The most prominent aspect of the thyroid cartilage corresponds to the vertebral level of

Anatomy

The larynx is located anteriorly to the fourth, fifth, and sixth cervical vertebrae. It is localized by the neighboring boundaries of the oropharynx superiorly, the hypopharynx posteriorly and inferiorly, and the trachea inferiorly. The larynx is composed of multiple cartilages, one bone (hyoid), and multiple ligaments and muscles. The laryngeal framework is composed of the cartilages and hyoid bone. The hyoid bone is not articulated to any other bone, but rather is connected distantly via muscular and ligamentous attachment. The hyoid is analogous to the “wishbone” or furcula of poultry.

The thyroid and cricoid cartilages compose the main framework. The thyroid cartilage has a notch superiorly that makes the protuberance of the Adam’s apple. The cricoid cartilage marks the narrowest portion of the adult airway, because it is the only complete circular ring of the airway (as opposed to the tracheal rings that are truly arches and not complete circular rings).

The epiglottis is composed of fibroelastic cartilage covered with a mucosal surface that tapers inferior to the petiole of the epiglottis. It has both a lingual surface that is considered within the boundaries of the oropharynx and a laryngeal surface that is part of the larynx. Anterior to the epiglottis is the preepiglottic space, a frequent route for spread of malignancy in this region. The vallecula signifies the base of the tongue and the base of the lingual surface of the epiglottis. This is a familiar landmark for the placement of a commonly used laryngoscope blade utilized to facilitate endotracheal intubation in the securing of a patient’s airway during surgery.

Posteriorly along the larynx are the arytenoids, a paired set of pyramid-shaped cartilages that rest upon the cricoid cartilage posteriorly. The arytenoids allow for a forward rocking and a rotational motion that lead to the adduction and abduction of the vocal folds. The three functions of the larynx—swallowing, phonation, and respiration—all rely on the motion of the vocal cords. The intrinsic muscles of the larynx allow for vocal fold motion of the vocal folds. The true motion is more than just adduction and abduction; it is a complex movement in all three dimensions. The vocal fold on either side is primarily composed of the thyroarytenoid muscle, ligament, and a fibroelastic epithelial layer.

The larynx is divided into three parts: the supraglottis, glottis, and subglottis. The supraglottis involves the tip of the epiglottis down along the aryepiglottic folds down to the false vocal folds (ventricular bands). The glottis is composed of the true vocal fold and posterior commissure. The subglottis begins at the inferior aspect of the true vocal folds at the junction of the squamous and respiratory epithelium down to the inferior edge of the cricoid cartilage (Figure 17-1).

Larynx

The adult larynx is found at the level of the first through the fifth cervical vertebrae, consisting of a number of articulated cartilages surrounding the upper end of the trachea (Fig. 13.6).52 “Adam's apple” is another common name for the larynx (more accurately, Adam's apple denotes the thyroid cartilage). The laryngeal cavity extends from just below the epiglottis to the lower level of the cricoid cartilage, where it becomes continuous with the trachea.

The primary function of the larynx is phonation, but it also has a protective function because the airway becomes quite narrow at this point. Structures found within the laryngeal cavity include the vestibular folds (i.e., the false vocal cords) and the vocal cords (i.e., the true vocal cords), which are two pearly white folds of mucous membrane.

The narrowest portion of the larynx in the adult is located at the true vocal cords.47 Larger aspirated objects will become lodged at this site. They can usually be dislodged by the abdominal thrust or chest thrust. In the child younger than 10 years, the narrowest portion of the larynx occurs at the level of cricoid cartilage.47,48 Should material be small enough to pass between the vocal cords, in the adult and in larger children, it will usually enter either the right or left main stem bronchus, a situation that is serious but not acutely life threatening.

Point Locations and Indications

A group of 42 points, running bilaterally along the spinal column, are distributed along the cervical, thoracic, lumbar, and sacral vertebrae (Figure 15-1).

Respiratory System

The degree of airway and respiratory compromise depends on the level of the SCI, as well as the presence of concomitant injuries. In cervical cord injuries, the level of spinal cord edema and hence dysfunction may ascend and make respiratory support necessary. In addition, injuries below the fifth cervical vertebra (C5) affect the intercostal muscles, which may decrease alveolar ventilation and also limit the effectiveness and strength of cough. In unstable thoracic spine injuries, particularly those associated with rib fractures and lung contusions, there is a high risk for the development of respiratory complications.

