of anterior cranial structures. The clinician should always counsel patients with anosmia regarding sensory loss, including potential risks associated with the lack of smell sensation (eg, inability to detect dangers such as smoke, spoiled foods, toxins).

Promptly complete evaluation and treatment of clear rhinorrhea in the patient in whom leakage of CSF is suspected. Initial testing of fluid for glucose suggests CSF but is not confirmatory. Presence of beta-transferrin is a more sensitive indicator of CSF rhinorrhea. Computed tomography with cisternography or radionuclide scans can be used to detect the site of CSF leakage from the anterior cranial fossa. Repair of leaks at the level of the cribriform plate may be achieved from the intracranial approach, intranasal (endoscopic) approach, or both, depending on the nature of the defect.

Positron emission tomography (PET) and functional MRI are promising modalities to assist in making the diagnosis of different types of hyposmia (central vs peripheral), as well as in delineation of the role of limbic structures as sites of odor recognition, memory, and integration of multisensory inputs.
 

 

 

 

 

 

Brachial Plexus

Overview

The brachial plexus (plexus brachialis) is a somatic nerve plexus formed by intercommunications among the ventral rami (roots) of the lower 4 cervical nerves (C5-C8) and the first thoracic nerve (T1).

The plexus is responsible for the motor innervation to all of the muscles of the upper extremity with the exception of the trapezius and levator scapula. The image below depicts the brachial plexus (BP).

The basic anatomical relationships of the brachial plexus (BP). The BP is subdivided into roots, trunks, divisions, cords, and branches. LC stands for lateral cord, PC stands for the posterior cord, and MC stands for the medial cord

The BP supplies all of the cutaneous innervation of the upper limb, except for the area of the axilla (which is supplied by the supraclavicular nerve) and the dorsal scapula area, which is supplied by cutaneous branches of the dorsal rami.

The BP communicates with the sympathetic trunk via gray rami communicantes, which join the roots of the plexus. They are derived from the middle and inferior cervical sympathetic ganglia and the first thoracic sympathetic ganglion.

Gross Anatomy

Brachial plexus architecture

The brachial plexus (BP) is subdivided into roots, trunks, divisions, cords, and branches. Several mnemonics can be used to remember this architecture (eg, Really Tired Drink Coffee Black). Typically, the brachial plexus is composed of 5 roots, 3 trunks, 6 divisions, 3 cords, and terminal branches, as seen in the image below

Brachial plexus with terminal branches labeled. MC is musculocutaneous (nerve), AXI is axillary, RAD is radial, MED is median, and ULN is ulnar.

Roots

The ventral rami of spinal nerves C5 to T1 are referred to as the "roots" of the plexus. The typical spinal nerve root results from the confluence of the ventral nerve rootlets originating in the anterior horn cells of the spinal cord and the dorsal nerve rootlets that join the spinal ganglion in the region of the intervertebral foramen.

The roots emerge from the transverse processes of the cervical vertebrae immediately posterior to the vertebral artery, which travels in a cephalocaudad direction through the transverse foramina. Each transverse process consists of a posterior and anterior tubercle, which meet laterally to form a costotransverse bar. The transverse foramen lies medial to the costotransverse bar and between the posterior and anterior tubercles. The spinal nerves that form the brachial plexus run in an inferior and anterior direction within the sulci formed by these structures.

Trunks

Shortly after emerging from the intervertebral foramina, the 5 roots (C5-T1) unite to form 3 trunks. The trunks of the BP pass between the anterior and middle scalene muscles.

The ventral rami of C5 and C6 unite to form the upper trunk. The suprascapular nerve and the nerve to the subclavius arise from the upper trunk. The suprascapular nerve contributes sensory fibers to the shoulder joint and provides motor innervation to the supraspinatus and infraspinatus muscles.

The ventral ramus of C7 continues as the middle trunk. The ventral rami of C8 and T1 unite to form the lower trunk .

Divisions

Each trunk splits into an anterior division and a posterior division. These separate the innervation of the ventral and dorsal aspect of the upper limb. The anterior divisions usually supply flexor muscles. The posterior divisions usually supply extensor muscles.

