Mervyn G. Thomas

Structural Grading of Foveal Hypoplasia Using Spectral-Domain Optical Coherence Tomography: A Predictor of Visual Acuity?

PURPOSE To characterize and grade the spectrum of foveal hypoplasia based on different stages of arrested development of the fovea. Grading was performed using morphologic findings obtained by ultra high-resolution spectral-domain optical coherence tomography. Best-corrected visual acuity (BCVA) was calculated for different grades. DESIGN Observational case series. PARTICIPANTS AND CONTROLS Sixty-nine patients with foveal hypoplasia (albinism, n = 34; PAX6 mutations, n = 10; isolated cases, n = 14; achromatopsia, n = 11) and 65 control subjects were examined. METHODS A 7×7-mm retinal area was sampled using a 3-dimensional scanning protocol (743×75, A scans×B scans) with ultra high-resolution spectral-domain optical coherence tomography (SOCT Copernicus HR; 3-μm axial resolution). Gross morphologic abnormalities were documented. B-scans at the fovea were segmented using a longitudinal reflectivity profile. Logarithm of the minimum angle of resolution BCVA was obtained. MAIN OUTCOME MEASURES Grading was based on presence or absence of foveal pit and widening of the outer nuclear layer (ONL) and outer segment (OS) at the fovea. Quantitative measurements were obtained for comparing atypical foveal hypoplasia in achromatopsia. Best-corrected visual acuity was compared with the grade of foveal hypoplasia. RESULTS Four grades of foveal hypoplasia were distinguished: grade 1, shallow foveal pit, presence of ONL widening, presence of OS lengthening; grade 2, grade 1 but absence of foveal pit; grade 3, grade 2 but absence of OS lengthening; grade 4, grade 3 but absence of ONL widening. There was significant difference in visual acuity (VA) associated with each grade (P<0.0001). Grade 1 was associated with the best VA (median VA, 0.2), whereas grades 2, 3, and 4 were associated with progressively poorer VA with a median VA of 0.44, 0.60, and 0.78, respectively. The atypical features seen with foveal hypoplasia associated with achromatopsia were characterized by decreased retinal and ONL thickness and deeper foveal depth. CONCLUSIONS A structural grading system for foveal hypoplasia was developed based on the stage at which foveal development was arrested, which helps to provide a prognostic indicator for VA and is applicable in a range of disorders associated with foveal hypoplasia. Atypical foveal hypoplasia in achromatopsia shows different characteristics. FINANCIAL DISCLOSURE(S) The author(s) have no proprietary or commercial interest in any materials discussed in this article.

High-Resolution In Vivo Imaging in Achromatopsia

PURPOSE To characterize the retinal changes in patients with achromatopsia using an ultrahigh-resolution (UHR) spectral-domain optical coherence tomography (OCT) to examine how human achromatopsia corresponds to its animal model. DESIGN Comparative case series. PARTICIPANTS AND CONTROLS Ultrahigh-resolution OCT (Copernicus; OPTOPOL Technology S.A., Zawiercie, Poland; 3-μm axial resolution) was used to obtain scans from 13 patients (26 eyes) with achromatopsia and from 20 controls (40 eyes). METHODS A 3-dimensional scan program (743×75; A×B scan) sampling a 7×7-mm retinal area centered at the fovea was used to obtain tomograms of the fovea. Individual B-scans at the fovea were exported and analyzed using ImageJ (Wayne Rasband, National Institute of Health) for reflectance profiles and morphologic abnormalities. MAIN OUTCOME MEASURES Gross morphologic changes in OCT were characterized. Specifically, inner segment and outer segment (IS/OS) junction and cone outer segment tip (COST) disruption was noted. Using the reflectance profiles, foveal depth, thickness of the outer nuclear layer (ONL), and retinal thickness (RT) were measured. RESULTS A characteristic so-called punched out hyporeflective zone (HRZ) was noted in 7 of 13 patients; this was age-dependent (P = 0.001). The area of the HRZ was asymmetric with the nasal area being significantly greater than the temporal area (P = 0.002). In all patients, there was disruption of the IS/OS junction at the foveal or parafoveal regions, or both. Five of 13 patients also had a disrupted COST reflectivity. There was significant (P = 1.1×10(-6)) ONL thinning in the achromats compared with controls, which was age-dependent (P = 0.0002). Foveal maldevelopment was seen in 9 of 13 patients. The achromats also had a significantly reduced foveal depth (P = 7.7×10(-6)) and RT (P = 1.46×10(-9)) compared with controls. CONCLUSIONS A range of signs in achromatopsia are described that can be detected using UHR OCT. The IS/OS junction and COST reflectivity disruption and presence of HRZ and ONL thinning are signs of cone photoreceptor degeneration. The latter 2 are age-dependent, which suggests that achromatopsia is a progressive disorder. In addition, foveal maldevelopment is described; this represents a fetal developmental defect linked to cone photoreceptor degeneration.