In lesions at the C4/C5 level, voluntary respiration is maintained but vital capacity is reduced by as much as 20% to 25%. These patients often require ventilatory support. In any lesion above C4, accessory, diaphragmatic, and intercostal muscle function may be lost and require total ventilatory support.

Introduction

Our endocrine system is a highly sophisticated hierarchical system that dictates efficiency as well as dynamic control of different processes in our body such as metabolism, growth, and sexual and emotional development. The thyroid gland is unique among the endocrine organs as it maintains a large store of hormone. The thyroid gland is a brown-red highly, vascular gland located anteriorly in the lower neck extending from the fifth cervical vertebra down to the first thoracic vertebra. It weighs approximately 25 g but can be slightly heavier in females and can change in size during menstruation and pregnancy. The estimation of the size of the thyroid gland can be easily achieved noninvasively by clinical examination (Standring, 2016).

The thyroid gland is wrapped in an outer false and an inner true capsule, which separates the gland dividing its lobes into lobules. Cystic follicles are the main structural units of the gland and are lined by single epithelial layer surrounding a colloid filled cavity. The principal (follicular) cells produce thyroglobulin (Tg), a dimeric protein used by the gland as a precursor to thyroid hormones. In addition to the principal follicular cells, parafollicular cells (clear cells) secrete calcitonin hormone which is involved in the calcium homeostasis process. An extensively dense network of fenestrated capillaries, lymphatic vessels, and sympathetic nerves oversees the follicles whose size changes according to their activity and hence the abundance of colloid material (Standring, 2016).

Pathophysiology

The spinal cord is the major conduit through which motor, sensory, and autonomic information travels between the brain and the body. Pairs of spinal nerves enter and exit throughout the length of the spinal cord and are named based on the vertebral level they originate from. Eight cervical, twelve thoracic, five lumbar, five sacral, and one coccygeal nerves make up the 31 spinal nerves on each side. Nerve roots that exit and enter the spinal cord excite groups of muscle cells, or myotomes, and receive sensory information from skin areas, or dermatomes. Nerve roots are numbered according to the vertebral level they exit and enter. For example, nerve roots that exit the spinal cord at the fifth cervical vertebra excite C5-innervated muscles. An SCI interrupts the conduction of both efferent and afferent signals, resulting in varying degrees of sensorimotor loss and functional deficit. It is important to understand that the level of vertebral injury does not always correlate with the neurologic level of injury. For example, the spinal cord in adults usually terminates at either the L1 or L2 vertebra. Vertebral fracture or disk herniation involving the lower lumbar or sacral levels would likely involve injury to the lumbosacral spinal nerves and not the spinal cord proper, as in cauda equina syndrome.49,130

The cervical spinal cord is encased within seven cervical vertebrae. At the first and second levels, the diameter of the spinal cord is small in relation to the size of the spinal canal. The cord occupies only one third of the canal at C1–2, but it occupies half of the canal at C7.57 Because range of motion (ROM) is greatest at the C5, C6, and C7 vertebrae, and because of the relationship between canal size and cord size in the lower cervical region, most injuries occur at those levels.

Traumatic SCI generally results in disruption of the spinal cord architecture, followed by a complex pattern of pathophysiologic processes that exacerbate the injury.82 Although they may be classified as functionally complete, most traumatic injuries do not result in complete disruption of the cord. Residual effects are often characterized by a peripheral rim of intact tissue with a central cystic cavity.17,69,82 At the level of injury, injuries to white matter and gray matter result in a combination of central and peripheral injuries. Damage to the white matter affects motor control and sensory input from the periphery to the brain below the level of injury. In addition, at and surrounding the injury level, evidence can be seen of lower motor neuron (LMN) injury resulting in flaccid paralysis at the level of injury.109,146 Muscles with LMN injury are also at risk for contractures25 as a result of shortening and scarring and are not amenable to functional electrical stimulation (FES). Coulet et al.25 have detailed the implications of SCI pathophysiology on upper extremity rehabilitation.

Vertebral Artery

The vertebra artery is divided into 4 segments. The first segment (V1) of the vertebral artery extends from its origin to the point of entrance into the foramen of the cervical transverse process, which is usually the sixth body. The vertebral artery is usually the most proximal and largest branch off of the subclavian artery. Multiple variations in the anatomic course and origins of the vertebral arteries have been described. The most common variation in vertebral artery origin is in the origination from the proximal subclavian artery. The origin of the left vertebral artery from the aortic arch between the left common carotid artery and left subclavian artery has been described in 2.4-5.8% of cases. When there is an origin of the vertebral artery from the arch, the vertebral artery usually enters the foramen of the transverse process of the fifth cervical vertebrae. With a normal origin of the left vertebral artery from the subclavian artery, the vertebral artery enters the transverse foramen of the sixth cervical vertebrae in nearly 88% of cases. The site of entrance at the level of C4 is seen in 0.5%, C5 in 6.6%, and C7 in 5.4%. Rare examples of origins of the left vertebral artery from the left common carotid artery, or external carotid artery, have been described. Also rare are variations in the origin of the right vertebral artery (less than 1%) from the aorta, carotid arteries, or brachiocephalic arteries.

