Neurophysiologic Testing in Infants With Usher Syndrome Type 1

Purpose: Usher syndrome type 1 needs to be diagnosed at early age, when speech therapy and cochlear implantation can stimulate communication in a deaf child who will confront an additional visual handicap. Molecular diagnosis is not routinely available. Visual function tests are not easily performed in young children. Before the age of six years, progressive retinal degeneration does not always show fundoscopic changes and few data are available regarding neurophysiologic abnormalities. This study was designed to describe electroretinogram (ERG) and visual evoked potentials (VEP) in a series of fourteen children with Usher syndrome type I.Methods: Monocular ERG and binocular VEP were performed using white flash stimuli in mesopic conditions in the conscious child. Age at first neurophysiologic study was between 17 months and 5 years. Analysis was focused on the amplitudes and latencies of a- and b- ERG waves, and the latency of the P100 component of VEP.Results: When recorded on and after the 29th month of age, ERG was absent in all patients, including six with normal fundus. The youngest child with absent electroretinogram was 17 month-old. Whatever the age, an abnormal fundus was always confirmed with an absent ERG. In three patients, a normal-shaped normal-latencies but low-amplitude ERG (a-wave lower than 30 µV; b-wave lower than 90 µV) was recorded within the 26th and 27th months. The P100 component of VEP was always present. Its latency was normal (range: 90-112 ms) in five patients and increased (range: 122-187 ms) in nine patients, including all those recorded before 31 months of age.Conclusions: An extinguished ERG with preserved VEP is an early sign of Usher Syndrome type I, even in the absence of fundoscopic signs of retinal degeneration