Disorders of the Eye and Ocular Motility

Monocular diplopia is intrinsic to a single eye and persists even when the other eye is covered. Common causes include refractive errors, cataracts, or keratoconus.

Binocular diplopia is caused by ocular misalignment (strabismus) and disappears when either eye is occluded. Binocular diplopia is further divided into:

1. Neurogenic: Resulting from cranial nerve palsies.
2. Restrictive: Resulting from physical limitations of the extraocular muscles, such as in thyroid eye disease or orbital pseudotumor.

Cranial Nerve Palsies

The coordination of eye movements relies on the oculomotor (III), trochlear (IV), and abducens (VI) nerves. Damage to these nerves produces predictable patterns of misalignment.

Oculomotor (III) Nerve Palsy

The oculomotor nerve controls most of the extraocular muscles and the levator palpebrae. A palsy typically presents with ptosis and a characteristic "down and out" eye position due to the unopposed action of the remaining lateral rectus and superior oblique muscles.

Trochlear (IV) Nerve Palsy

The trochlear nerve innervates the superior oblique muscle. A palsy results in vertical diplopia. Patients often adopt a compensatory head tilt to minimize the misalignment and alleviate the double vision.

Abducens (VI) Nerve Palsy

The abducens nerve innervates the lateral rectus muscle. A palsy leads to horizontal diplopia, which is typically most pronounced when the patient attempts to gaze toward the side of the lesion.

Internuclear Ophthalmoplegia (INO)

INO is a specific deficit caused by damage to the medial longitudinal fasciculus (MLF), the pathway connecting the abducens nucleus in the pons to the oculomotor nucleus in the midbrain. The hallmark of INO is impaired adduction of the ipsilateral eye during attempted lateral gaze. Multiple sclerosis is a common cause of this condition.

Supranuclear Disorders

Supranuclear disorders affect the higher-order control of gaze rather than the nerves that directly innervate the muscles.

Parinaud's Syndrome

Also known as dorsal midbrain syndrome, this condition results from damage to the posterior commissure, often due to pineal tumors or hydrocephalus. It presents with a classic constellation of signs:

• Loss of upward gaze (upgaze palsy).
• Convergence-retraction nystagmus.
• The "setting sun" sign (downward ocular deviation).
• Light-near dissociation of the pupils.

Nystagmus and Rare Oscillations

Nystagmus is defined as a rhythmic, involuntary oscillation of the eyes.

Jerk Nystagmus

Most clinical nystagmus is "jerk nystagmus," which consists of a slow drift away from the target followed by a fast, corrective saccade. It is categorized by the direction of the fast phase:

• Horizontal: Left or right.
• Vertical: Upbeat or downbeat.
• Torsional: Rotational movement.

Gaze-evoked nystagmus is a common form where the oscillations occur when the eyes are held in an eccentric (deviated) position.

Opsoclonus

A rare and dramatic disorder is opsoclonus, characterized by bursts of consecutive, rapid, and irregular saccades, sometimes referred to as "saccadomania". This can be associated with viral encephalitis or paraneoplastic syndromes (e.g., neuroblastoma).