Clinical case: Oculomotor nerve palsy and distal myopathy

Objectives

• The attachments, function and innervation of the extraocular muscles, especially as they relate to this case.
• What is meant anatomically by “optic disc swelling;” The clinical significance is of this condition.
• What is the difference between communicating and non-communicating hydrocephalus, and how they are typically differentiated radiologically.
• What is meant anatomically by “tonsillar herniation;” the clinical significance of this condition.
• The locations and contents of the cavernous sinus, and how compression of the sinus could result in some of the ocular signs and symptoms discussed in this case.

Extraocular muscles

This patient showed left ptosis, which suggests some paresis of the left levator palpebrae muscle and accordingly compromised function in the left oculomotor nerve (and its contained sympathetic fibers; cf. below). The patient also showed right hypertropia and exotropia, suggesting paresis in the inferior and medial recti muscles and compromised function in the right oculomotor nerve. More details on the extraocular muscles are presented directly below (see also Figure 5):

• Levator palpebrae superioris: originates from roof of orbit immediately anterior to optic canal - passes forward covering superior rectus - tendon passes into upper lid to attach to superior tarsus and ski; arising from the inferior surface of this muscle is a layer of visceral muscle known as the superior tarsal muscle, which also inserts into the tarsus (the tarsus is a layer of thickened connective tissue within the lids that maintains their shapes); somatic part of the muscle is innervated by superior division of oculomotor nerve and visceral part by sympathetic system; muscle raises upper eyelid; paralysis of smooth muscle results in upper lid drooping (ptosis can involve one or both parts of the muscle); total paralysis results in the inability to open the eye voluntarily.
• Superior Oblique: originates from posterior aspect of roof of orbit and becomes tendinous before reaching the front of orbit; at superior medial corner of front of orbit tendon is attached to the frontal bone by a fibrocartilaginous pulley (trochlea); at the trochlea the tendon turns sharply posteriorly, inferiorly and laterally, deep to the superior rectus to inserts lateral to and in the upper lateral quadrant of the posterior surface of the eyeball; innervated by trochlear nerve; acts to move eye outward and downward; also intorsion (testing the action of this muscle is counterintuitive; the patient is asked to look down and in; this is because only the superior oblique can perform this function).
• Superior, medial, lateral, inferior recti: all originate from annulus tendineus and all insert on the respective anterior quadrant of the eyeball; all are innervated by oculomotor nerve except lateral rectus, which is innervated by abducent nerve; superior rectus elevates and intorts eye whereas inferior rectus depresses and extorts eye; both also cause some adduction; medial rectus adducts eye whereas lateral rectus abducts eye.
• Inferior Oblique: originates from floor of orbit; inserts into lower lateral quadrant of posterior surface of eyeball, below insertion of superior oblique; elevates eye and causes abduction, also extorsion.

Optic disc swelling

The optic disc or optic nerve head is the point of exit for retinal axons exiting the eye. Because there are not any rods or cones overlying the optic disc, this spot corresponds to a small blind area in each visual field. Optic disc swelling (edema) refers to the swelling as seen with an ophthalmoscope that is associated with an increase in fluid within or surrounding the axons of the optic nerve (Figure 2). While unilateral disc edema is more common, bilateral disc edema can occur.

Papilledema is a common form of optic swelling that results from increased intracranial pressure. In this patient, the presence of the tumors in her brain (and especially those that could be exerting pressure on the cavernous sinus) could explain the appearance of the swollen discs, which, in her case, could also have been referred to as papilledema.

Hydrocephalus

Hydrocephalus is a condition in which there is an excessive accumulation of cerebrospinal fluid (CSF) within the brain. Hydrocephalus can occur congenitally or be acquired. There are two primary types of hydrocephalus: communicating and non-communicating. Diagnosis is typically made by examination (increased CSF pressure) and medical imaging.

Communicating hydrocephalus (non-obstructive) is caused by impaired cerebrospinal fluid reabsorption presumably associated with functional impairment of the arachnoid granulations. Non-communicating (obstructive) hydrocephalus (Figures 4&8&9) is caused by a CSF-flow obstruction generally leading to dilation of one or both lateral ventricles, which provides radiological evidence for obstructive rather than non-obstructive hydrocephalus. 

Tonsillar herniation

In tonsillar herniation (downward cerebellar herniation, transforaminal herniation, "coning"), the cerebellar tonsils are displaced inferiorly through the foramen magnum resulting in compression of the lower brainstem and upper cervical spinal cord (Figure 10). This pressure on the brainstem can result in dysfunction of the primary neural structures controlling respiratory and cardiac function, eventually resulting in death.

Cavernous sinus

The cavernous sinus is a dural venous sinus that is located on the left and right sides of the sella turcica (Figures 4B). The sinus receives blood from the superior and inferior ophthalmic veins through the superior orbital fissure and from superficial cortical veins, and has connections to the basilar plexus of veins posteriorly. The ICA (carotid siphon), and cranial nerves III, IV, V1, V2 and VI all pass through this blood filled space.

The cavernous sinus drains via the superior and inferior petrosal sinuses, ultimately into the internal jugular vein via the sigmoid sinus. Because the superior and inferior ophthalmic veins drain into the sinus, any increases in pressure in the sinus (as likely caused by a tumor in this case) will impede venous drainage from the eye and orbit. This will result in papilledema and also can cause exophthalmos (protruding eye). And, similarly, compression of the sinus can result in nerve compression (e.g., oculomotor nerve), which will manifest in extraocular muscle weakness, as found in this patient.