INFRARED VIEWER

Typical Number in Hospital: 1

Cost Bands: 3

References:

One of the standard tests in the investigation of dizziness or vertigo involves asking the patient to look in different directions while the clinician observes the patient's eyes in order to detect an abnormal eye movement called nystagmus. In the normal clinical environment, only patients with acute vestibular problems will exhibit this particular symptom since their eyes will be held steady by observation of the surroundings. A much more sensitive test is to do this in darkness, and there are several methods by which the patient's eye movement can be detected. The best method of achieving this is to use an electro-nystagmograph although an infrared viewer or Frenzel glasses also allow a subjective assessment of eye movement.

An infrared viewer is a hand-held battery-powered device which contains an infrared light source (which is invisible) to illuminate the patient's eyes. The reflected light is passed through an optical system containing a non-linear optical cell having the effect of doubling the frequency of the light, which brings the the light into the visible spectrum to provide a monochromatic image (usually an eerie green).

Horizontal and vertical nystagmus can be readily observed with this method although true rotary nystagmus (where the eyes rotate about their axis of view) is very difficult to detect. The device uses technology developed for the night sights used on rifles.