The Pathophysiology of Amblyopia: Electrophysiological Studies (original) (raw)

1980, Annals of the New York Academy of Sciences

Functional amblyopia may be defined as a loss of visual acuity caused by form deprivation and/or abnormal binocular interaction, for which no organic cause can be detected by the physical examination of the eye.' Amblyopia is usually associated with strabismus (turned eye), anisometropia (unequal refractive error), or form deprivation early in life, and thus represents a developmental disorder. There appears to be a critical period for the development of amblyopia in humans and it may be preventable if the causative factors are discovered and eliminated within this critical period.' Interestingly, amblyopia may also be reversible much later in life.* Over the past 15 to 20 years, there has been a renewed interest in the study of amblyopia, which is in large part attributable to the single-unit studies in cats and monkeys reared with experimentally induced amblyopia. These studies have shown that normal visual experience is essential for the development and maintenance of the physiological characteristics of cells in the visual cortex, and that disruption of the normal visual process during an early period of susceptibility by light or form deprivation, strabismus, or anisometropia, may result in a marked disturbance of the physiological organization of the visual cortex, cell shrinkage in the LGN, and severe a m b l y~p i a .~-~ However, it is not clear whether strabismus and form deprivation (due to lid suture or occlusion) affect similar cell populations in the visual pathway. There is evidence that the effects of monocular lid suture are selective for the Y or transient cells," while in kittens reared with unilateral strabismus, the X or sustained cells appear to be most affected."-" Although such methods of investigation are valuable in dealing with experimental animals, studies of human amblyopia have, for the most part been restricted to psychophysics and visual evoked potentials (VEP). In humans, with naturally occurring amblyopia, the VEP is generally the only direct method available for studying cortical responses to visual stimuli and may provide a valuable tool for the assessment of visual function in amblyopia for several reasons: (1) it provides an objective measure of function which may be useful in infants and nonverbal patients; (2) it may aid in localizing function and dysfunction; (3) it may be of value in assessing prognosis and monitoring therapy; and (4) it may be helpful in bridging the gap between psychophysics and physiology. Over the past few years we have been studying the VEP of observers with naturally occurring amblyopia using luminance and pattern stimuli. The present paper reviews some of the results. LUMINANCE EVOKED POTENTIALS There have been a large number of studies of the VEP in amblyopia using unpatterned stimuli; however, the results are quite equivocal. Some investigators