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Editor,—Occult macular dystrophy (OMD), idiopathic photoreceptor dysfunction, or central cone dystrophy is an unusual form of macular dystrophy where a progressive decline of visual acuity occurs with an essentially normal fundus and normal fluorescein angiography (FA) findings.1-3 The topography of the electroretinographic responses in the central visual field in three patients with OMD was examined by means of the multifocal electroretinogram (m-ERG).4 5 To evaluate the retinal pigment epithelium (RPE) and the choroidal circulation in OMD, indocyanine green videoangiography (ICG-V) was performed.6
A 77 year old pseudophakic woman presented with progressive decreased visual acuity bilaterally of 10 years’ duration. The best corrected Landolt visual acuity was 20/250 right eye and 20/300 left eye. Visual field testing revealed central scotomas in both eyes. Fundus photography (Fig 1A), FA, and ICG-V were normal. Photopic ERG showed borderline amplitudes while scotopic ERG was normal. The Farnsworth–Munsell 100 hue tests showed several errors without any specific axis. The m-ERG exhibited markedly diminished responses in a relatively small circumscribed area in the macula (Fig 2A).
A 68 year old patient had blurred visual acuity for 10 years. The best corrected visual acuity was 20/200 right eye and 20/600 left eye. Fundus photography (Fig 2A), FA, and ICG-V were normal. Photopic ERG showed borderline amplitudes while scotopic ERG was normal. The Farnsworth–Munsell 100 hue tests showed several errors without a specific axis. There were central scotomas in the visual fields bilaterally. The m-ERG revealed marked depression of responses only in the macula (Fig 2B).
Focal macular cone ERG is the key to the diagnosis of OMD.1 3 In our two patients, m-ERG activity was markedly diminished in a relatively small circumscribed area in the macula, suggesting limited functional defects in the fovea. Since no abnormality was found by ICG-V, or FA, the intact RPE may have a barrier effect to the underlying choroidal circulation.
The pathology of OMD may involve the macular cone system without affecting the RPE and the choroid. m-ERG can be useful for the differential diagnosis of OMD and can help map the topography of cone activity loss more precisely. m-ERG may aid in characterising the functional retinal topography in the near future.