Louise Allen

Full-field ERG responses recorded with skin electrodes in paediatric patients with retinal dystrophy

Purpose: Assess ERG responses recorded with skin electrodes in children with retinal dystrophies. Method: ERG responses were recorded using skin electrodes in 17 healthy children and 43 paediatric patients with retinal dystrophy. Subjects were aged 4–14 years. ERG responses were recorded to full-field stimuli similar to those recommended in the ISCEV standard. The type of retinal dystrophy was classified on the basis of standard clinical criteria and the ERG responses were compared with those of the age-matched controls. Results: ERG responses were abnormal in every patient. The specific type of ERG abnormality was also consistent with the clinical findings in the majority of patients. Rod responses were abnormal in every patient with a rod–cone dystrophy and cone responses were also abnormal in the majority of patients. Those patients with cone dystrophy or rod monochromatism had normal or near normal rod responses but sub-normal or absent cone responses. Patients with CSNB or XLRS had a sub-normal b-wave but normal amplitude a-wave. Conclusion: ERGs can be recorded successfully with skin electrodes in paediatric patients and responses can aid the diagnosis of the type of retinal dystrophy.

A role for VAX2 in correct retinal function revealed by a novel genomic deletion at 2p13.3 causing distal Renal Tubular Acidosis: case report

BackgroundDistal Renal Tubular Acidosis is a disorder of acid-base regulation caused by functional failure of α-intercalated cells in the distal nephron. The recessive form of the disease (which is usually associated with sensorineural deafness) is attributable to mutations in ATP6V1B1 or ATP6V0A4, which encode the tissue-restricted B1 and a4 subunits of the renal apical H+-ATPase. ATP6V1B1 lies adjacent to the gene encoding the homeobox domain protein VAX2, at 2p13.3. To date, no human phenotype has been associated with VAX2 mutations.Case presentationThe male Caucasian proband, born of a first cousin marriage, presented at 2 months with failure to thrive, vomiting and poor urine output. No anatomical problems were identified, but investigation revealed hyperchloremic metabolic acidosis with inappropriately alkaline urine and bilateral nephrocalcinosis. Distal Renal Tubular Acidosis was diagnosed and audiometry confirmed hearing loss at 2 years. ATP6V0A4 was excluded from genetic causation by intragenic SNP linkage analysis, but ATP6V1B1 completely failed to PCR-amplify in the patient, suggesting a genomic deletion. Successful amplification of DNA flanking ATP6V1B1 facilitated systematic chromosome walking to ascertain that the proband harbored a homozygous deletion at 2p13.3 encompassing all of ATP6V1B1 and part of VAX2; gene dosage was halved in the parents. This results in the complete deletion of ATP6V1B1 and disruption of the VAX2 open reading frame. Later ocular examinations revealed bilateral rod / cone photoreceptor dystrophy and mild optic atrophy. Similar changes were not detected in an adult harbouring a disruptive mutation in ATP6V1B1.ConclusionsThe genomic deletion reported here is firstly, the only reported example of a whole gene deletion to underlie Distal Renal Tubular Acidosis, where the clinical phenotype is indistinguishable from that of other patients with ATP6V1B1 mutations; secondly, this is the first reported example of a human VAX2 mutation and associated ocular phenotype, supporting speculation in the literature that VAX2 is important for correct retinal functioning.

Abnormalities of the scotopic threshold response correlated with gene mutation in X-linked retinoschisis and congenital stationary night blindness

STRs and dark-adapted ERGs were recorded in nine normal subjects, nine patients with XLRS, 11 patients with CSNB1 and one patient with CSNB2. In XLRS STR amplitude was significantly lower than normal at every intensity, but the response could be recorded in every patient and the maximum amplitude response was outside the 95% confidence limits in only four of the nine patients. STRs were significantly poorer in patients with CSNB and a responses was not measurable at any intensity in nine of the 11 patients with CSNB1. In both CSNB and XLRS the STR could only be recorded at higher stimulus intensities, suggesting reduced sensitivity of the STR. In XLRS onset and peak latencies were also significantly prolonged and the slope of the intensity–response functions for amplitude and onset latency differed significantly from normal. Maximum STR amplitude did not correlate with the maximum dark-adapted ERG response. The finding of abnormal STRs and dark adapted ERGs in all three dystrophies indicates that the different causative genes must have similar effects on the rod On-bipolar cell pathway. But there were also differences between the three clinical groups, particularly in the greater severity of the abnormality in CSNB1, which suggests that there may be multiple sites of abnormality.