Vestibular and non-vestibular nystagmus in examination of dizzy patients.

Observation of nystagmus is of great importance in the otoneurologic examination. By means of electronystagmography it is possible to examine the nystagmus under different visual conditions such as in darkness or with eyes closed. Physiological nystagmus is characterized by miniature eye movements with an amplitude of 4-5 minutes of one degree. With fixation, the amplitude decreases and the frequency increases. Vestibular nystagmus is a biphasic eye movement with slow and rapid phases of opposite directions. The intensity increases when the eye bulb is directed in the same direction as the fast phase or when fixation is hampered by darkness or eye closure. Nystagmus in which the fast phase changes direction according to the direction of gaze, i.e. gaze (paretic) nystagmus, can be congenital, but is often a result of paralysis of the eye muscles, of infection, intoxication or, if asymmetric in amplitude, of an acoustic neurinoma. Fixation nystagmus, miner's nystagmus, and nystagmus latens, is called optic nystagmus, and is thought to be caused by defects in the fixation mechanism. The main characteristic is that it changes direction if one or both eyes are closed and often interferes with the vestibular nystagmus. In the diagnosis of dizzy patients with optic nystagmus, the examination of the vestibular system must be carried out with other methods than nystagmus recordings.