The Functional Significance of Foveal Abnormalities in Albinism Measured Using Spectral-Domain Optical Coherence Tomography

PURPOSE The relationship between foveal abnormalities in albinism and best-corrected visual acuity (BCVA) is unclear. High-resolution spectral-domain optical coherence tomography (SD OCT) was used to quantify foveal retinal layer thicknesses and to assess the functional significance of foveal morphologic features in patients with albinism. DESIGN Cross-sectional study. PARTICIPANTS Forty-seven patients with albinism and 20 healthy control volunteers were recruited to the study. METHODS Using high-resolution SD OCT, 7×7×2-mm volumetric scans of the fovea were acquired (3-μm axial resolution). The B scan nearest the center of the fovea was identified using signs of foveal development. The thickness of each retinal layer at the fovea and foveal pit depth were quantified manually using ImageJ software and were compared with BCVA. MAIN OUTCOME MEASURES Total retinal thickness, foveal pit depth, photoreceptor layer thickness, and processing layer thickness in relation to BCVA. RESULTS Total photoreceptor layer thickness at the fovea was correlated highly to BCVA (P = 0.0008; r = -0.501). Of the photoreceptor layers, the outer segment length was correlated most strongly to BCVA (P<0.0001; r = -0.641). In contrast, there was no significant correlation between either total retinal thickness or pit depth and BCVA (P>0.05). This was because of an inverse correlation between total photoreceptor layer thickness and total processing layer thickness (P<0.0001; r = -0.696). CONCLUSIONS Neither the total retinal thickness nor the pit depth are reliable indicators of visual deficit, because patients with similar overall retinal thickness had widely varying foveal morphologic features. In albinism, the size of the photoreceptor outer segment was found to be the strongest predictor of BCVA. These results suggest that detailed SD OCT images of photoreceptor anatomic features provide a useful tool in assessing the visual potential in patients with albinism. FINANCIAL DISCLOSURE(S) The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Early signs of longitudinal progressive cone photoreceptor degeneration in achromatopsia.

Aims To characterise longitudinal progressive retinal changes in achromatopsia. Methods Ultrahigh-resolution spectral optical coherence tomography (Copernicus, 3 μm axial resolution) was used to obtain tomograms of the fovea from five children and three adults with achromatopsia. Each patient was scanned twice with a mean follow-up time of 16 months. Progressive changes in reflectivity at the inner segment/outer segment (IS/OS) junction, the central macular and outer nuclear layer thickness were analysed. Results Younger patients (<10 years; patient 1–5) showed progressive morphological changes at the IS/OS junction between visits 1 and 2. However, older patients (>40 years; patients 6–8) did not have any changes in the retinal morphology between visits 1 and 2. In patients 1 and 2, IS/OS discontinuities (visit 1) developed into a hyper-reflective zone confined to the fovea (visit 2). In patient 3, the hyper-reflective zone (visit 1) progressed to form an IS/OS disruption and early formation of a small hypo-reflective zone (visit 2). Patients 4 and 5 had a hypo-reflective zone (visit 1) which subsequently increased in size (visit 2). There was a decrease in central macular and outer nuclear layer thickness between visits 1 and 2 in children. Conclusions For the first time, we show progressive longitudinal changes in retinal morphology in achromatopsia. Early changes include subtle IS/OS reflectivity alterations. The dynamic retinal changes in younger patients provide evidence that it represents a progressive disorder, and implementation of gene therapy during the early stages of the disease may provide best prognosis.

