Abnormality of the pattern electroretinogram and pattern visual evoked cortical response in esotropic cats

In six 3-week-old kittens, the tendon of the lateral rectus muscle of one eye was sectioned under anaesthesia in order to induce an esotropic (nasally-directed) squint. At maturity, the pattern electroretinogram (PERG) and pattern visual evoked response (PVER) were recorded under anaesthesia to phase-reversal at 1.67 Hz of a 0.5c/deg square wave grating pattern of 75% contrast. Refraction was determined by retinoscopy and confirmed by recording the PERG for different trial lenses. The amplitude and time-to-peak of the PERG and PVER were compared between operated and unoperated eyes, and with the responses of one normal and two sham-operated cats. With stimulation of the operated eye, the amplitude of the PVER was consistently reduced by 50% compared with the non-operated eye, confirming that the cats had become amblyopic as a result of the squint. While the appreciable inter-eye variations in the amplitude of the PERG of the normal and sham-operated cats precluded identification of a change in the esotropic cats, there was a consistent prolongation of the time-to-peak of the PERG by 50% with stimulation of the operated eye. This result is consistent with a retinal component to strabismic amblyopia.