Semester 2 Anatomy - Anatomy Posterior Triangle Of The Neck

The phrenic nerve is formed from contributions from the C3, 4, and 5 ventral primary rami. It lies on the anterior scalene muscle, immediately deep to the subclavian vein. Since the phrenic nerve innervates the diaphragm, the patient's symptom (difficulty breathing) fits with an injury to the phrenic nerve.
The external branch of the superior laryngeal nerve travels with the superior thyroid artery to innervate the cricothyroid muscle. It is found medial to the scalene muscles. The recurrent laryngeal nerves are branches of the vagus that branch in the thorax. The right recurrent laryngeal loops around the subclavian artery and the left recurrent laryngeal loops around the aortic arch. Then, the two nerves ascend to the larynx, running medial to the scalenes. The sympathetic trunk lies in the prevertebral fascia behind the carotid sheath and in front of the prevertebral muscles. Finally, the vagus nerve runs in the carotid sheath, superficial to the scalene muscles.

The omohyoid muscle is an important landmark in both the anterior and posterior triangles of the neck. In the anterior triangle, the superior belly of the omohyoid muscle serves as the superolateral border of the muscular triangle and the anterior border of the carotid triangle. In the posterior triangle, the inferior belly of the omohyoid muscle divides the omoclavicular triangle from the occipital triangle. So, in both the anterior and posterior triangles, the omohyoid is an important muscle that subdivides the triangles.
All of the other listed muscles are associated with the anterior triangle only. Geniohyoid runs from the hyoid bone to the genu of the mandible-- it is a deep muscle on the floor of the mouth. It is found deep to the submental triangle. Mylohyoid is another muscle associated with the floor of the mouth--it is in the submandibular triangle and the submental triangle. Sternohyoid is a strap muscle -- it is in the muscular triangle. Finally, stylohyoid is a small muscle innervated by the facial nerve which retracts and elevates the hyoid. It is found in the submandibular triangle.

The subclavian artery runs between the anterior and middle scalene muscles. Since this is the major artery supplying blood to the upper limb, occluding this artery might cause the symptoms of pain and weakness. This patient has a thoracic outlet syndrome, which means that one of the arteries or nerves that is passing from the thoracic outlet into the upper limb or or lower neck is being compressed. Thoracic outlet syndrome may be caused by several things, including a cervical rib (when ribs start forming on C7, not T1) , hypertrophy of scalene muscles, or hyperabduction of the upper limb. Specifically, this patient has scalenus anticus syndrome, which means that there is a compression of structures between the scalene muscles.
The axillary artery is the continuation of the subclavian artery lateral to the 1st rib, and the brachial artery is the continuation of the axillary artery distal to teres major. These arteries and their branches are not receiving blood because the subclavian artery is compressed. The brachiocephalic trunk is the structure off the aortic arch which gives rise to the right subclavian and common carotid arteries, and the suprascapular artery is a branch off the thyrocervical trunk that contributes to the scapular anastomosis.

The inferior thyroid artery branches from the thyrocervical trunk to provide blood to the lower poles of the thyroid gland. This artery approaches the posterior side of the thyroid gland, crossing the recurrent laryngeal nerve and coming near the middle cervical sympathetic ganglion. This artery also supplies blood to the parathyroid glands, which are on the posterior surface of the thyroid. The inferior thyroid veins are associated with the lower parts of the gland--however, these veins are on the anterior side of the thyroid and are not as closely associated with these structures found deep to the gland. The superior thyroid artery and vein are associated with the superior pole of the thyroid. Remember--the important relationship here is that the superior thyroid artery travels with the external branch of the superior laryngeal nerve.

The phrenic nerve is formed from contributions from the C3, 4, and 5 ventral primary rami. It lies on the anterior scalene muscle, which means that the surgeon would need to watch for this nerve while transecting the anterior scalene. None of the other structures lie upon the anterior scalene. The roots of the brachial plexus come out between the anterior and middle scalenes, and the trunks lie lateral to the anterior scalene. The long thoracic nerve comes off the C5, 6 and 7 roots of the brachial plexus and can be seen piercing the middle scalene muscle. This nerve lies on the surface of serratus anterior in the thorax, innervating that muscle. The sympathetic trunk is found deep to the scalene muscles; it runs in the prevertebral fascia along the front of the vertebral column. The vagus nerve runs within the carotid sheath, superficial to the anterior scalene muscle. It passes between the subclavian artery and vein.

