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Supranuclear gaze palsy should be suspected in patients with bilateral and symmetric vertical or horizontal gaze difficulty. Use of specific maneuvers to bypass the supranuclear input to the ocular motor nuclei can help to establish a supranuclear localization to the problem and direct neuroimaging to the supranuclear pathways in the brain and brainstem.
This article briefly describes the anatomy and clinical findings of supranuclear gaze difficulties and the localization to allow appropriate neuroimaging (i.e., cranial magnetic resonance imaging [MRI]).
What does supranuclear mean?
Disturbances of horizontal gaze
The initiating supranuclear signal for horizontal gaze originates in the contralateral frontal lobe for fast eye movements (saccades) and in the ipsilateral parieto-occipito-temporal region for smooth pursuit movements.
This supranuclear pathway reaches the ipsilateral sixth-nerve (abducens) nucleus in the pons, and the final common pathway for horizontal gaze includes the pontine reticular formation (PRF) and the sixth-nerve nucleus. The sixth-nerve nucleus receives supranuclear and vestibular input to initiate horizontal eye movements. Thus vestibular input (e.g., doll's head maneuver) can "bypass" a supranuclear horizontal gaze palsy by directly innervating the nucleus.
The sixth-nerve nucleus itself contains the motor neurons for both the lateral rectus muscle and the internuclear neurons to the contralateral medial longitudinal fasciculus (MLF). The MLF interneurons synapse in the oculomotor subnucleus that innervates the medial rectus muscles. In this way, the right sixth-nerve nucleus can direct the eyes horizontally to the right via the peripheral sixth nerve to the right lateral rectus muscle and to the contralateral left medial rectus muscle via the third nerve.
Horizontal gaze palsy. A horizontal gaze palsy can thus result from either a contralateral frontal or ipsilateral pontine lesion. Frontal lobe lesions may impair ipsilateral horizontal smooth pursuit, whereas lesions of the posterior parietal cortex or temporo-occipito-parietal region may produce a decreased amplitude and velocity of smooth pursuit eye movements toward the side of the lesion.
Pontine lesions also can produce the horizontal gaze palsy (i.e., PRF or sixth-nerve nucleus). Thus, patients who are unable to move either eye beyond the midline to the left may have a left pontine lesion, and the eyes may be straight or may be deviated to the right. The patient may have partial horizontal gaze movement if the damage to the pontine structures is only partial.
Pontine nuclear (and infranuclear) lesions usually can be differentiated from supranuclear lesions (e.g., frontal lobe) with the oculocephalic or "doll's eyes maneuver." Passive horizontal rotation of the head directly stimulates the sixth-nerve nucleus via the vestibulo-ocular reflex and will overcome gaze palsies induced by frontal lobe lesions but will not overcome gaze palsies caused by pontine nuclear and infranuclear lesions.
Myasthenia gravis can mimic a horizontal (or vertical) gaze palsy and should be considered in the differential diagnosis. Typically, the doll's head maneuver will not be able to overcome the neuromuscular junction defect seen in myasthenia gravis. Other signs of myasthenia gravis to look for include ptosis, variability, fatigue, other ocular muscle involvement, and systemic weakness. Prostigmine or edrophonium (Tensilon, ICN) testing might reverse the gaze palsy in myasthenia gravis.
Patients with an unexplained supranuclear (or nuclear) horizontal gaze palsy probably should undergo neuroimaging (preferably MRI of the brain and brainstem with gadolinium enhancement). Patients with brainstem lesions may have other neurologic findings (e.g., fascicular sixth-nerve palsy, seventh-nerve palsy, Horner's syndrome, trigeminal dysfunction, hemisensory loss, or hemiparesis) to suggest topographic localization to the brain or brainstem.