Although, strictly, full-field ERG abnormalities imply generalised retinal dysfunction, X-linked retinoschisis (XLRS) may be referred to as a macular dystrophy. Patients typically present in the 1st or 2nd decade with reduced visual acuity, and macular abnormalities are present in most cases. Classically, these have a "spoke wheel" appearance due to the presence of foveal micro-cysts. The retinal cleavage may be revealed by optical coherence tomography . With time, these radial cystic changes may give way to nonspecific macular atrophy. Peripheral schisis lesions are only present in 50 % of patients, mostly in the infero-temporal quadrant. In older patients with nonspecific macular atrophy, but no peripheral schisis, the diagnosis may not be obvious and electrophysiology may be instrumental in making the diagnosis or suggesting a candidate gene. An electronegative maximal ERG is typically present (Fig.9.2D), although the degree of abnormality can vary and in rare cases b-wave amplitudes have been reported as normal . Rod-specific ERGs are usually undetectable or markedly reduced in keeping with the intraretinal cleavage  and typical maximal response ERG b-wave abnormality. Photopic and 30-Hz flicker ERGs are usually subnormal and delayed. The ON- b-wave may be reduced with preservation of the OFF-d-wave in some patients , but in others the OFF- d-wave may also be affected. Pattern ERGs are usually markedly reduced in keeping with macular dysfunction [18, 80]. The presence of an electronegative ERG in a patient with a macular lesion, even if atypical, may suggest XLRSi as a suitable candidate gene for analysis. Severe cases of XLRS may also have an abnormal a-wave reflecting some photoreceptor loss, possibly consequent upon haemorrhage or full-thickness detachment early in life.
Stargardt-fundus flavimaculatus (S-FFM)
• Fleck lesions in the posterior pole extending to the mid-periphery, usually with central atrophy
• PERG usually undetectable - severe macular dysfunction
• Some patients have generalised ERG abnormalities that may be of prognostic value
• Typically characterised in the early stages by a well-circumscribed vitelliform macular lesion
• Characterised by normal full-field ERGs but an absent or severely reduced EOG light rise
• Pattern ERGs are typically normal until the vitelliruptive stage, when there is visual acuity loss and PERG reduction
• Classically, a "spoke wheel" appearance at the fovea that is replaced by nonspecific macular atrophy with age
• Electronegative ERG appearance indicating dysfunction post-phototransduction or inner retinal
• Pattern ERGs are usually markedly reduced
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