The cervical sympathetic trunk lies in front of the prevertebral muscles in the prevertebral fascia. It is not part of the carotid sheath. All of the other structures listed are in the carotid sheath. The carotid sheath contains 3 vessels: internal carotid, common carotid, and internal jugular vein. It also contains 2 nerves: vagus and the superior ramus of ansa cervicalis.

Remember: if the tip of the shoulder is drooping, trapezius is denervated, and the accessory nerve has been damaged! The accessory nerve (CN XI) runs through the posterior triangle, so it is likely to be injured in posterior neck triangle operations. If the accessory nerve is damaged there, the trapezius will be denervated, but the sternocleidomastoid will be OK. Ansa cervicalis is a structure of the cervical plexus that innervates the strap muscles; if this structure was injured, you would note weakness in the strap muscles, not the trapezius. The facial nerve innervates the muscles of facial expression; it is not found in the posterior triangle. The hypoglossal nerve provides motor innervation to the tongue--it is also not in the posterior triangle. Finally, the suprascapular nerve innervates supraspinatus and infraspinatus. It is located in the posterior triangle, and it might get injured in posterior neck operations, but the symptoms here are consistent with an injury to the accessory nerve.

The recurrent laryngeal nerves have different relationships to structures on the right and left sides of the body. On the right side, the recurrent laryngeal nerve loops around the right subclavian artery at approximately the T1/T2 vertebral level. On the left side, the recurrent laryngeal nerve loops around the arch of the aorta, behind the ligamentum arteriosum, at approximately the T4/T5 level. After this, both the right and left recurrent laryngeals ascend in the tracheoesophageal groove to innervate all the intrinsic muscles of the larynx, except the cricothyroid. So, you should see that the right recurrent laryngeal has a special relationship to the right subclavian that would put it in danger during a surgical procedure.

The greater occipital nerve is a branch of the dorsal primary ramus of spinal nerve C2--it provides cutaneous innervation to the posterior scalp. None of the other listed nerves are dorsal primary rami. The great auricular nerve comes from the C2 and C3 branches in the cervical plexus; it provides sensory innervation to the ear and the skin below the ear. The lesser occipital nerve also comes from C2 in the cervical plexus--it innervates the skin behind the ear. The phrenic nerve is comprised of contributions from the ventral primary rami of C3, C4, and C5--it innervates the diaphragm. The supraclavicular nerves come from C3 and C4 in the cervical plexus. These cutaneous nerves provide sensory innervation to the skin of the root of the neck, upper chest, and upper shoulder

Subclavian steal syndrome occurs when there are abnormal blockages in vessels coming off of the aortic arch. Due to these blockages, there may be low blood flow to one arm through the subclavian artery. During high activity in the underperfused arm, the body needs to find a way to send blood to the arm to meet metabolic demands. It does this by shunting blood from the vertebral arteries.
Usually, blood flows from the left and right vertebral arteries to the basilar arteries to the brain. However, with subclavian steal syndrome, blood can flow from the left vertebral artery toward the right vertebral artery. Then, there will be retrograde flow from the right vertebral artery toward the subclavian artery, which should help supply blood to the arm. This means that blood that should be getting to the brain is going to the arm, so these patients might present with symptoms of dizziness and lightheadedness - they aren't getting enough blood to their brains. The other arteries listed cannot participate in similar shunts from the brain to the upper limb.
You should remember the specific association with subclavian steal syndrome and the vertebral arteries!

