How do you differentiate ocular apraxia from ptosis?

Differentiating Ocular Apraxia from Ptosis

Ocular apraxia and ptosis are fundamentally different conditions that can be distinguished by examining voluntary eye movements versus eyelid position: ocular apraxia involves inability to initiate voluntary saccades with preserved reflex eye movements, while ptosis is drooping of the upper eyelid below its normal position.

Key Distinguishing Features

Ocular Apraxia Characteristics

• Impaired voluntary eye movements with compensatory head thrusts or blinks to achieve gaze changes 1
• Preserved reflex eye movements including oculocephalic responses (doll's head maneuver) and spontaneous eye movements 2
• Normal eyelid position and function - the eyelid itself is not drooping 1
• In vertical ocular motor apraxia specifically, patients cannot voluntarily move eyes up or down but use combined blinks and head movements to achieve vertical gaze changes 1
• Neuroimaging may reveal brainstem lesions at the mesencephalic-diencephalic junction 1

Ptosis Characteristics

• Drooping of the upper eyelid - defined as upper eyelid less than 2 mm from midpupil 3
• Normal eye movements unless accompanied by other cranial nerve pathology 2
• Reduced upper visual field to 30 degrees or less in 97% of cases 3
• Assessment includes measuring levator function, amount of ptosis, and presence of eyelid retraction 2

Critical Examination Algorithm

Step 1: Assess Eyelid Position

• Measure the distance from upper eyelid margin to midpupil 3
• Ptosis present: Upper eyelid less than 2 mm from midpupil 3
• Normal eyelid position: Consider ocular apraxia if eye movement abnormalities present 1

Step 2: Test Voluntary vs. Reflex Eye Movements

• Command the patient to look in different directions (tests voluntary saccades) 2
• Perform oculocephalic rotation (doll's head maneuver) to test reflex movements 2
• Ocular apraxia: Impaired voluntary movements with preserved reflex movements 1
• Ptosis: Both voluntary and reflex movements typically normal unless third nerve palsy present 2

Step 3: Observe Compensatory Mechanisms

• Ocular apraxia patients use head thrusts, blinks, or combined head-eye movements to achieve gaze changes 1
• Ptosis patients may adopt chin-up head posture to see under drooping lid but do not use head thrusts for eye movements 2

Special Diagnostic Considerations

Apraxia of Eyelid Opening (ALO) - A Confounding Entity

This condition can mimic ptosis but is mechanistically different:

• Inability to voluntarily open the eyelids despite normal levator function 4, 5, 6
• Often occurs only on awakening from sleep, resolving after manual eyelid elevation 5, 6
• Can be unilateral (more common in women, mean age 59 years) 5, 6
• No true ptosis - the levator muscle functions normally once activated 4, 6
• Distinguished from true ptosis by the transient nature and normal eyelid position after manual elevation 6

Variable Ptosis Suggesting Myasthenia Gravis

• Fatigable ptosis that worsens with prolonged upgaze 2, 7, 8
• Variable strabismus that changes during examination 2
• Positive ice test: Applying ice pack for 2 minutes reduces ptosis by approximately 2 mm 2, 7, 8
• Cogan lid-twitch sign may be present 2
• Slow saccades distinguish this from ocular apraxia where saccades are absent or severely impaired 2

Pupillary Examination - Critical for Ptosis Etiology

• Dilated pupil with ptosis: Third nerve palsy requiring urgent neuroimaging 7, 8, 9
• Miotic pupil with mild ptosis: Horner syndrome 7, 8, 9
• Normal pupils: Consider myasthenia gravis, mechanical causes, or apraxia of eyelid opening 2, 8
• Ocular apraxia: Pupils are normal as this is a supranuclear disorder 1

Common Pitfalls to Avoid

• Mistaking apraxia of eyelid opening for true ptosis - ALO resolves with manual elevation and has normal levator function, while true ptosis persists 4, 6
• Failing to test reflex eye movements - this is essential to diagnose ocular apraxia, as voluntary movements are impaired but reflex movements preserved 2, 1
• Missing pupil-involving third nerve palsy - this represents a neurosurgical emergency requiring immediate imaging 7, 8, 9
• Not performing ice test in variable ptosis - this simple bedside test is highly specific for myasthenia gravis 2, 7, 8
• Overlooking compensatory head movements - these are characteristic of ocular apraxia, not ptosis 1

Neuroimaging Indications

Urgent Imaging Required

• Ptosis with pupillary involvement (dilated pupil) - MRI with gadolinium and MR angiography or CT angiography 7, 8, 9

Elective Imaging Indicated

• Ocular apraxia - MRI to identify brainstem lesions 1
• Ptosis with ophthalmoplegia, proptosis, or other neurological deficits 7, 8, 9

Imaging Not Routinely Indicated

• Isolated ptosis without other neurological symptoms 8, 9
• Apraxia of eyelid opening without associated neurologic or ocular findings 5, 6