Spatial and temporal sequence of corneal crystal deposition in nephropathic cystinosis

Establishing an objective biomarker for corneal cystinosis using a threshold-based Spectral domain optical coherence tomography imaging algorithm

PURPOSE

The purpose of the present study was to establish a semi-automated threshold-based image segmentation algorithm to detect and objectively quantify corneal cystine crystal deposition in ocular cystinosis with anterior segment optical coherence tomography (AS-OCT).

METHODS

This prospective, observational, comparative study included 88 eyes of 45 patients from the German Cystinosis Registry Study as well as 68 eyes of 35 healthy control subjects. All eyes were imaged with AS-OCT (Cirrus HD-OCT 5000, Carl Zeiss Meditec AG, Jena, Germany). As an initial step, B-scan images were subjectively analysed for typical changes in morphology in comparison to healthy controls. Based on the experience gained, an objective semi-automated B-scan image segmentation algorithm was developed using a grey scale value-based threshold method to automatically quantify corneal crystals.

RESULTS

On AS-OCT B-scans, corneal crystals appeared as hyperreflective deposits within the corneal stroma. The crystals were distributed either in all stromal layers (43 eyes, 49%) or confined to the anterior (23 eyes, 26%) or posterior stroma (22 eyes, 25%), respectively. The novel automatic B-scan image segmentation algorithm was most efficient in delineating corneal crystals at higher grey scale thresholds (e.g. 226 of a maximum of 255). Significant differences in suprathreshold grey scale 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.

CONCLUSION

Objective quantification of corneal cystine crystal deposition is feasible with AS-OCT and can serve as a novel biomarker for ocular disease control and topical treatment monitoring.

Examination of corneal deposits in nephropathic cystinosis using in vivo confocal microscopy and anterior segment optical coherence tomography: an age-dependent cross sectional study

Background

Presence of corneal cystine crystals is the main ocular manifestation of cystinosis, although controversial findings concerning the corneal layer with the highest density have been reported. The aim of this study was the analysis of the characteristics of crystal arrangement in different corneal layers and the assessment of corneal morphological changes with age.

Methods

A cross sectional study was carried out in three children and three adults who had nephropathic cystinosis and corneal cystine depositions. All patients underwent a comprehensive ophthalmological examination including best corrected distance visual acuity, slit-lamp examination, in vivo confocal microscopy and anterior segment optical coherence tomography. An evaluation of the depth of crystal deposits and crystal density in different corneal layers was also performed. Due to the low number of subjects no statistical comparison was performed.

Results

Anterior segment optical coherence tomography images revealed deposition of hyperreflective crystals from limbus to limbus in each patient. Crystals appeared as randomly oriented hyperreflective, elongated structures on in vivo confocal microscopy images in all corneal layers except the endothelium. In children the deposits occurred predominantly in the anterior stroma, while in adults, the crystals were mostly localized in the posterior corneal stroma with the depth of crystal deposition showing an increasing tendency with age (mean depth of crystal density was 353.17 ± 49.23 μm in children and it was 555.75 ± 25.27 μm in adults). Mean crystal density of the epithelium was 1.47 ± 1.17 (median: 1.5; interquartile range: 0.3–2.4). Mean crystal density of the anterior and posterior stroma of children and adults was 3.37 ± 0.34 (median: 3.4; interquartile range: 3.25–3.55) vs. 1.23 ± 0.23 (median: 1.2; interquartile range: 1.05–1.35) and 0.76 ± 0.49 (median: 0.7; interquartile range: 0.4–1.15) vs. 3.63 ± 0.29 (median: 3.7; interquartile range: 3.45–3.8), respectively. Endothelium had intact structure in all cases. Some hexagonal crystals were observed in two subjects.

Conclusions

In vivo confocal microscopy and anterior segment optical coherence tomography confirmed an age-related pattern of crystal deposition. In children, crystals tend to locate anteriorly, while in adults, deposits are found posteriorly in corneal stroma.

