Abstract
Retinal dystrophies include different hereditary disorders characterized by a progressive loss of the retinal pigment epithelium layer and the choriocapillaris. This spectrum of dystrophies may present with different phenotypic characteristics, but the multimodal analysis of the cases may be helpful in the diagnosis.
Keywords
Choroid, Multimodal imaging, Optical coherence tomography
Dystrophies
Retinal dystrophies include a variety of hereditary retinal disorders characterized by progressive atrophy of the retina, retinal pigment epithelium (RPE), and choriocapillaris complex. These dystrophies are generally not choroidal dystrophies, but usually retinal and/or RPE dystrophies that lead to secondary atrophy of the choriocapillaris. The most common disease in this group is retinitis pigmentosa (RP). Other common dystrophies include central areolar choroidal dystrophy (CACD), late onset retinal degeneration (LORD), and choroideremia (CHM). The lack of a standardized classification for these entities has led to the use of multiple terms for similar conditions. We describe the clinical and imaging characteristics of the main choroidal dystrophies.
Retinitis Pigmentosa
Epidemiology and Genetics
RP includes a heterogeneous group of generalized rod–cone dystrophies characterized by night blindness and progressive centripetal contraction of the visual field. RP occurs in 1 in 4000 people worldwide. RP may show different inheritance patterns, being autosomal recessive in 50–60% of cases, autosomal dominant in 30–40% of cases, and X-linked in 10–15% of cases. Mutation in more than 60 genes has been reported to cause RP.
Multimodal Imaging Analysis
Fundus examination
Typical findings of RP are pallor of the optic nerve, vascular narrowing, and generalized RPE diffuse moth-eaten pattern that starts with a gray discoloration and depigmentation of the RPE and migration of RPE within the retina creating a bone spicule pattern. Typically, the fovea is not involved in the early stages, and patients may preserve central visual acuity. However, the presence of concurrent vitreous opacities, cystoid macular edema, or epirretinal membrane has been described, involving the central visual acuity ( Fig. 5.1 ).
Fundus autofluorescence
The affected areas in the periphery show a mottled pattern of hypopigmentation. This pattern is surrounded by a hyperreflective zone both in the periphery and also in the macula demarcating the involved retina and the preserved retina. Macular changes could be very useful to differentiate foveal sparing or fovea-involving ( Fig. 5.1 ).
Optical coherence tomography
The optical coherence tomography (OCT) scans in RP are characterized by a progressive loss of the ellipsoid zone of the inner segments of the photoreceptors and the external limiting membrane starting at the peripheral retinal and progressing centripetally through the fovea. Progressive photoreceptor loss is subsequently reflected by a reduced thickness of the outer nuclear layer (ONL) (nuclei of photoreceptors) and outer photoreceptor-RPE complex. The analysis of these patients by OCT is helpful in order to rule out treatable findings such as cystoid macular edema and their management ( Fig. 5.2 ).
Different studies have shown a decrease on choroidal thickness in RP patients; however, other studies have shown no significant difference on the choroidal thickness compared with age-matched controls. In addition, the potential correlation of these changes on the functional outcomes of RP patients is not well understood, and some results are inconsistent.
Other features
The electroretinogram shows a marked reduction rod signal initially, then both rod and cone signals in later stages (reduced a- and b-waves); with further progression of the disease, the electroretinographic responses may be undetectable.
Prognosis and Complications
RP is a progressive entity that often leads to legal blindness. Due to this really poor prognosis, the differential diagnosis in cases with no family history is important in order to avoid an error in the diagnosis and its psychological implications. The influence of the choroid in the pathogenesis of RP has the choroidal decreased blood flow, as it seems to precede retinal changes. Decrease in choroidal and retinal blood flow has been shown with different techniques, including magnetic resonance imaging and confocal laser Doppler flowmetry; however, these vascular functional changes could not be correlated with tomographic changes in choroidal thickness. Evaluation of choroid could be an investigation as the suprachoroidal artificial retinal implants could be useful in such conditions.
Special Forms of Retinitis Pigmentosa
Usher syndrome
RP may be primary, affecting only the retina as we have described above, or could be associated with other systemic disorders. The most common association is deafness and RP, known as Usher syndrome.
Usher syndrome is defined as the combination of congenital hearing loss and RP. Usher syndrome has an autosomal recessive inheritance, and mutations in more than 15 genes have been described. Usher syndrome has been classified in three types:
Type 1: congenital sensorineural deafness, unintelligible speech, vestibular symptoms, and early onset of the retinal changes.
Type 2: nonprogressive deafness, absence of vestibular symptoms, and later onset of the retinopathy. This is the most common form.
Type 3: progressive deafness starting in the 2nd–4th decades and adult-onset retinopathy. This is the rarest form.
There is only one series of small cases analyzing the changes in choroidal thickness in cases diagnosed with Usher syndrome type 2, showing a reduction on the thickness of the choroid in these patients.
Further analysis is requested in order to better understand the influence of the choroidal changes in the pathogenesis of RP and Usher syndrome. Also, the recognition of clinical features in the daily practice that may predict the functional changes that has been demonstrated would be useful in the evaluation of these diseases and the potential effects of future therapeutic approaches.
Central Areolar Choroidal Dystrophy
Epidemiology and Genetics
This dystrophy usually presents in late childhood or early adulthood in the absence of symptoms. CACD commonly has an autosomal dominant inheritance caused by mutation in the PRPH2 gene. There are seven different mutations identified to cause CACD. There are also rare cases of autosomal recessive inheritance.
Multimodal Imaging
Fundus examination
The clinical findings in CACD have been classified in four stages. In stage I, slight parafoveal pigmentary changes of the RPE are seen, usually in the adolescent patient. Stage II is characterized by a round-to-oval, mildly atrophic hypopigmented area of 1.5 to several disc diameters. In stage III, ≥1 areas of well-circumscribed atrophy of the RPE and choriocapillaris appear outside the fovea. In stage IV, this area of well-defined chorioretinal atrophy affects the fovea. When the area of profound chorioretinal atrophy has reached the center of the macula and is about to involve central foveal function with a still fairly good visual acuity, lesions were categorized as stages III–IV ( Figs. 5.3 and 5.4 ). There is an absence of drusen or other concurrent retinal findings.
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