What Can Acquired Nystagmus Tell Us About Congenital Forms of Nystagmus?

Downbeat nystagmus: a clinical review of diagnosis and management

PURPOSE OF REVIEW

This review will extensively cover the clinical manifestations, causes, diagnostic evaluation, and management strategies of downbeat nystagmus (DBN).

RECENT FINDINGS

Historically, MRI to assess for structural lesions at the cervicomedullary junction has been the primary diagnostic test in the evaluation of DBN since the 1980s. In recent years, there is increasing awareness of nonstructural causes of DBN including gluten ataxia, nutritional deficiencies, and paraneoplastic syndromes, among others. Medical management with aminopyridines has become first-line therapy in addition to disease-specific therapies.

SUMMARY

DBN is a common form of acquired nystagmus and the differential diagnosis remains broad, including both benign and potentially fatal causes. For practical purposes, the causes can be categorized as structural vs. nonstructural with MRI as the ideal, initial diagnostic study to differentiate the two. General therapeutic options include pharmacotherapy to enhance Purkinje cell function, strabismus surgery or prisms to shift null points, and behavioural changes. Disease-specific treatment is necessarily broad, though a significant proportion of patients will be idiopathic.

Development and clinical utility of a novel diagnostic nystagmus gene panel using targeted next-generation sequencing

Infantile nystagmus (IN) is a genetically heterogeneous disorder arising from variants of genes expressed within the developing retina and brain. IN presents a diagnostic challenge and patients often undergo numerous investigations. We aimed to develop and assess the utility of a next-generation sequencing (NGS) panel to enhance the diagnosis of IN. We identified 336 genes associated with IN from the literature and OMIM. NimbleGen Human custom array was used to enrich the target genes and sequencing was performed using HiSeq2000. Using reference genome material (NA12878), we show the sensitivity (98.5%) and specificity (99.9%) of the panel. Fifteen patients with familial IN were sequenced using the panel. Two authors were masked to the clinical diagnosis. We identified variants in 12/15 patients in the following genes: FRMD7 (n=3), CACNA1F (n=2), TYR (n=5), CRYBA1 (n=1) and TYRP1 (n=1). In 9/12 patients, the clinical diagnosis was consistent with the genetic diagnosis. In 3/12 patients, the results from the genetic diagnoses (TYR, CRYBA1 and TYRP1 variants) enabled revision of clinical diagnoses. In 3/15 patients, we were unable to determine a genetic diagnosis. In one patient, copy number variation analysis revealed a FRMD7 deletion. This is the first study establishing the clinical utility of a diagnostic NGS panel for IN. We show that the panel has high sensitivity and specificity. The genetic information from the panel will lead to personalised diagnosis and management of IN and enable accurate genetic counselling. This will allow development of a new clinical care pathway for IN.

Anticonvulsant-induced downbeat nystagmus in epilepsy

We report data from two patients who developed reversible downbeat nystagmus (DBN) while using AEDs within the therapeutic range. All previous reported cases of epilepsy with drug-induced DBN related to toxic levels of AEDs were summarized, and DBN was found mostly occurring in those using a sodium channel blocking AED. We propose that in our cases, the DBN with therapeutic AED levels may be explained by additive effects of sodium channel blockers. Adverse drug effects should be considered as a cause of DBN in people with epilepsy treated with multiple AEDs.

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.

Nystagmus in infancy and childhood

Involuntary occular oscillations in infancy and childhood can be understood, diagnosed, classified and treated as a result of modern electrophysiology and neuroanatomical knowledge.

Infantile aperiodic alternating nystagmus

BACKGROUND

This study identifies the clinical and ocular motility characteristics of the periodic and aperiodic forms of infantile alternating nystagmus (IAPAN) and establishes the range of electrophysiological and clinical characteristics while providing clues to its presence and pathophysiology.

METHODS

Seventy-eight patients with ocular oscillations consistent with IAPAN were reported. Outcome variables were: age, follow-up in months, vision, strabismus, other eye and systemic abnormalities, head position, periodicity, cycle and null period duration, foveation time, waveforms, and cycle symmetry.

RESULTS

Age range was 1 to 67 years, 50% had pure periodic and aperiodic forms, 46% had albinism, 26% had binocular acuity of 20/40 or greater, 72% had strabismus, 35% had amblyopia, 31% had other eye disease, 14% had systemic disease, 87% had an anomalous head posture, and 65% had binocular directional asymmetry. The periodic cycle averaged 224 seconds and the aperiodic cycle ranged from 2 to more than 300 seconds. One in three patients with strabismus and nystagmus periodicity had a static head posture.

CONCLUSION

Fifteen percent of the infantile nystagmus syndrome population had either the periodic or aperiodic form. A changing null period is often clinically missed because of long or irregular cycles, decreased acuity, associated strabismus, and either a nonexistent or inconsistent head posture. The changing null period is easier to recognize using eye movement recordings or if the non-preferred eye is occluded and the preferred eye is examined with the head straight and gaze in primary position for at least 5 to 7 minutes. The recognition of this variant has profound treatment implications.