10.7 Cranial Neuropathies & Neuro-Ophthalmology

1. Facial Nerve Palsy

The first question in any facial weakness is central or peripheral, and the answer is written on the forehead.

The forehead is spared centrally because the upper facial muscles receive bilateral cortical innervation; the lower face does not. A patient who can raise both eyebrows but has a drooping mouth has a stroke until proven otherwise.

Bell palsy

Acute idiopathic peripheral facial palsy, probably from herpes simplex reactivation, evolving over hours to days. Associated features include hyperacusis (stapedius weakness), loss of taste on the anterior two-thirds of the tongue (chorda tympani), and reduced tearing.

Treatment: oral corticosteroids started within 72 hours improve the rate of complete recovery. Antivirals add little for mild-to-moderate disease and are considered in severe palsy. Eye protection is mandatory — artificial tears by day, lubricating ointment and taping or a shield at night — because an eye that will not close develops exposure keratopathy. Most patients recover substantially within weeks.

Features that argue against Bell palsy and require further evaluation: gradual onset over weeks, bilateral palsy, no improvement after three months, involvement of other cranial nerves, or a vesicular rash.

Bilateral facial palsy is essentially never Bell palsy — think Lyme disease, sarcoidosis, Guillain-Barre syndrome or HIV.

2. Disorders of Eye Movement

The pupil rule in third nerve palsy is one of the highest-yield facts in neurology. Parasympathetic pupillomotor fibers run superficially in the nerve, supplied by the pial circulation, while the somatic motor fibers run centrally and are supplied by the vasa nervorum.

• Compressive lesions (posterior communicating artery aneurysm, uncal herniation, tumor) squeeze the outside first and therefore involve the pupil early — often before ptosis and ophthalmoplegia are complete. This is a neurosurgical emergency requiring urgent CT or MR angiography.
• Ischemic (microvascular) palsies — diabetes, hypertension — infarct the core and characteristically spare the pupil, often with periorbital pain, and resolve over weeks.

Internuclear ophthalmoplegia — impaired adduction of one eye with nystagmus of the abducting eye — localizes to the medial longitudinal fasciculus. Bilateral internuclear ophthalmoplegia in a young adult suggests multiple sclerosis; unilateral in an older adult suggests brainstem stroke.

3. Pupillary Abnormalities and Anisocoria

The first step is to determine which pupil is abnormal, by comparing the anisocoria in light and in dark.

• Anisocoria greater in bright light — the larger pupil is abnormal (it fails to constrict): third nerve palsy, pharmacologic dilation, Adie tonic pupil, traumatic mydriasis.
• Anisocoria greater in darkness — the smaller pupil is abnormal (it fails to dilate): Horner syndrome, pharmacologic miosis.

Horner syndrome — ptosis, miosis and anhidrosis — reflects interruption of the oculosympathetic pathway. The exam value is in localizing it: painful Horner syndrome with neck pain suggests carotid artery dissection (an emergency); Horner syndrome with arm pain and hand weakness suggests a Pancoast tumor; Horner syndrome with brainstem signs suggests lateral medullary (Wallenberg) infarction.

Relative afferent pupillary defect (Marcus Gunn pupil) — detected with the swinging flashlight test — indicates asymmetric optic nerve or extensive retinal disease, not a pupillomotor efferent problem. It is a hallmark of optic neuritis and of severe retinal artery occlusion.

Adie tonic pupil is a large pupil with poor light reaction but slow, sustained near response and often diminished deep tendon reflexes; it is benign. Argyll Robertson pupils are small, irregular pupils that accommodate but do not react to light, classically in neurosyphilis.

4. Optic Nerve Disorders

Optic neuritis

Inflammatory demyelination of the optic nerve.

Classic presentation: a young adult, more often female, with subacute monocular vision loss over hours to days, pain with eye movement (present in the great majority and a key discriminator), impaired color vision (dyschromatopsia) disproportionate to acuity loss, a central scotoma, and a relative afferent pupillary defect. The disc is normal in retrobulbar cases — the classic phrase is that the patient sees nothing and the doctor sees nothing.

Significance: optic neuritis is frequently the presenting event of multiple sclerosis, and MRI of the brain with contrast stratifies subsequent risk based on the number of demyelinating lesions.

Treatment: intravenous methylprednisolone hastens recovery but does not change final visual outcome. Oral prednisone alone at conventional doses is avoided, as it was associated with an increased recurrence rate. Most patients recover useful vision.

Atypical features — bilateral simultaneous involvement, severe or poorly recovering vision loss, or marked disc swelling — should raise neuromyelitis optica spectrum disorder (aquaporin-4 antibody) or MOG antibody disease, both of which are treated differently from multiple sclerosis.

Other optic neuropathies

• Non-arteritic anterior ischemic optic neuropathy — sudden painless monocular vision loss on waking, an altitudinal field defect, and a crowded optic disc in the fellow eye. Associated with hypertension, diabetes, sleep apnea and nocturnal hypotension.
• Arteritic anterior ischemic optic neuropathy — from giant cell arteritis. Older patient, markedly elevated inflammatory markers, jaw claudication, scalp tenderness, and often preceding transient visual loss. Start high-dose corticosteroids immediately; the fellow eye is at imminent risk, and biopsy remains informative for up to two weeks.
• Toxic and nutritional optic neuropathy — ethambutol (a heavily tested cause, producing loss of color discrimination), methanol, B12 deficiency.

5. Papilledema and Idiopathic Intracranial Hypertension

Papilledema means disc swelling caused by raised intracranial pressure and is bilateral by definition. Distinguish it from other causes of a swollen disc:

Idiopathic intracranial hypertension (an enumerated blueprint topic) classically affects young women with obesity, presenting with headache, papilledema, transient visual obscurations, pulsatile tinnitus and sixth nerve palsy.

Diagnostic sequence, in order:

1. Neuroimaging first (MRI with venography) to exclude a mass and cerebral venous sinus thrombosis
2. Lumbar puncture showing elevated opening pressure with normal cerebrospinal fluid composition

Do not reverse that order — lumbar puncture before imaging risks herniation if a mass is present.

Precipitants to ask about: vitamin A and retinoids (isotretinoin), tetracyclines, growth hormone, and withdrawal of corticosteroids.

Treatment: weight reduction, acetazolamide, and — for fulminant vision loss — optic nerve sheath fenestration or shunting. Vision, not headache, is what is being protected, so formal perimetry guides urgency.

6. Vestibular Neuronitis and Acute Vertigo

Enumerated under cranial neuropathy as vestibular neuronitis (acute labyrinthitis).

Acute persistent vertigo lasting days with nausea, gait instability and horizontal nystagmus that beats away from the affected ear, without hearing loss (labyrinthitis adds hearing loss). The essential task is excluding a posterior circulation stroke, since a cerebellar infarct can present identically.

The HINTS examination — Head Impulse, Nystagmus type, Test of Skew — is more sensitive than early MRI for stroke in acute vestibular syndrome. Findings that indicate a central cause are a normal head impulse test, direction-changing nystagmus, or skew deviation. A normal head impulse test in a patient with severe ongoing vertigo is a danger sign, not a reassuring one.

Additional red flags: inability to walk unaided, severe headache or neck pain, and any other focal neurologic finding. Treatment of vestibular neuronitis is short-course vestibular suppressants followed promptly by vestibular rehabilitation, since prolonged suppressant use impairs central compensation.