Let’s briefly review what we learned in Part 1. Fusion Development Nystagmus, or FMNS, is not the type of nystagmus you’re used to seeing — when there is oscillation easily visible to the naked eye of the observer. You’re more like to notice it when doing ophthalmoscopy, and would certainly see it when doing visuoscopy, in the form of central unsteady fixation. The primary characteristic of FMNS is a slow nasalward drift or bias with a fast re-foveating microscaccade.
These FMNS flicks are common in children with hyperopic anisometropia, when trying to fixate with the eye exhibiting more hyperopia, as in frame B above. The magnitude of FMNS is correlated more closely with stereoacuity than with visual acuity. The temporal flicks of FMNS to correct nasal fixation drift may mimic microstrabismus during cover testing, but are actually an ocular motor abnormality that results from decorrelated visual experience between the two eyes. The Nidek MP-1 computes and plots this cloud of fixation instability as a simultaneous integration of subjective perimetry and objective fundus imaging.
Awhile back we used the term “zero retinomotor value” to denote that central, steady fixation was located at the center of the fovea. Well, it’s not purely zero, as we know that steady state fixation requires 2 microhertz oscillation in order to prevent images from fading. But in terms of the cloud of fixation instability, it would ideally be a very small cloud or region of points inside 1 degree. As Bridgeman defines it, fixation is best conceived as steady gaze maintained by compensatory eye movements.
In low vision rehabilitation, when the macula itself become organically dysfunctional tissue, there is obvious benefit in helping the cortex to direct the eye to a new preferred retinal locus. That was the original impetus for developing scanning laser ophthalmoscopy, and in this instance what Nidek had in mind in developing the MP-1.
Keith Main has a nice example of auditory biofeedback training in the context of utilizing cortical plasticity to change the PRL in AMD as Figure 14 on page 88 of this gorgeous Ph.D. thesis: main_keith_l_201012_phd
So if you don’t have a fancy auditory biofeedback setup in your office to help patients with fixation instability learn to get both eyes hangin’ around on the same cloud, what therapeutic interventions might counteract the FMNS adaptation? More on that in Part 3.