SUMMARY The osmotic fragility of erythrocytes from patients with genetically classified forms of retinitis pigmentosa (RP) has been studied. The mean fragility was increased in autosomal dominantly inherited RP, where the dystrophy was expressed regionally in the retina, with both rods and cones affected. In contrast it was normal in patients with the dominantly inherited disease, which leads to a diffusely distributed dystrophy of, predominantly, rod photoreceptor cells. Raised osmotic fragility of erythrocytes has also been observed in female patients with multiplex (recessive) RP and in female carriers of the X-linked form of the disease.Retinitis pigmentosa (RP) is the name given to a group of inherited diseases, generally of unknown aetiology, which lead to the degeneration of rod photoreceptor cells; cones may also be affected. In our studies on RP in man we have adopted the premise that there might be a generalised metabolic lesion that is expressed overtly in photoreceptors because of their unique composition, function, and/or environment. ' The response of erythrocytes to hypotonic insult provides a simple monitor of general body homoeostasis, since it is influenced not only by the metabolic status of the cells but also by the composition of plasma.2 We have therefore routinely included the test in our studie's on patients with RP, and here we report our initial findings. Erythrocyte osmotic fragility varies with the age of the donor and the turnover time of the cell population."-5 The latter is reflected in heterogeneity of cell fragilities within individual samples.' In the course of the study we also detected sex-related differences.5 We have therefore considered these variables in the evaluation of our results.Several distinct inheritance patterns of RP are recognised. They include autosomal dominant, Xchromosome-linked (X-hemizygote), and autosomal recessive. The last is most clearly distinguished when brothers and sisters express the disease but there is no other affected relative, a category referred to.by Jay7 as mixed multiplex. As in our previous study,8 we Corrcspondcncc to Dr M J Vo.adcn. have used this terminology, referring also to RP patients with no known affected relative as simplex.
Materials and methodsThe patients reported on here attended the Genetic and Electrodiagnostic Clinics of Moorfields Eye Hospital, London, where the category of retinal dystrophy was defined genetically, clinically, and where possible by psychophysical and electrophysiological tests.9-1 In particular, patients with autosomal dominantly inherited disease were divided into those with a diffuse loss of, principally, rod vision (D type), and those with more regionalised retinal lesions and significantly affected cone as well as rod vision (R type) (Ernst WJK, Lyness AL, personal communication).Osmotic fragility was determined with hypotonic, phosphate-buffered saline, pH 7-4, as described by Dacie and Lewis.'' Heparinized blood (50 ,tl) was mixed with 5-0 ml buffered saline (12 tubes with dilutions ranging fro...