A case of corneal cystinosis in a patient with rickets and chronic renal failure

A 22-year-old man diagnosed with nephropathic cystinosis at the age of 4 years was found to have progressive bilateral corneal crystal deposition. He presented with severe photophobia and decreased visual acuity. Ocular cystinosis was diagnosed on observing the typical crystals. Optical coherence tomography showed multiple areas of stromal hyperreflectivity due to crystal deposits within the corneal stroma. Ex vivo transmission electron microscopy of the cornea showed pathognomonic crystal deposits in corneal stromal keratocytes. Using polymerase chain reaction sequencing of the entire coding region, we identified five gene mutations, including two unreported mutations.

Successful treatment of the murine model of cystinosis using bone marrow cell transplantation

Cystinosis is an autosomal recessive metabolic disease that belongs to the family of lysosomal storage disorders. The defective gene is CTNS encoding the lysosomal cystine transporter, cystinosin. Cystine accumulates in every organ in the body and leads to organ damage and dysfunction, including renal defects. Using the murine model for cystinosis, Ctns(-/-) mice, we performed syngeneic bone marrow cell (BMC), hematopoietic stem cell (HSC), and mesenchymal stem cell transplantation. Organ-specific cystine content was reduced by 57% to 94% in all organs tested in the BMC-treated mice. Confocal microscopy and quantitative polymerase chain reaction revealed a large quantity of transplanted BMC in all organs tested, from 5% to 19% of the total cells. Most of these cells were not from the lymphoid lineage but part of the intrinsic structure of the organ. The natural progression of renal dysfunction was prevented, and deposition of corneal cystine crystals was significantly improved in the BMC-treated mice. HSC had the same therapeutic effect as whole BMC. In contrast, mesenchymal stem cell did not integrate efficiently in any organ. This work is a proof of concept for using HSC transplantation as a therapy for cystinosis and highlights the efficiency of this strategy for a chronic, progressive degenerative disease.

The ocular anomalies in a cystinosis animal model mimic disease pathogenesis

Cystinosis is a lysosomal storage disorder characterized by abnormal accumulation of cystine, which forms crystals at high concentrations. The causative gene CTNS encodes cystinosin, the lysosomal cystine transporter. The eye is one of the first organs affected (corneal lesions and photophobia in the first and visual impairment in the second decade of life). We characterized the ocular anomalies of Ctns-/- mice to determine whether they mimic those of patients. The most dramatic cystine accumulation was seen in the iris, ciliary body, and cornea of Ctns-/- mice. Consistently, Ctns-/- mice had a low intraocular pressure (IOP) and seemed mildly photophobic. Retinal cystine levels were elevated but increased less dramatically with age. Consistently, the retina was intact and electroretinogram (ERG) profiles were normal in mice younger than 19 mo; beyond this age, retinal crystals and lesions appeared. Finally, the lens contained the lowest cystine levels and crystals were not seen. The temporospatial pattern of cystine accumulation in Ctns-/- mice parallels that of patients and validates the mice as a model for the ocular anomalies of cystinosis. This work is a prerequisite step to the testing of novel ocular cystine-depleting therapies.

In vivo confocal microscopy and polarizing microscopy of the cornea in a patient with nephropathic cystinosis

The clinicopathological and in vivo confocal microscopic characteristics of the corneas from a patient with infantile cystinosis is reported. Crystals were demonstrated in the epithelium and stroma of this patient.

Ophthalmic manifestations and histopathology of infantile nephropathic cystinosis: report of a case and review of the literature

Cystinosis is a rare autosomal recessive metabolic disorder characterized by the intracellular accumulation of cystine, the disulfide of the amino acid cysteine, in many organs and tissues. Infantile nephropathic cystinosis is the most severe phenotype. Corneal crystal accumulation and pigmentary retinopathy were originally the most commonly described ophthalmic manifestations, but successful kidney transplantation significantly changed the natural history of the disease. As cystinosis patients now live longer, long-term complications in extrarenal tissues, including the eye, have become apparent. A case of an adult patient with infantile nephropathic cystinosis is reported. He presented with many long-term ocular complications of cystinosis. After 4 years of follow-up, the patient died from sepsis. Pathology of the phthisical eyes demonstrated numerous electron-transparent polygonal spaces, bounded by single membrane, in corneal cells, retinal pigment epithelial cells, and even choroidal endothelial cells. The ophthalmic manifestations and pathology of infantile nephropathic cystinosis are discussed and reviewed in light of the current report and other cases in the literature.

