Central ocular motor control and supranuclear disorders

Distinguish a nerve palsy from a gaze disorder

A cranial nerve palsy weakens a particular set of muscles. A gaze palsy affects coordinated movement of both eyes toward a direction. Internuclear ophthalmoplegia (INO) disrupts communication between ocular motor nuclei. These distinctions help localize a brainstem lesion and identify an acute stroke presentation that cannot be explained by an isolated extraocular muscle problem.

Examine primary alignment, versions, ductions, saccades, smooth pursuit, convergence and the vestibulo-ocular reflex when safe. Describe speed and accuracy, not just the final eye position. Ask about onset, diplopia, imbalance, weakness, altered sensation and speech. Acute neurological symptoms require emergency assessment even if the ocular abnormality initially appears small.

Horizontal gaze circuit

For a horizontal saccade, cortical control reaches the contralateral pontine gaze network. The paramedian pontine reticular formation (PPRF) drives the ipsilateral abducens nucleus. Abducens motor neurons abduct the ipsilateral eye, while internuclear neurons cross and ascend in the contralateral medial longitudinal fasciculus (MLF) to activate medial rectus neurons in the third-nerve nucleus. Thus rightward gaze requires right lateral rectus and left medial rectus to work together.

A right sixth-nerve lesion principally weakens right abduction. A right abducens nuclear lesion impairs conjugate right gaze because both the motor and internuclear outputs are affected. A PPRF lesion affects horizontal saccadic drive, with the pattern of other movements helping refine localization. Avoid calling every inability to look right a right sixth-nerve palsy.

In a left MLF lesion, the left eye adducts poorly during rightward gaze and the right eye may show abducting nystagmus. Adducting saccades may be slowed even when final range appears nearly normal. Convergence can be preserved but is variable, so its absence does not automatically exclude INO. Multiple sclerosis and vascular lesions are important causes, with age and onset guiding the differential.

One-and-a-half syndrome

A lesion affecting the ipsilateral horizontal gaze pathway plus the ipsilateral MLF produces one-and-a-half syndrome. With a left-sided lesion, neither eye moves normally toward the left, and the left eye also cannot adduct normally on rightward gaze; the remaining conspicuous horizontal movement is right-eye abduction. Associated facial weakness suggests extension within the pons and helps localize the lesion.

Vertical and supranuclear control

Vertical and torsional saccadic control involves the rostral interstitial nucleus of the MLF; the interstitial nucleus of Cajal contributes to gaze holding. Dorsal midbrain disease can cause impaired upgaze, convergence-retraction movements and light–near pupillary dissociation. Assess for hydrocephalus, pineal-region lesions and other causes with appropriate imaging rather than diagnosing a benign isolated motility disorder.

Supranuclear palsy means that control above the ocular motor nuclei is impaired. Reflex eye movements can remain more preserved than voluntary movements, particularly early in progressive supranuclear palsy. Slow vertical saccades may precede a major range limitation. Associated falls, axial rigidity and cognitive or behavioural changes guide neurological assessment. A preserved reflex is a localization clue, not a guarantee of a particular degenerative diagnosis.

Saccadic dysmetria and gaze-evoked nystagmus can point toward cerebellar disease. Impaired smooth pursuit is less specific because age, attention and medication affect it. Differentiate nystagmus from saccadic intrusions such as square-wave jerks or opsoclonus. Opsoclonus is multidirectional chaotic saccadic activity and may require urgent investigation for inflammatory, infectious or paraneoplastic disease.

Safe neurological examination

The vestibulo-ocular reflex counter-rotates the eyes with head movement. In a patient with possible cervical injury, do not perform an oculocephalic manoeuvre until the neck is cleared. In an unconscious patient, pupil findings and brainstem reflexes belong to a broader emergency neurological assessment; sedation and systemic illness can confound interpretation. An ophthalmic finding alone does not establish brain death.

Case localization

A patient suddenly cannot look left with either eye, and the left eye also fails to adduct on right gaze. Localize a combined left pontine gaze/MLF problem and arrange urgent neurological assessment. Another patient has isolated right abduction weakness with otherwise coordinated movements: investigate the sixth-nerve pathway and its cause. The anatomical explanation makes the investigation plan coherent and avoids a vague diagnosis of diplopia.

Source: EBO neuro-ophthalmology curriculum.