The second segment (V2) of the vertebral artery extends superiorly through the foramen of the transverse processes in a vertical course until it reaches the transverse process of C2. The third segment (V3) of the vertebral artery extends from the exit of C2 to its entrance into the spinal canal. After leaving the transverse foramen of C2, it courses laterally and posteriorly to pass through the transverse foramen of C1. The vertebral artery then extends posterior and medially in a horizontal groove on the upper surface of the posterior arch of C1. The vertebral artery turns abruptly as it nears the midline and pierces the posterior atlantooccipital membrane and enters into the vertebral canal. Anomalous connections in this region are uncommon but include the proatlantal intersegmental artery, which can communicate between the internal or external carotid artery and the vertebral artery at this level. Local duplication or fenestration of the V3 segment can occur. The occipital artery also can arise from the V3 segment. A persistent first intersegmental artery can occur where the vertebral artery courses below the C1 arch after exiting the transverse foramen of C2 and enters the spinal canal without passing through the C1 transverse foramen (3-4%). The origin of the posterior inferior cerebellar artery (PICA) may also be anomalously low between C1 and C2.

The fourth segment (V4) segment pierces the dura and extends through the foramen magnum where it lies anterior to the medulla and eventually joins the contralateral vertebral artery to form the basilar artery. Major branches arising off the vertebral artery include multiple muscular branches from the extracranial segments to supply the deep muscles of the neck and meningeal branches. The posterior meningeal branch arises from the vertebral artery above the level of C1, and below the foramen magnum, and supplies the falx cerebelli and the medial portion of the dura of the occipital posterior fossa. Just before joining to form the basilar artery, each of the vertebral arteries give off a branch that will become the anterior spinal artery, which extend downward and medially to join in the midline with a corresponding branch from the other vertebral artery. The posterior spinal arteries can originate from the posterior inferior cerebellar arteries or from the intracranial portion of the vertebral arteries.

Etiopathogenesis

Paraplegia is one of the major mortifying complications. Occasional case reports of midcervical quadriplegia in the postoperative period after posterior fossa procedures performed in the sitting position have been reported in the literature. Hitselberger and House mentioned five unreported cases of midcervical quadriplegia after cerebellopontine angle neuroma resection performed in the sitting position.1 Acute focal pressure on the spinal cord, along with the flexion of the neck, was the postulated mechanism. Matjasko and colleagues reported a case of quadriparesis due to sitting position in a patient with severe cervical stenosis.2

Wilder highlighted the fact that acute flexion of the neck in a patient under general anesthesia in the sitting position may cause stretching of the cord at the level of fifth cervical vertebra3 due to which regional cord perfusion is compromised, especially if intraoperative hypotension occurs.

According to a previous report, quadriplegia can also be a major complication of lumbar spinal surgery. Several possible mechanisms have been held responsible for cervical spinal cord damage.4

First, neuromuscular blocking drugs cause a decrease in the tone in the cervical musculature during general anesthesia so that already present spondylitic bars may cause compression of the spinal cord. Haisa and Kondo5 demonstrated slight intervertebral disk bulge which occurred during surgery because of acute neck flexion that led to compression and cord stretching, leading to myelomalacia.

Second, blood flow may be impaired in the upper spinal cord as a result of prone position with hyperflexion. Wilder3 suggested that flexion of the cervical spine during general anesthesia may produce enough spinal cord stretching to alter the autoregulation by mechanically affecting the spinal cord vessels.

When the neck position is changed from neutral position to full flexion, the entire cervical cord is elongated by 10% of its initial length as compared to the neutral position.6 Hence the longitudinal arteries are stretched and constricted with this elongation.

Anatomically, the cervical spinal cord receives its major share of blood from the longitudinal trunks (anterior and posterior spinal arteries), supplied by radicular branches of vertebral arteries.7,8 Turnbull et al.9 demonstrated that the cervical cord receives zero to eight radicular arteries.

