Purpose: Recent studies show that patients with Usher syndrome type 2 (USH2) have abnormal cone structure and density in the central retina. This occurs in the presence of normal acuity, opening the quest for additional sensitive functional measures of central cone function in USH. We tested here whether focal macular cone electroretinogram (fERG) could be such a tool.
Methods: This retrospective study of central cone function loss was based on data from 47 patients with USH2 from the Ophthalmology Department of the Policlinico Gemelli/Catholic University in Rome. The analysis focused on the decrease of the fERG, obtained in response to a 41-Hz sinusoidal modulation of a uniform field presented to the central 18°, generated by red light-emitting diodes (LEDs) and superimposed on an equiluminant steady adapting background. fERG decrease was compared with the decrease of best-corrected visual acuity and Goldmann kinetic perimetry V4E field.
Results: fERG follow-up data document a severe and precocious loss of central cone function in USH2 patients, preceding losses in other measures of cone function. fERG is already reduced to 40% of control at the beginning of the second decade of life, and by 25 years of age, all USH2 patients have fERGs less than 30% of control values.
Conclusions: fERG represents a sensitive tool to evaluate central cone function in USH2, anticipating the decline of other central cone function measures, such as visual acuity and Goldmann perimetry.