A novel interaction between FRMD7 and CASK: evidence for a causal role in idiopathic infantile nystagmus

Idiopathic infantile nystagmus (IIN) is a genetically heterogeneous disorder of eye movement that can be caused by mutations in the FRMD7 gene that encodes a FERM domain protein. FRMD7 is expressed in the brain and knock-down studies suggest it plays a role in neurite extension through modulation of the actin cytoskeleton, yet little is known about its precise molecular function and the effects of IIN mutations. Here, we studied four IIN-associated missense mutants and found them to have diverse effects on FRMD7 expression and cytoplasmic localization. The C271Y mutant accumulates in the nucleus, possibly due to disruption of a nuclear export sequence located downstream of the FERM-adjacent domain. While overexpression of wild-type FRMD7 promotes neurite outgrowth, mutants reduce this effect to differing degrees and the nuclear localizing C271Y mutant acts in a dominant-negative manner to inhibit neurite formation. To gain insight into FRMD7 molecular function, we used an IP-MS approach and identified the multi-domain plasma membrane scaffolding protein, CASK, as a FRMD7 interactor. Importantly, CASK promotes FRMD7 co-localization at the plasma membrane, where it enhances CASK-induced neurite length, whereas IIN-associated FRMD7 mutations impair all of these features. Mutations in CASK cause X-linked mental retardation. Patients with C-terminal CASK mutations also present with nystagmus and, strikingly, we show that these mutations specifically disrupt interaction with FRMD7. Together, our data strongly support a model whereby CASK recruits FRMD7 to the plasma membrane to promote neurite outgrowth during development of the oculomotor neural network and that defects in this interaction result in nystagmus.

Clinical and oculomotor characteristics of albinism compared to FRMD7 associated infantile nystagmus.

PURPOSE Previous studies have found no difference between nystagmus characteristics associated with idiopathic infantile nystagmus (IIN) and that associated with albinism. The present aim is to compare the oculomotor characteristics and other associated clinical features of albinism and a genetically homogenous group of IIN volunteers where the nystagmus is associated with FRMD7 mutations. METHODS Oculomotor characteristics and related clinical features between albinism (n = 52) and idiopathic nystagmus associated with FRMD7 mutations (FRMD7-IIN, n = 83) were compared. The nystagmus characteristics compared included amplitude, frequency, intensity of nystagmus, foveation characteristics, and waveform type. Other clinical features compared were strabismus, stereopsis and anomalous head posture. RESULTS The FRMD7-IIN group contained a higher proportion of pendular waveform types compared with the albinism group (P < 0.0001). Nystagmus frequency was significantly lower in albinos (mean = 3.3 Hz, SD = 0.13 Hz) compared with the FRMD7-IIN group (mean = 4.3 Hz, SD = 0.18 Hz) (F = 14.5, P < 0.0001). Strabismus and anomalous head posture was seen in higher proportions in the albinism group, and stereopsis was worse compared with the FRMD7-IIN group (P ≪ 0.0001). CONCLUSIONS Differences in nystagmus characteristics associated with albinism and those associated with FRMD7 mutations leading to IIN are described for the first-time. These findings may provide useful information in the future elucidation of mechanisms underlying the nystagmus associated with albinism and idiopathic infantile nystagmus.

