Ocular motor cranial nerves

Oculomotor nerve palsy

Damage to the oculomotor nerve interrupts motor input to the majority of extraocular muscles, including most of the recti and the inferior oblique, as well as the levator palpebrae superioris, the sphincter pupillae, and the ciliary muscles. If the abducens and trochlear nerves are unaffected, the actions of the lateral rectus and superior oblique muscles respectively will be unopposed. As such, an isolated oculomotor nerve palsy presents as a “down-and-out” shifted eye, an eye directed inferiorly and laterally. An affected individual will also present with eyelid ptosis, or drooping of the eyelid. A fixed, dilated pupil (mydriasis) may also be present if the lesion affects the parasympathetic fibers within the oculomotor nerve.

Weber syndrome

Weber syndrome, otherwise known as medial midbrain syndrome, presents with a constellation of symptoms referred to as a superior alternating hemiplegia. It tends to result from a vascular lesion, typically of the paramedian branches of the posterior cerebral artery (PCA), and affects the medial region of the cerebral peduncle at the level of the superior colliculus and oculomotor nerve in the midbrain. Because of this, patients present with oculomotor paralysis on the same side of the lesion due to damage to the ipsilateral oculomotor nerve, and contralateral hemiplegia due to damage to the upper motor neuron fibers of the corticospinal tract within the crus cerebri.

Oculomotor deficits may include:

• dilation of the pupil
• lack of pupillary constriction in response to light (a fixed pupil)
• drooping of the eyelid(ptosis)
• deviation of the eye in the down and out direction.

Claude syndrome

Claude syndrome, otherwise known as a central midbrain lesion, is characterized by damage to the oculomotor nerve, the red nucleus, and cerebellothalamic fibers in the midbrain.

Damage to oculomotor nerve leads to ipsilateral paralysis of eye movement: this presents with the eye directed in the down and out eye position, with a dilated, fixed pupil and drooping lid.

Damage to the red nucleus and cerebellothalamic tract fibers is associated with contralateral ataxia and a cerebellar-associated tremor.

If the lesion extends to affect medial lemniscus and trigeminothalamic fibers, an affected individual may also present with a loss or reduction in position, vibration, and fine touch sensation in the contralateral arm and parts of the contralateral face respectively.

Benedikt syndrome

Benedikt syndrome describes a constellation of symptoms resulting from the presence of a large midbrain lesion that encompasses the regions damaged in both Weber syndrome and Claude syndrome. As such, deficits include contralateral hemiplegia due to damage to corticospinal fibers; ipsilateral eye movement paralysis with a fixed, dilated pupil and drooping lid due to oculomotor nerve damage; and cerebellar tremor and rubral ataxia due to damage to cerebellothalamic fibers and the red nucleus.

Uncal herniation

The presence of large or rapidly expanding supratentorial lesion (i.e. a lesion above the tentorium cerebelli), medial portion of the temporal lobe especially the uncal portion herniate across the tentorium. Herniating tissue puts pressure on the ipsilateral oculomotor nerve and crus cerebri. When this occurs, parasympathetic fibers within the oculomotor nerve are typically affected first: as such, an affected person may initially present with pupils that constrict uncharacteristically slowly in response to light. As herniation becomes more severe, however, ocular symptoms can progress to include fixed dilation of pupils and weakness and eventual paralysis of the oculomotor nerve and eye movement, resulting in deviation of the ipsilateral eye in the lateral and downward direction.

Weakness or even paralysis on the contralateral side of the body may also be observed due to compression of the corticospinal fibers within the ipsilateral crus cerebri. Combination of ipsilateral eye paralysis and contralateral hemiplegia makes this a superior alternating hemiplegia. Again, the contralateral hemiplegia is due to fact that the corticospinal fibers have not yet decussated to the side of the body that they are intended to innervate, so compression of corticospinal tract fibers prior to their decussation in the medulla leads to loss of motor innervation to the opposite side of the body.

Kernohan syndrome, or Kernohan phenomenon, occurs when uncal herniation results in a shift of the midbrain to the opposite side. Rather than affecting the oculomotor nerve and crus cerebri on the same side of the brain, shifting of the midbrain leads to stretching of the ipsilateral oculomotor nerve (i.e. on the side of the herniation) and compression of the contralateral crus cerebri against the opposing edge of the tentorium cerebelli, damaging the corticospinal fibers within. This situation presents with ipsilateral paralysis of the oculomotor nerve and ipsilateral hemiplegia.

