Acquired cone dysfunction.

A reduction of visual acuity is probably the most common presenting symptom in ophthalmology and either a refractive error or a lesion of the eye or the visual pathways can gener- ally be found to account for it.Occasionally no cause for the poor vision can be found by routine clinical techniques, but more sophisticated methods of examination may reveal other evidence of retinal dysfunction allowing a diagnosis to be made.Sloan and Brown (I962) described five patients with poor vision in whom psychophysical and, to a lesser extent, electrophysiological tests demonstrated a specific defect of the photopic (cone) visual system.Goodman, Ripps, and Siegel (1963) presented a large series of such cases and noted that, although the condition may be congenital, it can develop postnatally.They recognized three grades of severity of the condition and remarked on the wide variety of diagnoses that had been applied to these patients.Berson, Gouras, and Gunkel (1968) described a family in which a progressive dysfunction of the cone system was dominantly inherited.Goodman, Ripps, and Siegel (1966) described a patient in whom the condition was markedly asymmetrical although abnormalities in the more normal eye could be demonstrated.This paper, which reviews a series of patients referred for the investigation of unex- plained loss of visual acuity, emphasizes the importance of electroretinography in the demonstration of a specific disorder of the photopic visual system. Materials and methods (I) SUBJECTSThese were patients referred to the Electrodiagnostic Clinic of Moorfields Eye Hospital for t investigation of an unexplained loss of visual acuity, which was the presenting symptom in 298 of the first 2,000 patients referred to the clinic.The results of the tests showed that fifteen of these patients had a defect of the photopic system, affecting both eyes in thirteen of them and one eye only in two.The salient findings in the patients with both eyes affected are shown in the table.The patients in whom one eye was affected are not described as they were inadequately studied.The visual acuity and visual fields of these patients were assessed by the referring clinicians and a wide variety of testing conditions were therefore involved.In no case was there a significant ametropia nor a significant defect of the visual field.(2) COLOUR VISION This was tested separately in each eye using the Hardy-Rand-Ritter (HRR, American Optical Company) pseudo-isochromatic test plates.The illumination was the battery of fluorescent tubes ('Warmlight', Crompton) used as the light source for electro-oculography.The colour defect was recorded according to the test plate instructions and when there was no colour discrimination this was recorded as 'none'.