Infantile Motor Nystagmus

What is it?

Nystagmus that appears within the first 6 months of life
One of two forms of infantile nystagmus syndrome -- the other form is Infantile Sensory Nystagmus (See Infantile Sensory Nystagmus )
Cause is unknown
May be an isolated finding or associated with strabismus, developmental delay, other impairments
Diagnosis is based on the pattern of nystagmus and associated ophthalmic features, early onset in life, lack of abnormalities to suggest an alternative diagnosis
Nystagmus pattern is identical to that of Infantile Sensory Nystagmus, which is caused by an underlying retinal or anterior visual pathway lesion (See Infantile Sensory Nystagmus )

Core clinical features
- Reduced visual acuity because of reduced foveation time, but no oscillopsia (illusory oscillation of viewed objects)
- Horizontal hybrid jerk-pendular nystagmus in straight ahead gaze, converting to horizontal jerk nystagmus on side gaze
- Distinctive slow phase waveform recognizable only on eye movement recordings as an increasing velocity exponential pattern
- Nystagmus trajectory remains horizontal in upgaze and downgaze (“uniplanar nystagmus”)
- Oscillation amplitude diminishes midway into eccentric gaze to one side (“eccentric null zone”)
Possible clinical accompanying features
- Face turn that optimizes visual acuity by placing eyes in an eccentric null zone where oscillations are of lower amplitude
- Covering one eye often increases the abducting fast phase amplitude in the uncovered eye (“latent nystagmus”)
- Cover test may reveal that one or both eyes descend when uncovered (“dissociated vertical deviation”)
- Head nodding (titubation)
- Convergence reduces nystagmus amplitude and makes reading vision better than distance vision
- Esotropia
Rare variant consists of alternating cycles of right-beating and left-beating nystagmus in primary gaze (“periodic alternating nystagmus”)
Tip: this form of nystagmus may rarely have a vertical or rotary trajectory

Exclude infantile sensory nystagmus
Refer for electroretinography if ophthalmoscopy suggests retinopathy or if vision is impaired
Order brain MRI if other neurologic abnormalities are present
Refer for extraocular muscle surgery if vision improves with face turn

Nystagmus lasts for a lifetime
Optical devices may aid vision, but are usually declined because they are cumbersome
Medications are not effective
Eye muscle surgery may eliminate the face turn and improve vision in properly selected patients
Trap: large extraocular muscle recessions and muscle tendon procedures do not reduce nystagmus amplitude or improve vision
Trap: botulinum toxin injected into the extraocular muscles or retrobulbar space does not improve vision or reduce nystagmus amplitude and causes intolerable side effects