Timothy C. Hain, MD
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Latent nystagmus is a variant of congenital nystagmus although some authors prefer to reserve the term congenital nystagmus for other variants. It is mainly encountered in persons with strabismus (eyes turning in or out) and amblyopia (uncorrectable loss of vision in one eye associated with lack of use of that eye in childhood). In the most common form, the latent nystagmus appears only when one eye is covered. When both eyes are viewing, no nystagmus is seen. This is the reason why it is called "latent" nystagmus.
The most common variety can be recognized because the eyes nearly always move with their slow-phase towards the nose for the viewing eye. In other words, the direction of the nystagmus in both eyes changes with the viewing eye. The figure below shows a right beating nystagmus, associated with left eye viewing. Recording method is infrared.
Latent nystagmus can be very confusing to people attempting to do an ENG. The eyes may jump left or right seemingly at random, and even worse, the unfavored eye commonly deviates to one side depending on which eye is viewing.
The author has encountered interesting situations where patients with latent nystagmus have gone unrecognized by otherwise quite competent neurotologists and neuro-ophthalmologists. This is rather silly as all it really takes is thinking of the diagnosis and then checking to see which way the eyes jump when one eye or the other is covered.
In a person with a strong latent nystagmus, it can be difficult to make much of anything out of the ENG. The author has encountered patients who were misdiagnosed by ENT doctors as having more serious conditions (such as a perilymph fistula), because the otherwise quite expert examiner simply did not recognize a strong latent nystagmus.
In persons with a mixture of LN and vestibular disorders, there may be an odd situation where one eye is still when fixating, and the other has nystagmus when fixating. This is probably due to an "Alexander's law" for latent nystagmus.
LN appears to be due to growing up without both eyes viewing. There is atrophy of neural pathways that support binocular viewing. People lose the ability to process and fuse input from both eyes together. Often one eye develops a reduction in visual acuity that cannot be corrected with glasses -- amblyopia.
Because LN is due to loss of neural pathways, it cannot be "fixed". There is no medication or procedure that can grow back neurons (yet anyway).
The amblyopia can sometimes be prevented by forcing children to view out of their less favored eye, using patching.
There are a few variants of latent nystagmus.
The most common variant of latent nystagmus is difficult to see with both eyes viewing, and becomes apparent only when one or the other eye is blocked. This is just called "latent nystagmus". With a little practice, persons with latent nystagmus can make their eyes jump to the left or right on command, by looking out of one or the other or both eyes. This was first shown by Van Vliet with his "pseudoscope" (1973), and later by Dell'Osso in a patient with a false eye (Dell'Osso, et al. 1987). We routinely observe this with our video-frenzel goggles -- just ask people to "look" out of one or the other eye, in complete darkness.
Manifest latent nystagmus is latent nystagmus that can be seen even with both eyes viewing. MLN can be acquired, presumably due to a change in the eye that one habitually views.
In voluntary latent nystagmus, people purposefully view out of either eye, and thus make their eyes jump in either direction at will. This "talent" is sometimes used for secondary gain - -i.e. to get out of military service or to claim that the eyes are jumping after a traumatic event.
Dissociated vertical deviation or DVD is a vertical strabismus characterized by a slow upward rotation of one eye without movement of the other.
Often persons with latent nystagmus develop a torsional deviation to their eye when they look away from center, which has to be corrected when they return fixation to center. This can be a way of spotting latent nystagmus using video frenzel goggles, during the saccade test.
Some patients have torsional LN -- their eyes twist in opposite directions depending on the eye that is viewing. Generally they are not very concerned by this, although of course, it must reduce their visual acuity. A video of torsional CN is shown here.
Movie of jerk type torsional Congenital nystagmus in light during fixation (19 meg).
Gabapentin is often useful in reducing the speed of congenital nystagmus. Gabapentin increases an inhitory neurotransmitter used in oculomotor function. We nearly always offer the option of taking gabapentin to patients with CN in our clinic setting in Chicago. It seems likely that pregabalin would work too, but we have not tried it as yet. Pregabilin costs more than gabapentin, but is easier to dose.
Recently, it has also been reported that Memantine (an agent which acts on asparate, glutamate and dopamine) is also helpful (Mclean et al, 2007). Glutamate is a major excitatory neurotransmitter. Memantine was used in doses of 40 mg -- greater than is commonly prescribed for other uses. We have never found a patient who responded to memantine, but we have not tried it often.
Surgical treatment is also sometimes offered -- mainly involving moving the eye so that the "null" is located more centrally. We have not found this to be very helpful, and think in general that it is a bad idea.