Retinitis Pigmentosa Rod Cone Dystrophy

Retinitis pigmentosa (RP) is a heterogeneous group of disorders characterised by progressive dysfunction affecting the rod more than the cone photoreceptors (a rod-cone dystrophy).

Typically patients present with night blindness and visual field constriction. Central vision may or may not be involved. Classical signs include bone-spicule pigmentation, vessel attenuation and disc pallor, but the fundus may be normal in the early stages of disease, and this is often the case in young children. Fundus appearance may be a poor indicator of the severity of dysfunction.

The rod-specific ERG, reflecting rod-system sensitivity, arises in the inner retina and is usually subnormal or undetectable in RP. The most direct ERG measure of rod photoreceptor function is the a-wave of the bright flash (maximal) ERG and in RP this will also be affected with associated reduction of the b-wave (Fig. 9.3). The normal maximal ERG is of high amplitude, usually allowing easy recording. This can be exploited in paediatric cases where surface electrodes may be used effectively to assess rod photoreceptor activity (see below). Photopic cone-mediated ERGs are less severely affected than rod-driven ERGs in RP, but usually show delayed implicit time and amplitude reduction, best seen in the 30-Hz flicker response. The degree of central retinal involvement in RP can be indicated by the PERG; some patients with RP and preserved central vision have an almost un-detectable full-field ERG but a normal PERG P50 component (Fig. 9.3) consistent with macular sparing [40,72,73]. However, the PERG P50 component can be abnormal in the presence of normal visual acuity [40, 72, 73] and may have some prognostic value in predicting involvement of central vision. Although difficult for young children to perform, multifocal ERG may play a similar role to PERG in the assessment of macular function.

In X-linked RP, accounting for about 5-20% of all familial cases [35], the ERG is usually un-detectable or grossly subnormal from an early age (Fig.9.3D). Asymptomatic female heterozygotes may show a prominent tapetal reflex and there may be intraretinal bone-spicule pigmentation, although the fundus can be normal. The incidence of ERG abnormality in X-linked carriers is high and may involve amplitude or implicit time [3, 9] in both rod and/or cone ERGs. The ERG findings in heterozygotes may show significant interocular asymmetry, unlike

Maximum

30 Hz flicker

Photopic

Pattern ERG

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