Cords

The cords are referred to as the lateral, posterior, and medial cord, according to their relationship with the axillary artery, as seen in the image below.

The cords pass over the first rib close to the dome of the lung and continue under the clavicle immediately posterior to the subclavian artery.

The anterior divisions of the upper and middle trunks unite to form the lateral cord, which is the origin of the lateral pectoral nerve (C5, C6, C7).

The anterior division of the lower trunk forms the medial cord, which gives off the medial pectoral nerve (C8, T1), the medial brachial cutaneous nerve (T1), and the medial antebrachial cutaneous nerve (C8, T1).

The posterior divisions from each of the 3 trunks unite to form the posterior cord.

The upper and lower subscapular nerves (C7, C8 and C5, C6, respectively) leave the posterior cord and descend behind the axillary artery to supply the subscapularis and teres major muscles. The thoracodorsal nerve to the latissimus dorsi (also known as the middle subscapular nerve, C6, C7, C8) also arises from the posterior cord, as seen in the image below.

Musculocutaneous nerve branch

These are mixed nerves containing both sensory and motor axons (as seen in the image below). The musculocutaneous nerve is derived from the lateral cord. The musculocutaneous nerve leaves the BP sheath high in the axilla at the level of the lower border of the teres major muscle and passes into the coracobrachialis muscle. It innervates the muscles in the flexor compartment of the arm. It carries sensation from the lateral (radial) side of the forearm.

Ulnar nerve branch

The ulnar nerve is derived from the medial cord. Motor innervation is mainly to intrinsic muscles of the hand (as seen in the image below). Sensory innervation is from the medial (ulnar) 1.5 digits (little finger and one-half of the ring finger).

Median nerve branch

The median nerve is derived from both the lateral and medial cords. Motor innervation is to most flexor muscles in the forearm and intrinsic muscles of the thumb (thenar muscles), as seen in the image below. Sensory innervation is to the lateral (radial) 3.5 digits (thumb, index, middle, and half of the ring finger).

Axillary nerve branch

The axillary nerve is derived from the posterior cord. The axillary nerve leaves the BP at the lower border of the subscapularis muscle and continues along the inferior and posterior surface of the axillary artery as the radial nerve. The axillary nerve serves as motor innervation to the deltoid and teres minor muscles, as seen in the image below. These act at the glenohumeral joint. Sensory innervation is from the skin just below the point of the shoulder. The axillary nerve continues as the superior lateral brachial cutaneous nerve of the arm.

Radial nerve branch

The radial nerve is also derived from the posterior cord. The radial nerve continues along the posterior and inferior surface of the axillary artery. The radial nerve innervates the extensor muscles of the elbow, wrist, and fingers, as seen in the image above. Sensory innervation is from the skin on the dorsum of the hand on the radial side.

Terminal Branches

Five "terminal" branches and numerous other "pre-terminal" or "collateral" branches leave the plexus at various points along its length.

Dorsal scapular nerve

The dorsal scapular nerve is derived from the C5 root just after its exit from the intervertebral foramen. It serves as the motor nerve to the rhomboids major and minor muscles

Long thoracic nerve

The long thoracic nerve is derived from C5, C6, and C7 roots immediately after their emergence from the intervertebral foramina. The long thoracic nerve crosses the first rib and then descends through the axilla behind the major branches of the plexus. It innervates the serratus anterior muscle.

Phrenic nerve

The phrenic nerve arises from C3, C4, and C5 root levels, chiefly from the C4 nerve root. It crosses the anterior scalene from lateral to medial and extends into the thorax between the subclavian vein and artery.

Subclavius muscle nerve

The nerve to the subclavius muscle is a small filament, which arises from the upper trunk. It descends to the subclavius muscle in front of the subclavian artery and the lower trunk of the plexus.

Suprascapular nerve

The suprascapular nerve arises from the upper trunk formed by the union of the fifth and sixth cervical nerves. It innervates the supraspinatus muscles and infraspinatus muscles. It runs laterally beneath the trapezius and the omohyoideus and enters the supraspinatus fossa through the suprascapular notch, below the superior transverse scapular ligament; it then passes beneath the supraspinatus and curves around the lateral border of the spine of the scapula to the infraspinatus fossa.