The clinical and molecular genetic features of idiopathic infantile periodic alternating nystagmus

Periodic alternating nystagmus consists of involuntary oscillations of the eyes with cyclical changes of nystagmus direction. It can occur during infancy (e.g. idiopathic infantile periodic alternating nystagmus) or later in life. Acquired forms are often associated with cerebellar dysfunction arising due to instability of the optokinetic-vestibular systems. Idiopathic infantile periodic alternating nystagmus can be familial or occur in isolation; however, very little is known about the clinical characteristics, genetic aetiology and neural substrates involved. Five loci (NYS1-5) have been identified for idiopathic infantile nystagmus; three are autosomal (NYS2, NYS3 and NYS4) and two are X-chromosomal (NYS1 and NYS5). We previously identified the FRMD7 gene on chromosome Xq26 (NYS1 locus); mutations of FRMD7 are causative of idiopathic infantile nystagmus influencing neuronal outgrowth and development. It is unclear whether the periodic alternating nystagmus phenotype is linked to NYS1, NYS5 (Xp11.4-p11.3) or a separate locus. From a cohort of 31 X-linked families and 14 singletons (70 patients) with idiopathic infantile nystagmus we identified 10 families and one singleton (21 patients) with periodic alternating nystagmus of which we describe clinical phenotype, genetic aetiology and neural substrates involved. Periodic alternating nystagmus was not detected clinically but only on eye movement recordings. The cycle duration varied from 90 to 280 s. Optokinetic reflex was not detectable horizontally. Mutations of the FRMD7 gene were found in all 10 families and the singleton (including three novel mutations). Periodic alternating nystagmus was predominantly associated with missense mutations within the FERM domain. There was significant sibship clustering of the phenotype although in some families not all affected members had periodic alternating nystagmus. In situ hybridization studies during mid-late human embryonic stages in normal tissue showed restricted FRMD7 expression in neuronal tissue with strong hybridization signals within the afferent arms of the vestibulo-ocular reflex consisting of the otic vesicle, cranial nerve VIII and vestibular ganglia. Similarly within the afferent arm of the optokinetic reflex we showed expression in the developing neural retina and ventricular zone of the optic stalk. Strong FRMD7 expression was seen in rhombomeres 1 to 4, which give rise to the cerebellum and the common integrator site for both these reflexes (vestibular nuclei). Based on the expression and phenotypic data, we hypothesize that periodic alternating nystagmus arises from instability of the optokinetic-vestibular systems. This study shows for the first time that mutations in FRMD7 can cause idiopathic infantile periodic alternating nystagmus and may affect neuronal circuits that have been implicated in acquired forms.

Potential of Handheld Optical Coherence Tomography to Determine Cause of Infantile Nystagmus in Children by Using Foveal Morphology