A tumor of the thyroid gland could disrupt the preganglionic fibers traveling in the cervical sympathetic trunk. The cervical sympathetic trunk lies in front of the preverterbral muscles in the preverterbral fascia--it is in the right location to be compressed by a thyroid tumor. Disrupting the cervical sympathetic trunk leads to Horner's syndrome, a disorder with a variety of characteristic symptoms. Sympathetic nerves innervate the dilator pupillae muscle, which allows the eye to dilate. If these sympathetic nerves are lost in a patient with Horner's syndrome, the pupil will contract due to unopposed parasympathetic action on sphincter pupillae. Ptosis (a droopy eyelid) is seen because the superior tarsal muscle, which raises the eyelid involuntarily, is innervated sympathetically. If this muscle is paralyzed, the eyelid droops. You might also observe enophthalmos (a sunken eye), possibly due to the paralysis of a smooth muscle in the floor of the orbit. Without the sympathetic nerve supply, the vasculature of the face cannot constrict. So, the arterioles in the patient's face are vasodilated, making the face red. Sympathetic nerves also innervate sweat glands; if these nerves are interrupted, the patient will not sweat and the face will appear very dry. These are all characteristic symptoms of Horner's syndrome, and you should be familiar with all of these!
Parasympathetic fibers innervate the glands of the face, cause the pupil to constrict, and allow for accommodation of the eye for close vision; they are not involved with Horner's syndrome. The internal carotid nerve carries postsynaptic sympathetic fibers from the superior cervical ganglion to the nasal cavity. The thyroid gland is near the level of the middle cervical ganglion, not the superior cervical ganglion.

The roots of the brachial plexus are between the anterior and middle scalene muscles; the trunks of the brachial plexus which come from these roots travel in the posterior triangle. So, the trunks of the brachial plexus are not related to the anterior surface of the anterior scalene muscle. The phrenic nerve is comprised of contributions from the ventral primary rami of C3, C4, and C5. This nerve courses along the anterior surface of the anterior scalene. The brachiocephalic vein also crosses the anterior surface of the anterior scalene as it reaches its connection with the internal jugular vein. The transverse cervical artery is a branch of the thyrocervical trunk which crosses the anterior scalene. Finally, the cervical part of the thoracic duct dumps into the brachiocephalic vein on the left side of the neck. So, on the left side of the neck, the thoracic duct lies on the anterior part of the anterior scalene.

The subclavian artery crosses between the anterior and middle scalene muscles, which means that it is also posterior to the sternocleidomastoid muscle. Lateral to the sternocleidomastoid muscle, the subclavian artery runs just above the clavicle. Eventually, this artery descends toward the upper limb. So, it is in the right position to be injured in the stabbing. The question also mentions that the wound was life-threatening, so you should really be looking for an answer about an artery.
The brachial plexus is also in the area of the stab wound, but a brachial plexus injury would not be life-threatening. The internal jugular vein is located medial and posterior to the sternocleidomastoid muscle--it is not likely that this vein would be cut. The axillary artery is a continuation of the subclavian artery beneath the level of the first rib, which is inferior to the area of interest. The thoracic duct, which dumps into the brachiocephalic vein on the left side, might be injured in the stabbing, but this would not be a life-threatening problem.

The subclavian vein lies anterior to the right and left anterior scalene muscles. It can be found coursing over the anterior scalene muscles on both the right and left sides. The subclavian arteries lie posterior to the anterior scalene muscles--they travel between the anterior and middle scalenes on both sides of the neck. The thoracic duct is found on the left side only--it would not be on the right side of the neck. On the left side, the thoracic duct enters the left brachiocephalic vein where it is formed by the union of the subclavian and internal jugular veins. So, the thoracic duct does contact the anterior scalene muscle, but only on the left side of the neck. The thyrocervical trunk is a branch of the first part of the subclavian artery, medial to the anterior scalene. It is not in contact with the anterior scalene. Finally, the vagus nerve travels with the carotid vessels in the carotid sheath. It is anterior to the anterior scalene, but not in immediate contact with the muscle. This means that subclavian vein is the best answer.

The deep cervical chain of lymph nodes runs closely with the internal jugular vein. The internal jugular vein does not drain all of the thyroid gland--the superior and middle thyroid veins drain into the internal jugular vein, but the inferior thyroid veins drain directly into the brachiocephalic veins. The internal jugular vein does not drain into the external jugular vein. The external jugular vein is a smaller vein on the lateral neck which drains into the subclavian vein. The internal jugular vein meets the subclavian vein, and the two vessels join to form the brachiocephalic vein. The prevertebral fascia is found deep in the neck, over the vertebrae and the prevertebral muscles. So, the internal jugular vein is very superficial to this fascia. Finally, the sternocleidomastoid muscle crosses over the internal jugular vein.