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The "H in Space" test is a quick way to grossly evaluate the function of cranial nerves 2, 3, 4, and 6. However, further mechanisms for eye movement exist outside than the above mentioned for voluntary eye movement. Namely, the semicircular canals of the vestibules and the extraocular muscles of the eye function together physiologically to help control gaze locked when the head is in motion. Nystagmus is an aberration in this stabilization of the ocular muscles when in motion.

Clinical Features

Nystagmus is a rapid, uncontrolled, rhythmic oscillation of the eye. While most commonly this occurs side to side, it can also occur up and down, circularly, or in any direction.

Differential Diagnosis

Congenital Nystagmus

  • Optic Nerve Hypoplasia
  • Macular Disease

Acquired Nystagmus

  • Neurological Damage
  • Psychoactive drugs
  • Alcohol
  • Head Trauma
  • Brain Tumor
  • Stroke
  • Multiple Sclerosis
  • Thalamic Hemorrhage


While the "H in space" test may be the quickest way to assess for gross extraocular muscle dysfunction, the Caloric Reflex Test (also known as Vestibular Caloric Stimulation) is the best clinical test for the Vestibulo-Optic reflex. Water irrigated into the external auditory canal will exaggerate nystagmus if present. Furthermore, the temperature of the irrigated water will control the direction. Cold water will exaggerate a nystagmus to move contralaterally, and warm water will exaggerate a nystagmus to move ipsilaterally. This can be remembered by the mnemonic "COWS".

COWS: Cold Opposite, Warm Same.


Stabilize patient and manage primary course of illness. If possible, reverse toxicology.


See Also

External Links