Lateral pectoral nerve

The lateral pectoral nerve arises from the lateral cord of the brachial plexus, from the fifth, sixth, and seventh cervical nerves. It passes across the axillary artery and vein, pierces the coracoclavicular fascia, and is distributed to the deep surface of the pectoralis major. It sends a filament to join the medial anterior thoracic and forms with it a loop in front of the first part of the axillary artery. This nerve innervates the clavicular head of the pectoralis major muscle.

Medial pectoral nerve

The medial pectoral nerve arises from the medial cord from the eighth cervical and first thoracic nerve. It passes behind the first part of the axillary artery, curves forward between the axillary artery and vein, and unites in front of the artery with a filament from the lateral nerve. It then enters the deep surface of the pectoralis minor, where it divides into a number of branches, which supply the muscle. Several branches of the medial pectoral nerve pierce the muscle and end in the pectoralis major, which supply the muscle.

The medial and lateral pectoral nerve often join together to act as a single nerve innervating both the pectoralis major and minor muscles.

Medial brachial cutaneous nerve

The medial brachial cutaneous nerve is the smallest branch of the brachial plexus and, arising from the medial cord, receives its fibers from the eighth cervical and first thoracic nerves. It passes through the axilla, at first lying behind and then medial to the axillary vein, and communicates with the intercostobrachial nerve. It descends along the medial side of the brachial artery to the middle of the arm, where it pierces the deep fascia, and is distributed to the skin of the back of the lower third of the arm, extending as far as the elbow, where some filaments are lost in the skin in front of the medial epicondyle, and others over the olecranon. It communicates with the ulnar branch of the medial antebrachial cutaneous nerve. It carries sensation from the lower medial portion of the arm.

Medial antebrachial cutaneous nerve

The medial antebrachial cutaneous arises from the medial cord of the brachial plexus. It derives its fibers from the eighth cervical and first thoracic nerves, and at its commencement is medial to the axillary artery. It gives off, near the axilla, a filament, which pierces the fascia and supplies the integument covering the biceps brachii, nearly as far as the elbow. The nerve then runs down the ulnar side of the arm medial to the brachial artery, pierces the deep fascia with the basilic vein, about the middle of the arm, and divides into a volar and an ulnar branch.

Microscopic Anatomy

Blood supply of the brachial plexus

The blood supply of the brachial plexus is based largely on the subclavian (which becomes the axillary) artery and its branches, and variations exist. Generally, the vessels involved are the vertebral, the ascending and deep cervical, and the superior intercostal arteries. The cord and rootlets of the cervical nerves are supplied by the anterior and posterior spinal branches of the vertebral artery. The trunks of the plexus are supplied by muscular branches of the ascending and deep cervical arteries and superior intercostals, and occasionally from the subclavian itself.

Natural Variants

Many variant forms of the brachial plexus exist, with none representing most patients. Kerr catalogued 29 forms of the brachial plexus among some 175 cadaver specimens dissected between 1895 and 1910.[2] In the early part of the last century, one author described a total of 38 variations of the plexus. Up to 53.5% of plexuses in cadaver studies possess significant anatomic variation from the "classic" description of the brachial plexus. A study published in September 2003 found variants in 107 of 200 fetuses examined. The authors pointed out that morphological variations were more common in female fetuses and right sides.[3]

Prefixed brachial plexus (cephalic or high)

This occurs when the C4 ventral ramus contributes to the brachial plexus. Contributions to the plexus usually come from the C4-C8.

Postfixed brachial plexus (caudal or low)

This occurs when the T2 ventral ramus contributes to the brachial plexus. Contributions to the plexus usually come from C6-T2.

Other Considerations

Lesions of the Brachial Plexus

Knowledge of the muscles innervated by branches of the brachial plexus and the actions of these muscles and areas of anesthesia and/or paraesthesia allows the physician to determine the localization of a given lesion.

