Retinal Morphology in Sjögren-Larsson Syndrome on OCT: From Metabolic Crystalline Maculopathy to Early-Onset Macular Degeneration

Discovery of novel diagnostic biomarkers for Sjögren-Larsson syndrome by untargeted lipidomics

AIM

Sjögren-Larsson syndrome (SLS) is a rare neurometabolic disorder that mainly affects brain, eye and skin and is caused by deficiency of fatty aldehyde dehydrogenase. Our recent finding of a profoundly disturbed brain tissue lipidome in SLS prompted us to search for similar biomarkers in plasma as no functional test in blood is available for SLS.

METHODS AND RESULTS

We performed plasma lipidomics and used a newly developed bioinformatics tool to mine the untargeted part of the SLS plasma and brain lipidome to search for SLS biomarkers. Plasma lipidomics showed disturbed ether lipid metabolism in known lipid classes. Untargeted lipidomics of both plasma and brain (white and grey matter) uncovered two new endogenous lipid classes highly elevated in SLS. The first biomarker group were alkylphosphocholines/ethanolamines containing different lengths of alkyl-chains where some alkylphosphocholines were > 600-fold elevated in SLS plasma. The second group of biomarkers were a set of 5 features of unknown structure. Fragmentation studies suggested that they contain ubiquinol and phosphocholine and one feature was also found as a glucuronide conjugate in plasma. The plasma features were highly distinctive for SLS with levels >100-1000-fold the level in controls, if present at all. We speculate on the origin of the alkylphosphocholines/ethanolamines and the nature of the ubiquinol-containing metabolites.

CONCLUSIONS

The metabolites identified in this study represent novel endogenous lipid classes thus far unknown in humans. They represent the first plasma metabolite SLS-biomarkers and may also yield more insight into SLS pathophysiology.

Non-vasogenic cystoid maculopathies

Besides cystoid macular edema due to a blood-retinal barrier breakdown, another type of macular cystoid spaces referred to as non-vasogenic cystoid maculopathies (NVCM) may be detected on optical coherence tomography but not on fluorescein angiography. Various causes may disrupt retinal cell cohesion or impair retinal pigment epithelium (RPE) and Müller cell functions in the maintenance of retinal dehydration, resulting in cystoid spaces formation. Tractional causes include vitreomacular traction, epiretinal membranes and myopic foveoschisis. Surgical treatment does not always allow cystoid space resorption. In inherited retinal dystrophies, cystoid spaces may be part of the disease as in X-linked retinoschisis or enhanced S-cone syndrome, or occur occasionally as in bestrophinopathies, retinitis pigmentosa and allied diseases, congenital microphthalmia, choroideremia, gyrate atrophy and Bietti crystalline dystrophy. In macular telangiectasia type 2, cystoid spaces and cavitations do not depend on the fluid leakage from telangiectasia. Various causes affecting RPE function may result in NVCM such as chronic central serous chorioretinopathy and paraneoplastic syndromes. Non-exudative age macular degeneration may also be complicated by intraretinal cystoid spaces in the absence of fluorescein leakage. In these diseases, cystoid spaces occur in a context of retinal cell loss. Various causes of optic atrophy, including open-angle glaucoma, result in microcystoid spaces in the inner nuclear layer due to a retrograde transsynaptic degeneration. Lastly, drug toxicity may also induce cystoid maculopathy. Identifying NVCM on multimodal imaging, including fluorescein angiography if needed, allows guiding the diagnosis of the causative disease and choosing adequate treatment when available.

Clinical and biochemical footprints of inherited metabolic disorders. VII. Ocular phenotypes

Ocular manifestations are observed in approximately one third of all inherited metabolic disorders (IMDs). Although ocular involvement is not life-threatening, it can result in severe vision loss, thereby leading to an additional burden for the patient. Retinal degeneration with or without optic atrophy is the most frequent phenotype, followed by oculomotor problems, involvement of the cornea and lens, and refractive errors. These phenotypes can provide valuable clues that contribute to its diagnosis. In this issue we found 577 relevant IMDs leading to ophthalmologic manifestations. This article is the seventh of a series attempting to create and maintain a comprehensive list of clinical and metabolic differential diagnoses according to system involvement.

