cone disease

There is considerable clinical heterogeneity in this disorder.  Reduced vision may cause symptoms beginning during adolescence but some individuals may have good vision until the 6^th decade or even later.  Even those who complain of changes in acuity may still have 20/20 to 20/25 vision but it may also be much worse, in the range of count fingers.  There can be considerable asymmetry in acuity between the two eyes but there is little known about the rate of vision loss.  Mild dyschromatopsia is often present with deficits in both red-green and red-green discrimination but total color blindness has also been reported.  Full field ERGs usually show no rod or cone deficits.  However, multifocal ERG changes suggest dysfunction of the cones in the macula.  Spectral-domain optical coherence tomography can reveal disruptive changes at the photoreceptor inner/outer segment line and in cone outer segment tips.  Disruption of the external limiting membrane and decreased foveal thickness have also been reported.  The retina appears normal to clinical examination even in advanced stages of disease and fluorescein angiography likewise shows no abnormalities.