Brachial plexus injuries have numerous causes, such as labor and delivery. A plexus injury at birth is likely caused by a stretch or tear of the child's brachial plexus during the delivery process. The abducted arm of the infant can get pinned against the child's head. This injury results in incomplete sensory and/or motor function of the injured nerve. Traumatic brachial plexus injuries may occur due to motor vehicle accidents, bike accidents, ATV accidents, or sports.

Nerve injuries vary in severity from a mild stretch to the nerve root tearing away from the spinal cord.

Avulsion

The nerve is torn away from its attachment at the spinal cord; this is the most severe type of injury.

Rupture

The nerve is torn, but not at the spinal cord attachment.

Neuroma

Scar tissue has grown around the injury site, putting pressure on the injured nerve and preventing the nerve from sending signals to the muscles.

Neurapraxia

The nerve has been stretched and damaged but not torn.

Diagnoses Related to Brachial Plexus Injuries

Diagnoses related to brachial plexus injuries include Erb palsy, Klumpke palsy, thoracic outlet syndrome, Burner syndrome, and Parsonage-Turner syndrome.

Erb palsy

This condition involves the upper root nerves of C5, C6.
The patient often presents with the arm extended and wrist fully flexed (waiter's tip).
Motor deficits are loss of abduction, flexion, and rotation at the shoulder (axillary, suprascapular, upper, and lower subscapular nerve); weak shoulder extension; and weak elbow flexion and supination of the radioulnar joint (musculocutaneous and radial nerve).
The patient is susceptible to shoulder dislocation due to loss of rotator cuff muscles .
Sensory deficits are to the posterior and lateral aspect of arm (axillary nerve ), radial side of the forearm (musculocutaneous nerve), and thumb and first finger (superficial branch of radial nerve; digital branches of the median nerve ).

Klumpke palsy
This condition involves the lower root nerves of C8 and T1.
This is caused by a rare injury of the lower brachial plexus, usually following breech delivery.
Motor deficits are opposite the thumb (thenar branch of the median nerve), loss of adduction of the thumb (ulnar nerve), loss of abduction and adduction of metatarsophalangeal joints; flexion at the metatarsophalangeal and extension of the interphalangeal joints (deep branch of ulnar and median), and weak flexion of proximal interphalangeal joints and distal interphalangeal joints (ulnar and median nerve).
Sensory deficit is to the ulnar side of the forearm, hand, and ulnar one and a half digits (ulnar and medial antebrachial cutaneous nerve).

Thoracic outlet syndrome
This condition is caused by compression of the neurovascular structures (subclavian vessels, brachial plexus, cervical ganglia, and vertebral artery) in the cervicoaxillary region. It may be congenital or acquired.
Bony causes include the following: long transverse process of the seventh cervical vertebra, cervical rib, anomalous first rib, clavicle fracture, and first rib fracture.
Soft tissue causes include the following: congenital bands, poor posture, mass lesions, cervical strain, hypertrophy, and/or injury of the anterior and middle scalene muscles or the pectoralis minor muscle. 

Burner syndrome
This condition is thought to be related to brachial plexus stretch and/or nerve root compression. It generally involves the upper trunk of the brachial plexus, particularly C5-C6.
This condition is usually a result of traction injury with axial distraction or downward force on the shoulder combined with lateral bend of the neck away from the shoulder.
Brachial plexus stretch injuries are found in young athletes, with neck pain being less common.
Nerve root compression is found in older athletes (college), generally associated with cervical disc disease or cervical stenosis. Neck pain is more common. 

Parsonage-Turner syndrome (brachial neuritis)
This condition is generally associated with a viral prodrome, immunizations, and significant pain.
Acute excruciating unilateral shoulder pain is present, followed by flaccid paralysis of shoulder and parascapular muscles several days later.
This condition varies greatly in manifestation and nerve involvement. Due to the extreme pain involved, patients usually present acutely. Often, the affected arm is supported by the uninvolved arm and is held in adduction and internal rotation.
The patient may have atrophy of the affected muscles.
Pain may occur with palpation and active and passive range of motion.
Reflexes may be reduced/absent, depending on the nerves involved.
Sensory loss is usually not prominent, although it may be detectable.
This condition may cause scapular winging.