Evolution of ocular manifestations in nephropathic cystinosis: a long-term study of a population treated with cysteamine

BACKGROUND

Nephropathic cystinosis is characterized by an accumulation of cystine crystals within most body tissues. Renal transplantation and oral cysteamine have improved the general prognosis of the disease, and ocular manifestations are now the most common complication. This long-term follow-up study describes the sequence of ocular manifestations in patients with nephropathic cystinosis treated with oral and topical cysteamine.

METHODS

Data were recorded for all patients with cystinosis examined between 1980 and 2000. For each patient, photophobia and visual acuity were evaluated and slit-lamp and fundus examinations were performed. For some patients, an electroretinogram was also performed.

RESULTS

Twenty-nine patients were observed during this period. They received oral and topical cysteamine. Photophobia and loss of visual acuity generally began by 10 years of age but were severe only after 15 years of age. Peripheral corneal epithelial infiltration appeared in the first few years of life. Infiltration evolved toward the depth and center of the cornea during the second decade of life. Retinopathy was present in 51.7% of the patients, with 3 cases of maculopathy and 3 cases of flattening on electroretinogram.

CONCLUSIONS

Photophobia and corneal infiltration, although generally severe after 15 years of age, could be treated with topical cysteamine and corneal transplantation. Retinal infiltration, previously described as frequent and potentially blinding, is currently observed in only half of these patients, with mild visual impairment. This could be related to the treatment with oral cysteamine reaching the retinal vascularization.

Early-onset chronic renal failure as a presentation of infantile nephropathic cystinosis

We report an 18-month-old girl who presented in chronic renal failure after an illness characterised by protracted diarrhoea, poor weight gain and anaemia. There were no symptoms and signs suggestive of a renal Fanconi syndrome, but a diagnosis of nephropathic cystinosis was suggested by renal biopsy and confirmed by an elevated leucocyte cystine concentration. We suggest that the diagnosis of cystinosis should be considered in any child with chronic renal failure of unknown aetiology.

Clearance of corneal crystals in nephropathic cystinosis by topical cysteamine 0.5%

A 2-year-old girl with nephropathic cystinosis was successfully treated with topical cysteamine 0.5% to one eye. Clearance of crystals from the treated cornea was virtually complete after three months. The possibilities and limitations of this form of treatment are discussed.

Update on nephropathic cystinosis

The cystine that accumulates within cystinotic lysosomes comes primarily from proteins which have been degraded within this organelle. The individual amino acids have specific transport mechanisms to exit the lysosome. The lysosomal cystine transporter is defective in all types of cystinosis. When cells from patients with nephropathic and benign cystinosis were fused, the defect was not corrected and the cystine level remained elevated. This strongly indicates that the genetic defects are allelic (i.e., on the same chromosome). Cysteamine is a weak base which enters the cystinotic lysosome and reacts with cysteamine. forming a mixed disulfide of half-cystine and cysteamine. This mixed disulfide rapidly exits the lysosome via the transport system for cationic amino acids which is normal in cystinosis. Because of the success of renal transplantation, many cystinosis patients are alive in their twenties and even early thirties. Unfortunately, these patients have developed damage to other organs including thyroid, eye, central nervous system, pancreas, and muscle. Cysteamine and its analog, phosphocysteamine, are very beneficial to cystinosis patients, especially when started early in life. These drugs may prevent the need for transplantation. It is too early to know if they will prevent damage to other organs.

Corneal thickness in nephropathic cystinosis

Cystinosis is a rare autosomal recessive metabolic disorder in which non-protein cystine accumulates within cellular lysosomes owing to a defect in lysosomal cystine transport. The pathognomonic ocular manifestation of cystinosis is the deposition of distinctive iridescent crystals in the cornea, not associated with any inflammatory response or recognised change in corneal function. We measured corneal thickness in nine patients with infantile nephropathic cystinosis. We also studied a corneal button from one of these patients who underwent corneal transplantation. All nine patients had increased corneal thickness in comparison with an age matched control population. Electron microscopy analysis of the cystinotic button revealed structural changes of both epithelium and endothelium layers. Increased corneal thickness in patients with nephropathic cystinosis may reflect subclinical corneal oedema.