ENG and VNG testing in persons with strabismus
Strabismus means that there is a full ocular range, but that the eyes are misaligned -- either turned inward (eso) or outward (exo). Roughly 2% of the population have strabismus.
Strabismus is a large source of error's in ENG testing.
ENG results depend greatly on whether or not one is allowing vision (e.g. the oculomotor tests), or not (e.g. the caloric/positional tests).
In the light there are two main considerations:
1. Can the person see out of both eyes ?
Many persons with strabismus also have amblyopia -- an uncorrectable decreased vision in one eye. If the person is looking out of their amblyopic eye, they will be unable to see. People who can't see, don't track very well.
2. Are we looking at the viewing eye or the deviated eye ?
If one is measuring the deviated eye, it may "run into" the side of orbit, or just go beyond the measurement range of the VENG system, and one should largely ignore that eye in that direction.
3. Is there a congenital nystagmus (i.e. "latent" nystagmus) as is common in persons with congenital eso.
There is a large literature on latent nystagmus. Latent nystagmus is a type of congenital nystagmus. The direction of the horizontal jerk nystagmus can vary. Latent can create a DP as well as nystagmus that varies according to the viewing condition. See this movie of latent nystagmus. Optokinetic nystagmus in Latent is generally asymmetrical according to the viewing eye (it rides the slow phase).
Supplemental material : Video of latent nystagmus
In the dark, such as when doing the caloric or positional testing, the main consideration is simply whether or not the eyes are close enough to the center so that one can trust the (V)ENG system. In the dark, latent nystagmus is generally minimal.