Abstract

Cystinosis is a rare lysosomal storage disease caused by a mutation of the CTNS gene that codes for the lysosomal cystine transporter cystinosin. Functional impairment of cystinosin leads to an accumulation of cystine crystals in several organs and in different ophthalmic structures. Current gold standard treatment of ocular cystinosis is the oral administration of cysteamine. The purpose of the present work is to describe clinical, demographic and functional findings in Germany’s largest cystinosis collective and to evaluate data gained by multimodal imaging. The present work describes for the first time an anterior segment optical coherence tomography (AS-OCT) based objective biomarker for corneal cystine crystal deposition will be described and crystal deposition in chorioretinal structures as imaged by spectral domain optical coherence tomography (SD-OCT) will be characterized. In the following their potential value as a marker for systemic disease control is elucidated. The study is carried out at the University eye hospital of the Ludwig-Maximilian University in Munich in collaboration with the interdisciplinary cystinosis clinic at the RoMed Clinic Rosenheim. Patients were recruited from the German national Cystinosis registry study. A complete ophthalmological examination was performed, including ETDRS best corrected visual acuity, slitlamp biomicroscopy, dilated funduscopy, AS-OCT (Cirrus HD-OCT 5000, Carl Zeiss Meditec AG, Jena, Germany) and SD-OCT (Spectralis; Heidelberg Engineering GmbH, Heidelberg, Germany). Corneal crystals were graded using slitlamp biomicroscopy and AS-OCT. The first objective semi-automated B-scan image segmentation algorithm was developed using a grey scale value-based threshold method to automatically quantify corneal crystals. Deposition of retinal and choroidal crystals was graded employing a novel semi-quantitative grading system using SD-OCT, named chorioretinal cystine crystal score (RCCCS). It is the first description of a quantitative evaluation of retinal crystals on a large scale. To quantify patient-reported quality of vision, patients completed a standardized and clinically validated 30-item questionnaire addressing different visual symptoms (e.g. glare, blurred vision). Moreover, genetic testing to differentiate mutations of the cystinosin gene was performed. A total of 112 eyes of 56 patients (mean age 23 years ± 13 SD, median age 18,5 years; male:female ratio = 32:24; aged between 1-75 years) from the German Cystinosis Registry were included. 88 corneas of 45 patients were imaged. Another 68 AS-OCT Scans from 35 healthy subjects were used as controls. A total of 74 retina scans from 38 patients were evaluated. Cystine crystals present as hyperreflective, gold yellow deposits in ocular structures like the conjunctiva, cornea, iris, retina and choroid. On AS-OCT B-scans, corneal crystals appeared as hyperreflective deposits within the corneal stroma. The novel automatic B-scan image segmentation algorithm was most efficient in delineating corneal crystals at higher greyscale thresholds. Significant differences in suprathreshold greyscale pixels were observable between cystinosis patients and healthy controls (p<0.001). In addition, the algorithm was able to detect an age-dependent depth distribution profile of crystal deposition. Using SD-OCT of the retina cystine crystals were detectable in all neuroretinal layers and the choroid. Crystals in the choriocapillaris were the most common manifestation. The new defined parameter, RCCCS, based on the number of hyperreflective spots in the distinct retinal layers, was negatively correlated with the dose of cysteamine intake (R=0.533, p=0.001) and positively with Cystatin C, a stable parameter of renal function (R=0.496, p=0.016). Moreover, the value of the RCCCS affected subjective quality of vision. Genetic analysis indicated pronounced crystal deposition in patients with heterozygous mutations containing the 57-kb-deletion allele of the CTNS gene. The survey of the RCCCS allowed the establishment of a first clinical grading system. The newly established measuring methods for corneal and retinochoroidal crystal deposition can help to evaluate clinical data and to better understand the pathophysiology of cystinosis. Objective quantification of corneal cystine crystal deposition is feasible with AS-OCT and can present an age-related depth distribution. Via AS-OCT it was possible to develop a classification of cystinosis with respect to the different depth distribution patterns of the corneal cystine crystals. In future studies it may serve as a longterm parameter for ocular disease control and topical treatment surveillance. The presented SD-OCT based grading system might serve as a biomarker for systemic disease control. The established clinical grading system enables the quantification of the degree and the course of the illness.