Because of the very critical sources of blood supply to the spinal cord, any pathology that interferes with the arterial supply can cause ischemia, infarction, or both. The vertical extensity of spinal cord infarction depends upon the limit of vascular occlusion and collateralization.

Though spinal arteries are least susceptible to atherosclerosis, but multiple aortic atheromas may occasionally be the cause of infarction or ischemia. Various other possible sources of infarction are emboli originating from disk substance, 10infection, 11or stasis of blood because of extreme neck rotation or hyperflexion in the prone position.12

The vertebral body in its anterior most and lateral extents receive nutritional vessels termed anterior central arteries, sourcing from the anterior branches of paired vertebral arteries at the same or adjacent level.7,8 In contrast to the bony territory of the spinal cord, the radicular arteries which are the major sources of blood to the cervical spinal cord originate eccentrically and unilaterally form the paired vertebral arteries, passing through the root sheath of dura mater along with the nerve roots to the cervical cord with increasing obliquity from the cranial to caudal end, and contribute to blood supplying the longitudinal arterial trunk of the spinal cord.7,8 Therefore, the levels of cord ischemia are not the same as the levels of the vertebral column.

Few case reports also describe the use of oxidized cellulose (Surgicel) as a hemostat in spine surgeries which may swell up and cause spinal cord compression resulting in paraplegia. Oxidized cellulose is used commonly in many surgical fields as a hemostat. However, its tendency to swell up once placed causes increase in the compression risk of the cord in closed or bone walled spaces.

Dorsal Approach to the Occipitocervical Region

The dorsal approach is most commonly used when fusion of the OC region is indicated. This approach has been described by different authors, including Grantham and associates4 and Wertheim and Bohlman.5 Key in the approach is positioning of the patient to allow safe intubation and protect the neural elements. Longitudinal traction should be applied preoperatively to provide stability during the intubation process. The patient is then log-rolled into the prone position. Support for the head may also be provided using a Mayfield three-point headrest. Radiography or intraoperative fluoroscopy is used to confirm the alignment of the occiput to the atlas and the remainder of the cervical spine. The skin is then prepared, and the subcutaneous tissues are injected with a solution of epinephrine 1 : 500,000. A midline incision is made, extending from the external occipital protuberance to the spinous process of the fourth or fifth cervical vertebra. The spinous process of the C2 is the most prominent of the spinous processes encountered during the approach. The spinous process of C2 is bifid, allowing the short external rotators of the head to be attached to the cervical spine. Once the skin is incised, the incision is extended into the deep fascia and then into the ligamentum nuchae. It is very important to remain in the midline to avoid excessive bleeding. This placement can be confirmed by palpating the alignment of the spinous processes and by visualizing the avascular midline plane of the ligamentum nuchae. By staying in the midline, the paramedian venous plexuses are avoided. The paravertebral muscles are stripped off the spinous processes and the lamina subperiosteally to avoid excessive bleeding.

Although some may believe that it is safe to use Cobb elevators in dissecting the muscles subperiosteally off the lamina, the authors do not recommend this technique. The fact that the laminae are weaker in this region than in the lumbar spine may lead to fracture of the lamina because of excessive force, as well as increased blood loss caused by uncontrolled stripping of the musculature. However, a Cobb elevator may be used to gently retract the muscles, placing them under tension, while the muscles are stripped off of the lamina using a freer elevator or cautery in a controlled manner. At the base of the skull, full-thickness scalp flaps are reflected along the occipital ridge about 2 to 3 cm laterally. The extensive lateral dissection along the lamina of the cervical spine should be to the groove, which indicates the junction of the lamina along with the articular facet. Once the occipital exposure is completed, special care must be taken during the dissection of the arches of C1. The vertebral artery runs on the rostral surface of the arch and the lateral third of the arch (see Figs. 45-1 and 45-2). To expose this area safely, only 1 cm on each side of the dorsal arch of C1 is dissected. In this area, it is important to elevate the muscles subperiosteally. Cauterizing in this area is not recommended because of the thin membrane that attaches the base of the skull to the arch of atlas. Once the bony occipital protuberance, the dorsal arch of the atlas, and the remainder of the laminae of the cervical spine have been exposed, arthrodesis may be completed. This may be performed using the technique described previously by Grantham or using modifications introduced by other authors.4 With this technique, 24-gauge stainless-steel wires are used along with an iliac crest bone graft that is contoured to span the distance from the occiput and the upper cervical laminae after the laminae and occiput are decorticated with a bur. Occipital plates or rods that are inserted into the lateral mass of C1 and C2 using screws may also be used to provide more rigid fixation.

Injection of the Cervical Nerve Root