Trochlear nerve palsy

Damage to the trochlear nerve interrupts motor input to the superior oblique muscle. If the oculomotor and abducens nerves are unaffected, the actions of the recti and inferior oblique muscles will be unopposed. Characteristic finding is a hypertropia due to superior oblique palsy and patient may have a head tilt towards the opposite shoulder to minimize the vertical diplopia which is a hallmark of an isolated trochlear nerve palsy. .

Abducens nerve palsy

Damage to the abducens nerve interrupts motor input to the lateral rectus muscle. If the oculomotor and trochlear nerves are unaffected, the actions of the medial, superior, and inferior recti and superior and inferior oblique muscles will be unopposed. Isolated nerve palsy presents with horizontal binocular diplopia with ipsilateral impaired abduction and esotropia (inward turning of eye).

Medial pontine syndrome

Median pontine syndrome typically occurs when there is occlusion of the paramedian branches of the basilar artery and subsequent ischemia of the medial aspect of the pons. This can result in damage to a number of structures, including: the corticospinal fibers, resulting in contralateral hemiplegia; the medial lemniscus, resulting in contralateral diminution or potential loss of vibration, proprioception, and fine touch sensation; abducens nucleus, or abducens nerve, resulting in ipsilateral paralysis of the lateral rectus muscle and subsequently diplopia, or a potential loss of conjugate gaze toward the side of the lesion via interruption of communication between the abducens nucleus of one side of the brain with the oculomotor nucleus on the opposite side. The involvement of medial longitudinal fasciculus may cause anterior internuclear ophthalmoplegia.

Parinaud syndrome

Parinaud syndrome, otherwise known as gaze palsy, typically results from the presence of a tumor in the region of the pineal gland. A tumor in this location may arise from a variety of different types of cells, and as such may be a germinoma, astrocytoma, pineocytoma, or pineoblastoma.

As the tumor grows, it has the potential to occlude the cerebral aqueduct and put pressure on the superior colliculi. Occlusion of the cerebral aqueduct can lead to enlargement of the third ventricle and eventually hydrocephalus. Damage to superior colliculi is associated with upward gaze paralysis: this means an affected person cannot move the eyes up or down, a presentation sometimes referred to as sunset eye sign, setting-sun sign, or sunsetting of the eyes. Eventual impingement of the oculomotor and trochlear nuclei by the tumor can result in a more severe paralysis of eye movement; and if the medial longitudinal fasciculus (MLF) becomes involved, an affected person may also present with nystagmus.

Clinical case

A patient appears at your clinic complaining of diplopia, periorbital pain, and headache. The patient is an overweight, 64 year-old male with a medical history significant for hypertension and type II diabetes mellitus. Although he has been prescribed the antihypertensive medication lisinopril–an angiotensin-converting enzyme (ACE) inhibitor, a drug frequently prescribed to treat patients with co-occurring hypertension and diabetes–and antidiabetic medications including metformin (a drug which acts to decrease glucose production by the liver, decrease intestinal glucose absorption, and increase insulin sensitivity) and canagliflozin (an inhibitor of SGLT-2, a transporter otherwise responsible for reabsorbing glucose in the proximal tubule of the kidney), finger-stick testing quickly reveals that his disease is poorly controlled: the patient’s hemoglobin A1C (glycosylated hemoglobin) is revealed to be 8.0. Consistent with this finding, the patient reveals upon questioning that he lives alone and often misplaces or forgets to take his medications. Upon physical examination, you note that his left eyelid is drooping and his left eye points in the down and out direction. Pupillary light reflex testing reveals a symmetrical response of pupillary constriction.

This patient is presenting with a pupil-sparing oculomotor nerve palsy, characterized by the loss of somatic motor function (i.e. loss of function of the levator palpebrae superioris; medial, superior, and inferior recti; and inferior oblique muscles) with retention of special visceral afferent function of the constrictor pupillae muscle.

CN III palsy with sparing of the pupil is associated with ischemic neuropathy, which typically occurs secondary to microvascular disease. As such, it is frequently associated with hypertension, diabetes, or hyperlipidemia. Pupil-sparing CN III palsy, however, may also be associated with a compressive lesion, such as an aneurysm; although it should be noted that such space-occupying lesions can potentially progress to eventually affect the pupil as well. The presence of an aneurysm should be suspected when the patient does not have a history of diabetes, vascular disease, or other associated risk factors. Vasculitis, such as temporal (i.e. giant cell) arteritis, should also be considered as a potential etiology in older adults.