New Treatments for Infantile and Other Forms of Nystagmus

Our objective was to translate the past 40 years of infantile nystagmus syndrome (INS) research (i.e., ocular motor recording and control-systems analysis) into a therapeutic approach. Our eye movement recordings use infrared reflection, mag­ netic search coil, and high-speed digital video sys­ tems. Each eye was calibrated during monocular fixation (fellow-eye occluded). We analyzed and displayed all data using software developed and written in our laboratory in the MATLAB (Math­ Works, Natick, MA) environment, including the eXpanded Nystagmus Acuity Function (NAFX). Analysis of '" 1 000 INS subjects over 40 years revealed waveform characteristics that can be exploited therapeutically. Analysis of postoperative INS data suggested that tenotomy and resuture (at the original insertions) of the extraocular muscles in the plane of the IN would improve foveation. The NAFX across normal gaze angles showed both peak-value increases and NAFX-curve broadening. We have concluded that all patients with nystagmus should have eye movement recording and analysis. The resulting accurate diagnosis and documenta­ tion of INS characteristics (undetectable by clinical observation) identify the best therapy. NAFX analy­ sis allows estimation of postoperative acuities and determination of the most appropriate therapies. This eye movement-based procedure is the first to provide both the physician and patient with a post­ therapeutic estimation of specific improvements 87 in visual function that is not possible from acuity measurements alone. The most common types of benign nystagmus seen in infancy consist of the mixture found in the infantile nystagmus syndrome (INS), also known as congenital nystagmus, followed by the fusion malde­ velopment nystagmus syndrome (FMNS), also known as latent/manifest latent nystagmus; additional types include the nystagmus blockage syndrome (NBS) and the spasmus nutans syndrome (SNS).1 FMNS shares some clinical characteristics with INS, but it is always associated with strabismus and has a directional change with ocular cover, beating toward the fixating eye; it is rarely only present with occlusion of one eye. NBS is a special case of INS in which a purposive esotropia either damps the INS or converts it to a low-amplitude FMNS; both of the resulting waveforms allow higher acuity. SNS is a rare, disconjugate pendular nystagmus with head-nodding and tilt.2 Accurate diagnosis of each type is necessary, because each has a different mecha­ nism and relationship to strabismus, and each requires different therapy. This review concentrates on INS and the conclusions and recommendations that have resulted from ocular motility analysis. Commonly held clinical impressions about INS and FMNS have been found to be less than helpful, while others are simply wrong. INS can appear at birth but is usually noted in early infancy (at the time of development of both motor and visual function) and persists throughout life. This is an ocular motor disorder that may (>50%) or may not be associated with afferent visual abnormalities 88 NEW THERAPIES FOR CONGENITAL NYSTAGMUS (e.g., albinism, aniridia, retinal dystrophies, optic nerve/foveal hypoplasia, retinal disease, ammetropia, or congenital cataracts). Therefore, all INS patients should have a thorough examination of the eye (includ­ ing slit-lamp examination for iris transillumination and fundus examination) and afferent visual system. This includes testing monocular and binocular vision, refraction, and ophthalmoscopy. Additional tests may also be indicated, such as color vision and visual fields, dark adaptation, electroretinography, visual evoked potentials, or orbital and central nervous system imag­ ing such as MRI. A combination of visual and ocular motor system evaluations helps in understanding sys­ temic diagnosis, visual system prognosis, and poten­ tial treatment options.