Nystagmus is an involuntary, oscillating, rapid movement of the eyeball. While most people with nystagmus are born with the neurologic condition or develop it early in life, it can be acquired as a result from disease or trauma (brain tumor, diabetic neuropathy, head injury).
Nystagmus that is present all the time is referred to as manifest nystagmus. Latent nystagmus is when the condition occurs only when one eye is occluded. In manifest-latent nystagmus, the condition is present all of the time, but worsens when an eye is covered.
Since occluding one eye increases the eye movement, resulting in worse acuity than the patient’s normal, functional acuity it is difficult to check visual acuity in the traditional manner. When a patient has nystagmus visual acuity should be tested using a high power convex lens such as a + 10.00 diopter, instead of an occluder, over the non-tested eye. The high plus power lens will fog the non-tested eye enough so that the patient will not be able to see the chart and allow light to enter the eye keeping the eye movement from increasing.