What causes skew diplopia on superior temporal gaze in a patient?

Skew Deviation on Superior Temporal Gaze

Skew deviation is the primary cause of vertical diplopia on superior temporal gaze, resulting from disruption of the vestibulo-ocular pathways in the posterior fossa—specifically affecting the brainstem, cerebellum, or vestibular end-organ. 1

Pathophysiology and Clinical Features

Skew deviation represents a vertical strabismus that arises from disorders affecting:

• End-organ vestibular pathways within the inner ear (such as acute vestibular neuronitis) 1
• Supranuclear utriculo-ocular pathways within the posterior fossa, including both brainstem and cerebellum 1

The characteristic presentation includes:

• Vertical misalignment that may be comitant or incomitant, typically ranging from 1-30 prism diopters (median 5 PD) 2
• Ocular torsion: The hypertropic eye demonstrates fundus incyclotorsion while the hypotropic eye shows fundus excyclotorsion 1
• Ocular tilt reaction: Head tilt toward the shoulder of the hypotropic eye, with both eyes rotating in the direction of head tilt 1
• Vertical visual field tilt in the same direction as the head tilt 1

Critical Differential Diagnosis

You must distinguish skew deviation from fourth nerve (superior oblique) palsy because skew deviation demands urgent neuroimaging and may indicate stroke, demyelination, or mass lesion requiring immediate treatment. 1

Key distinguishing features:

• Fundus torsion pattern: In skew deviation, the hypertropic eye is incyclotorted (versus excyclotorted in fourth nerve palsy) 1
• Upright-supine test: Hypertropia may reduce by 50% in supine position for chronic skew deviation (sensitivity 80%, specificity 100%), though this is unreliable in acute presentations (onset within 2 months) 1
• Three-step test limitations: Both conditions can show similar patterns with head tilt away from hypertropic side, making the classic three-step test insufficient for differentiation 1

Etiologies Requiring Urgent Recognition

The most common causes include:

• Ischemic stroke (52% of cases)—affecting thalamus, brainstem, or cerebellum 2
• Acute vestibular neuronitis 1
• Demyelinating disease 1
• Brainstem or cerebellar tumors 2
• Post-surgical injury (intracranial or extracranial procedures) 2

Accompanying Neurologic Signs

85% of patients with skew deviation present with at least one additional neurologic sign, most commonly: 2

• Nystagmus (most frequent)
• Gaze paresis
• Ataxia
• Saccadic pursuit abnormalities
• Internuclear ophthalmoplegia

However, 11% of patients present with isolated skew deviation as the only clinical sign with no pertinent brain imaging abnormalities, making clinical suspicion critical. 2

Mandatory Diagnostic Workup

Urgent MRI of the brain and brainstem with and without contrast is required to evaluate for stroke, demyelination, or mass lesions. 1, 3, 4

Specific imaging considerations:

• MRI is preferred over CT for evaluating posterior fossa pathology 3
• Look for lesions in the thalamus, brainstem, or cerebellum 2
• Careful scrutiny of upper brainstem structures is essential, as subtle infarctions may be initially missed 5

When to Image Immediately

Neuroimaging should be obtained urgently if: 1, 4

• Any additional neurological signs or symptoms are present
• Small vertical fusional amplitudes in isolated cases without trauma history
• Young patients or those without vasculopathic risk factors
• Elevated intracranial pressure signs
• Other cranial neuropathies

Specialist Referral

Refer to neuro-ophthalmology and neurology for: 4

• Evaluation of vestibular-ocular coordination problems
• Specialized testing for ocular torsion and vertical misalignment
• Assessment of ocular tilt reaction
• Distinguishing central from peripheral vestibular pathology

Consider otolaryngology consultation if peripheral vestibular pathology (vestibular neuritis, labyrinthitis) is suspected. 4

Clinical Pitfalls to Avoid

• Do not assume benign etiology even with isolated vertical diplopia—11% of skew deviation cases have no other neurologic signs 2
• Do not rely solely on the three-step test to exclude skew deviation, as it can mimic fourth nerve palsy 1
• Do not delay imaging in patients with poor fusional amplitudes or atypical features 1
• Recognize that vertical misalignment ≤3 PD may present as blurred vision rather than frank diplopia 2

Prognosis and Management

• Resolution occurs in 42% of patients who have follow-up, usually within 3 months but sometimes requiring up to 12 months 2
• Prism spectacles successfully relieve diplopia in 62% of patients with persistent symptoms 2
• Accompanying neurologic signs (especially ataxia) persist in 44% and may be more debilitating than the diplopia itself 2