Introduction: Retinitis pigmentosa (RP) is associated with the formation of a posterior subcapsular cataract (PSC). As only a small part of the crystalline lens is usually affected, it is sometimes difficult to determine to what extent the visual loss is caused by the PSC alone.
Patients and methods: The methodology was developed in analogy to a scoring system for posterior capsule opacification by Tetz et al. Following dilation of the pupil, standardized photographs of the anterior segments were obtained utilizing a Zeiss photoslitlamp model 40 SL/P. The PSC was scored by evaluating retroillumination photographs. The individual PSC index was calculated by multiplying the density of the opacification (graded from 0 to 4) by the area involved in the central 4 mm zone of the pupil (calculated between 0 and 1). For testing the reliability of the evaluation system in part 1 of this study, 11 RP patients with different grades of PSC were examined by three independent observers. In part 2 of this study 37 eyes of 24 RP patients, aged 47.2 +/- 11.8 years, were evaluated and the PSC index was correlated with different parameters (visual acuity, age, visual fields, eletroretinography). RESULTS PART 1: The cataract-density grades were between 1 and 4 in the 11 patients. In relation to the central 4-mm pupillary zone between 13 and 100% of the area were opacified. Cataract indices (density x area) were between 0.13 and 4.0 (Mean values: Examiner 1:1.41 +/- 1.49; Examiner 2:1.28 +/- 1.46; Examiner 3:1.22 +/- 1.44; differences not significant: P = 0.77). PART 2: After an average duration of RP of 23 years, the average cataract index of the 24 patients was 1.72 +/- 1.35. There was no correlation between cataract index and ERG or visual fields (r < 0.2; P > 0.4); however, there was a good correlation to visual acuity (r = -0.72; P = 0.0001). Patients with early onset of RP (before 20th year of life) presented on average with an higher cataract index (2.06 +/- 1.67) compared to patients with late manifestation (0.61 +/- 0.44), but equivalent duration of RP.
Conclusions: The evaluation system offers a reliable and reproducible method for measuring PSC density and extension in RP patients. The method can serve as a useful tool for documenting PSC development and help to define the indications for cataract surgery in RP.