Characterization of West African Crystalline Macular Dystrophy in the Ghanaian Population

OBJECTIVE/PURPOSE

West African Crystalline Maculopathy (WACM) is characterized by the presence of macular hyper-refractile crystal-like deposits. Although the underlying pathophysiology has not been elucidated, a few biological drivers have been proposed. We analysed a large WACM case series to gain a more robust understanding of its features and etiology.

DESIGN

Prospective, Cross-sectional cohort study.

SUBJECTS/PARTICIPANTS

Participants with WACM were selected from the large cohort recruited into the Ghana Age-Related Macular Degeneration Study (Ghana AMD Study).

METHODS

Demographic and detailed medical histories, full ophthalmic examinations, digital colour fundus photographs and optical coherence tomography (OCT) images were obtained. All WACM cases were evaluated by three retina experts. Crystal numbers, location, and distribution were determined. Associations between WACM and Caucasian AMD risk variants were assessed using Firth's bias-reduced logistic regression, including age and gender as covariates.

MAIN OUTCOME MEASURES

Phenotypic features of, and genetic associations with, WACM.

RESULTS

WACM was identified in 106 eyes of 53 participants: 22 were bilateral and 24 unilateral. Grading for AMD was not possible in one eye of seven WACM participants; therefore, laterality was not assessed in these subjects. Thirty-eight participants were female, and 14 male; gender was unrecorded for one participant. Mean age was 68.4 years (range 45-101). OCT demonstrated typical WACM crystals, which were more easily identified at high contrast and predominantly located at the inner limiting membrane (ILM). In eyes with co-pathology, crystals localised deeper in the inner retina with wider retinal distribution over co-pathology lesions. There was no age or gender association. A significant association was observed between the complement factor H (CFH) 402H risk variant and WACM.

CONCLUSION

This study confirms localization of crystals adjacent to the ILM, and distribution over lesions in eyes with co-pathology. Evaluation of OCT images under high contrast allows improved identification. WACM may be associated with the CFH-CFHR5 AMD-risk locus identified amongst Caucasians; however, it is also possible that combination of crystals and the CFH 402H allele increases the risk for developing late AMD. Further analyses using larger sample sizes are warranted to identify causalities between genotype and WACM phenotype.

Genetic diseases mimicking multiple sclerosis

Multiple sclerosis (MS) is an inflammatory neurodegenerative disorder manifesting as gradual or progressive loss of neurological functions. Most patients present with relapsing-remitting disease courses. Extensive research over recent decades has expounded our insights into the presentations and diagnostic features of MS. Groups of genetic diseases, CADASIL and leukodystrophies, for example, have been frequently misdiagnosed with MS due to some overlapping clinical and radiological features. The delayed identification of these diseases in late adulthood can lead to severe neurological complications. Herein we discuss genetic diseases that have the potential to mimic multiple sclerosis, with highlights on clinical identification and practicing pearls that may aid physicians in recognizing MS-mimics with genetic background in clinical settings.

Sjögren-Larsson syndrome: The mild end of the phenotypic spectrum

Sjögren-Larsson syndrome (SLS) is a rare inborn error of lipid metabolism. The syndrome is caused by mutations in the ALDH3A2 gene, resulting in a deficiency of fatty aldehyde dehydrogenase. Most patients have a clearly recognizable severe phenotype, with congenital ichthyosis, intellectual disability, and spastic diplegia. In this study, we describe two patients with a remarkably mild phenotype. In both patients, males with actual ages of 45 and 61 years, the diagnosis was only established at an adult age. Their skin had been moderately affected from childhood onward, and both men remained ambulant with mild spasticity of their legs. Cognitive development, as reflected by school performance and professional career, had been unremarkable. Magnetic resonance spectroscopy of the first patient was lacking the characteristic lipid peak. We performed a literature search to identify additional SLS patients with a mild phenotype. We compared the clinical, radiologic, and molecular features of the mildly affected patients with the classical phenotype. We found 10 cases in the literature with a molecular proven diagnosis and a mild phenotype. Neither a genotype-phenotype correlation nor an alternative explanation for the strikingly mild phenotypes was found. New biochemical techniques to study the underlying metabolic defect in SLS, like lipidomics, may in the future help to unravel the reasons for the exceptionally mild phenotypes. In the meantime, it is important to recognize these mildly affected patients to provide them with appropriate care and genetic counseling, and to increase our insights in the true disease spectrum of SLS.