The K+ channel expressed by the KCNJ10 gene (Kir4.1) has previously demonstrated 
importance in retinal function in animal experiments. Recently, mutations in 
KCNJ10 were recognised as pathogenic in man, causing a constellation of 
symptoms, including epilepsy, ataxia, sensorineural deafness and a renal 
tubulopathy designated as EAST syndrome. We have studied the impact of KCNJ10 
mutations on the human electroretinogram (ERG) in four unrelated patients with 
EAST syndrome. Corneal ganzfeld ERGs were elicited in response to flash stimuli 
of strengths of 0.001–10 phot cd s/m2 presented scotopically, and 0.3–10 phot cd 
s/m2 presented photopically. ERG waveforms from light-adapted retinae of all 
patients showed reduced amplitudes of the photopic negative response (PhNR) (P < 
0.001). The photopic ERGs showed a delay in b-wave time to peak, but the 
photopic hill, i.e. the relative variation of time to peak and amplitude with 
luminance flash strength, was preserved. Scotopic ERGs to flash strengths 0.01 
to 0.1 phot cd s/m2 showed a delay of up to 20 ms before the onset of the b-wave 
in two patients compared to controls. Stimulus–response functions were fitted by 
Michaelis–Menten equations and showed significantly lower retinal sensitivity in 
two patients than in controls (P < 0.001). Our study for the first time in the 
human ERG shows changes in association with KCNJ10 mutations affecting a Muller 
cell K+ channel. These data illustrate the role of KCNJ10 function in the 
physiology of proximal and possibly also the distal human retina.