Multimodal imaging of Bietti's crystalline dystrophy.

Bietti's crystalline dystrophy (BCD) is a rare autosomal recessive retinal dystrophy characterized by deposition of crystals in the retina. The purpose of this article is to describe retinal abnormalities in BCD using multimodal imaging. An 18-year-old girl presented with decrease of vision and nyctalopia. She was assessed with color fundus picture, red-free photographs, short-wave autofluorescence, spectral-domain optical coherence tomography (OCT) and en face OCT and was diagnosed to have BCD based on typical presentation. Retinal crystals were better visualized on en face OCT as compared to conventional B scan OCT.

Bietti's crystalline dystrophy (BCD) is a rare inherited retinal dystrophy, which usually presents with progressive visual loss in the third or fourth decade.[1] The disease is characterized by multiple small intraretinal crystal deposits. This causes retinal atrophy leading to peripheral visual field and visual loss often causing legal blindness by the fifth or sixth decade.[2] The crystalline deposits are seen throughout the body including lymphocytes and skin fibroblasts.[3] The gene responsible for BCD is cytochrome P450 4V2 (CYP4V2) which codes a protein involved in fatty acid metabolism.[4] We describe a case of BCD in a young girl using multimodal imaging highlighting the different aspects of disease with special emphasis on visibility of the characteristic retinal crystals.

Case Report

An 18-year-old girl presented with a 5-year history of gradually progressing decreased vision and night blindness in both eyes. The family history was unremarkable. Best-corrected visual acuity was 20/80 and 20/100 in the right and the left eye, respectively. The anterior segment was normal in both eyes with no crystals visible at the corneal limbus. Dilated fundus examination revealed multiple pinpoint crystalline deposits at the posterior pole mainly in the macular area [Fig. 1a and b]. Large choroidal vessels were visible around the macular area suggesting retinal pigment epithelium (RPE) atrophy. The improved contrast of red-free photography allowed for the discrimination of a greater number of crystals [Fig. 1c and d]. Short-wave autofluorescence (SWAF) showed well defined sharply demarcated patches of hypoautofluorescence (indicative of RPE atrophy) around the macular area involving the peripapillary zone in both of the eyes [Fig. 1e and f]. The center of the macula was spared, which accounted for the relatively preserved visual acuity. The area outside of these patches of hypoautofluorescence showed granular hyperfluorescence.

Figure 1

Colour photograph of the right and left eye (a and b) of a patient with Bietti's crystalline dystrophy. Red-free photographs of the right and the left eye (c and d) show the greater number of crystals due to better contrast. Short-wave autofluorescence of the right and the left eye (e and f) shows well-defined areas of hypoautofluorescence encroaching on the fovea. Note: the granular hyperautofluorescence outside these areas, which indicate distressed retinal pigmented epithelial cells

A spectral-domain optical coherence tomography (SD-OCT) scan through the fovea of both eyes showed relatively preserved foveal contour and retinal layers [Fig. 2a and d]. The ellipsoid zone was disrupted in both of the eyes, the right eye being affected more [Fig. 2b and e]. The choriocapillaris was thinned out with prominent large choroidal vessels on SD-OCT scans in both of the eyes. Retinal thinning and disorganization of the outer retinal layers were appreciable corresponding to the areas of RPE atrophy seen on SWAF imaging (blue arrows). The crystals were appreciable on conventional B scans of SD-OCT as small faintly hyperreflective dots (arrow heads). The en face OCT images at the level of RPE showed hyperreflective crystals more prominently in the right eye [Fig. 2c and f]. The dark areas seen on en face imaging represented preserved RPE in the center of macula. A diagnosis of BCD was made. The prognosis was explained to the patient, and regular follow-up was advised.

Figure 2

Spectral-domain optical coherence tomography images of the right (a-c) and the left (d-f) eyes. Infrared images showing the location of the line scan (a and d). The line scan of the right eye (b) shows outer retinal thinning while the scan of the left eye (e) shows disruption of the outer retinal layers. Retinal crystals were imaged at the level of the retinal pigment epithelium in both eyes. En face imaging more clearly shows the hyperreflective crystals surrounded by the darkly imaged intact retinal pigment epithelium. Arrows show outer retinal layers disruption while arrowheads show crystals

Discussion

BCD, first described by Bietti in 1937,[1] is a rare autosomal recessive retinal dystrophy characterized by deposition of crystals made of cholesterol. These are seen in corneo-limbal region and in the retina. The loss of RPE and sclerosis of choroidal vessels follows this crystal deposition.[1] Patients usually present after the second decade of their life with complaints of decrease of vision, night blindness, and paracentral scotoma.

The RPE, photoreceptors and choroidal atrophy, progresses centrifugally as well as centrally in BCD leading to progressive visual loss.[5] The RPE atrophy is much better appreciated on SWAF pictures. SWAF imaging displayed a distinct patchy atrophy in an annular pattern around the posterior pole in our case. The granular hyperfluorescence outside these patches of RPE atrophy indicates “distressed” RPE, which is at the risk of degeneration with time. The crystals decrease in number with progression of RPE atrophy, which is reflected by fewer crystals in the areas of RPE atrophy in this case.

OCT changes have also been described in BCD.[6] The affected areas on OCT initially have loss of the outer retinal layers, which is followed by retinal and choroidal thinning.[7] Reports have documented crystals as hyperreflective dots on conventional SD-OCT B scans. Crystals were however only faintly visible on SD-OCT B scans in our case.[56] The crystals were better evident on en face OCT at the level of RPE. Crystals at the level of the choroid have been reported in patients with BCD using en face OCT,[7] though the crystals were not seen at the level of choroid in our case.

Conclusion

This report emphasizes the superiority of red-free and en face OCT in the detection of crystals over colour fundus photographs and OCT B scans, respectively, in patients with BCD.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.