Inner retinal function in hereditary retinal dystrophies

Hereditary retinal dystrophies are most often disorders of photoreceptors a nd/or the retinal pigment epithelium. Structures secondary to the photorece ptor layer such as bipolar, horizontal, amacrine and ganglion cells are sec ondarily involved. In later stages of the disease a mild to moderate loss o f inner retina occurs, but the second and third neurons remain surprisingly viable even in late and severe stages of retinal dystrophies. The function of the inner retina in patients suffering from hereditary retinal dystroph ies is not easy to determine because it depends on the input of photorecept ors. The electroretinogram (ERG) offers several possibilities in this respe ct: b-wave, off-response (off-ERG), oscillatory potentials, scotopic thresh old response of the flash ERG and the pattern ERG (PERG). We looked at two ERG tests: the PERG and the off-ERG. The PERG is an indicator of ganglion c ell function. Its amplitude is related to the photoreceptor input determine d by the flash ERG and visual field testing. But in cases of an undetectabl e flash ERG response the PERG can be recorded in some patients, but not in others. This may be an indication of a different effect on inner retinal fu nction in different groups of patients. On- and off-responses are related t o the function of depolarizing and hyperpolarizing bipolar cells. Evaluatio n of 301 patients with various retinal dystrophies revealed that most hered itary disorders primarily affect the photoreceptors or the pigment epitheli um. In some patients, alterations of the on- and off-response amplitudes or implicit times were indicative of inner retinal disorders and different pa thophysiologic mechanisms. However, interpretation has to be made carefully , as on- and off-responses may be influenced by dysfunction of photorecepto r synapses to bipolar cells, bipolar cells, Muller cells and intercellular matrix.