OBJECTIVE To investigate the feasibility of handheld (HH) ultra-high-resolution spectral-domain optical coherence tomography (SD-OCT) in young children with nystagmus, to determine its sensitivity and specificity in classifying foveal abnormalities, and to investigate its potential to determine the cause of infantile nystagmus with the use of foveal morphology. DESIGN Prospective, case-control study. PARTICIPANTS AND CONTROLS A total of 50 patients with nystagmus and 50 healthy control subjects (mean age, 3.2 years; range, 0-8 years). METHODS Each patient was scanned using HH SD-OCT (Bioptigen Inc., Research Triangle Park, NC) without sedation, and foveal morphology was classified into 1 of 4 categories: (1) typical foveal hypoplasia (predicting clinical diagnosis of albinism, PAX6 mutations, or isolated foveal hypoplasia); (2) atypical foveal hypoplasia (predicting achromatopsia); (3) other foveal changes (corresponding to retinal dystrophies); and (4) normal fovea (predicting idiopathic or manifest latent nystagmus). An independent interpretation of the HH SD-OCT scans by masked examiners was performed, and the sensitivity and specificity of the predicted diagnosis were calculated. MAIN OUTCOME MEASURES The success rate of image acquisition and sensitivity and specificity of the HH SD-OCT in classifying foveal abnormalities. RESULTS In 94% of examinations, HH SD-OCT was successful. Twenty-three patients had typical foveal hypoplasia (category 1). Of these patients, 21 were diagnosed with albinism and 2 were diagnosed with PAX6 mutations. Five patients were classified as atypical (category 2) and diagnosed with achromatopsia. Six patients had other abnormal foveal morphology (category 3) and were diagnosed with retinal dystrophy. Sixteen patients had normal foveal morphology (category 4). Of these patients, 12 were diagnosed with idiopathic nystagmus and 4 were diagnosed with manifest latent nystagmus. Sensitivities of HH SD-OCT for classifying typical or atypical foveal hypoplasia, other abnormal foveal morphology, and normal morphology were 92.8%, 86.7%, 41.1%, and 88.4%, respectively, with specificities of 91.4%, 94.8%, 97.7% and 95.1%, respectively. CONCLUSIONS We demonstrate excellent feasibility of HH SD-OCT in the diagnosis of conditions associated with infantile nystagmus. The HH SD-OCT classification of foveal abnormalities was highly sensitive and specific. This classification was used to determine the underlying cause of infantile nystagmus. Handheld SD-OCT in early childhood can facilitate focused investigations and earlier diagnosis. This is important in an era when potentially time-sensitive treatment, such as gene therapy, is imminent.

The Role of FRMD7 in Idiopathic Infantile Nystagmus

Idiopathic infantile nystagmus (IIN) is an inherited disorder in which the nystagmus arises independently of any other symptoms, leading to the speculation that the disorder represents a primary defect in the area of the brain responsible for ocular motor control. The inheritance patterns are heterogeneous, however the most common form is X-linked. FRMD7 resides at Xq26-27 and approximately 50% of X-linked IIN families map to this region. Currently 45 mutations within FRMD7 have been associated with IIN, confirming the importance of FRMD7 in the pathogenesis of the disease. Although mutations in FRMD7 are known to cause IIN, very little is known about the function of the protein. FRMD7 contains a conserved N-terminal FERM domain suggesting that it may provide a link between the plasma membrane and actin cytoskeleton. Limited studies together with the knowledge of the function of other FERM domain containing proteins, suggest that FRMD7 may play a role in membrane extension during neuronal development through remodeling of the actin cytoskeleton.

Abnormal retinal development associated with FRMD7 mutations

Idiopathic infantile nystagmus (IIN) is a genetically heterogeneous disorder, often associated with FRMD7 mutations. As the appearance of the retina is reported to be normal based on conventional fundus photography, IIN is postulated to arise from abnormal cortical development. To determine whether the afferent visual system is involved in FRMD7 mutations, we performed in situ hybridization studies in human embryonic and fetal stages (35 days post-ovulation to 9 weeks post-conception). We show a dynamic retinal expression pattern of FRMD7 during development. We observe expression within the outer neuroblastic layer, then in the inner neuroblastic layer and at 9 weeks post-conception a bilaminar expression pattern. Expression was also noted within the developing optic stalk and optic disk. We identified a large cohort of IIN patients (n = 100), and performed sequence analysis which revealed 45 patients with FRMD7 mutations. Patients with FRMD7 mutations underwent detailed retinal imaging studies using ultrahigh-resolution optical coherence tomography. The tomograms were compared with a control cohort (n = 60). The foveal pit was significantly shallower in FRMD7 patients (P < 0.0001). The optic nerve head morphology was abnormal with significantly decreased optic disk area, retinal nerve fiber layer thickness, cup area and cup depth in FRMD7 patients (P < 0.0001). This study shows for the first time that abnormal afferent system development is associated with FRMD7 mutations and could be an important etiological factor in the development of nystagmus.