The use of video in assessing and illustrating abnormal eye movements in young children

A model of neuronopathic Gaucher disease

Gaucher disease (GD) is a lysosomal disorder involving the accumulation of glucocerebroside in the liver, spleen, bones and brain. Some patients exhibit only systemic disease (type I), but others have additional neurological signs which may lead to rapid neurodegeneration in infancy (type II) or take a more intermediate course (type III). Types II and III are collectively known as neuronopathic Gaucher disease (NGD). Systemic disease can now be treated by enzyme replacement therapy (ERT), but its efficacy in NGD is limited. Two infants who presented with bulbar palsy and failure to thrive were enzymatically diagnosed at 8 months with NGD. They were started on high-dose ERT (120 IU/kg every 2 weeks). Both underwent serial oculomotor assessment and an audiological battery, including visual reinforcement audiometry, otoacoustic emissions, and the auditory brain stem response (ABR). Biochemical markers showed an incomplete systemic response to ERT, but neurological deterioration was relentless, leading to death at 16 and 25 months. Oculomotor testing revealed a complete absence of saccadic eye movements and progressive bilateral sixth nerve palsy in one. Audiological assessment revealed progressive deterioration of ABRs, but with normal peripheral hearing and otoacoustic emissions. Both infants showed neurological deterioration in spite of high-dose ERT. The audiological findings suggested a loss of inner hair cell pathway function with preserved outer hair function, similar to what is seen in auditory neuropathy. The unusual pattern of audiological and oculomotor abnormalities is consistent with an excitotoxic mechanism predisposing nerve cells to glucocerebroside toxicity. Such excitotoxic damage may be amenable to direct therapeutic intervention.

Spontaneous reversal of nystagmus in the dark

AIM

To report five children with horizontal jerk nystagmus in whom eye movement recordings in the dark revealed a spontaneous reversal in the direction of the nystagmus beat. Three patients were blind in one eye and were diagnosed as having a manifest latent nystagmus (MLN), and two patients had strabismus and congenital nystagmus (CN).

METHODS

Eye movements were recorded using DC electro-oculography with simultaneous video recording, including infrared recording in total darkness.

RESULTS

Four patients had decelerating velocity slow phase jerk nystagmus when recorded under natural lighting conditions; the fifth case had accelerating velocity and linear slow phase jerk nystagmus. Under absolute darkness, nystagmus reversed in direction of beat with a mixture of linear and decelerating velocity slow phase waveforms. One child with unilateral anophthalmos could wilfully reverse the beat direction of his nystagmus by trying to look with his blind eye in the light and dark.

CONCLUSIONS

These observations support the theory that LN/MLN beat direction is determined by the "presumed" viewing eye and may be consciously controlled. The spontaneous reversal of beat direction in the dark suggests eye dominance is predetermined. Eye movement recordings identified mixed nystagmus waveforms indicating that CN (accelerating velocity slow phases) and LN/MLN (linear/decelerating velocity slow phases) coexist in these subjects.

Vertical or asymmetric nystagmus need not imply neurological disease

AIM

To indicate that congenital idiopathic nystagmus (CIN) and sensory defect nystagmus (SDN) can be vertical or asymmetric in some children.

METHODS

Of 276 children presenting with nystagmus for electrophysiological testing, 14 were identified as having CIN or SDN, yet had a nystagmus which was either vertical (n=11) or horizontal asymmetric (n=3). Flash electroretinograms and flash and pattern visual evoked potentials (VEPs) were recorded in all patients. Eye movement assessment, including horizontal optokinetic nystagmus (OKN) testing, was carried out in 11/14 patients.

RESULTS

Eight patients (seven with vertical, one with asymmetric horizontal nystagmus) had congenital cone dysfunction. One patient with vertical and another with asymmetric nystagmus had cone-rod dystrophy. One patient with vertical upbeat had congenital stationary night blindness. Two patients (one downbeat, one upbeat nystagmus) had normal electrophysiological, clinical, and brain magnetic resonance imaging findings and were classified as having CIN. One patient with asymmetric nystagmus showed electrophysiological and clinical findings associated with albinism. Horizontal OKN was present in 80% of patients tested, including the three cases with horizontal asymmetric nystagmus. This is atypical in both CIN and SDN, where the OKN is usually absent.

CONCLUSIONS

Vertical and asymmetric nystagmus are most commonly associated with serious intracranial pathology and its presence is an indication for neuroimaging studies. However, such nystagmus can occur in children with retinal disease, albinism, and in cases with CIN. These findings stress the importance of non-invasive VEP/ERG testing in all cases of typical and also atypical nystagmus.

Saccadic strategies in children with hemianopia

Multiple hypometric (undershooting) saccades are generally reported as a compensatory strategy in adults with homonymous hemianopia. However, hypermetric (overshooting) saccades have been reported to develop spontaneously as a beneficial strategy in response to predictable targets. We examined the saccades of 10 children (aged 5 to 16 years) with homonymous hemianopia to determine the type of compensatory eye-movement strategies employed 6 months to 16 years after hemianopia onset. Homonymous hemianopia was identified using perimetry and/or pattern visual evoked potentials and supported with results of neuroimaging. Eye movements were recorded using bitemporal electrooculography. Saccades were elicited to a red light source in a semipredictable paradigm. We found that hypermetria was not a consistent compensatory strategy in our patients. In spite of the predictability of our paradigm and the long follow-up period, multiple hypometric saccades into the blind field appeared to be the preferred strategy.

Intermittent horizontal saccade failure ('ocular motor apraxia') in children

BACKGROUND

Ocular motor apraxia (OMA) in childhood is a poorly understood condition involving a failure of horizontal saccades. OMA is thought to be rare but the literature indicates wide clinical associations. OMA is often identified by abnormal head movements, but failure of reflexive quick phases has been reported in all but a few patients. The extent of this oculomotor disorder was examined in a large group of children with diverse clinical backgrounds.

METHODS

The degree of quick phase failure during horizontal vestibular and optokinetic nystagmus was measured using DC electro-oculography and video in 74 affected children, aged 17 days to 14 years.

RESULTS

All children showed an intermittent failure of nystagmic quick phases, except for total failure in one case. Other visuomotor abnormalities were common including saccadic hypometria (85%), low gain smooth pursuit (70%), neurological nystagmus (28%), strabismus (22%), and vertical abnormalities (11%). Non-ocular abnormalities were common including infantile hypotonia (61%), motor delay (77%), and speech delay (87%). There was a wide range of clinical associations including agenesis of the corpus callosum, Joubert syndrome, Dandy-Walker malformation, microcephaly, hydrocephalus, vermis hypoplasia, porencephalic cyst, megalocephaly, Krabbe leucodystrophy, Pelizaeus Merzbacher disease, infantile Gaucher disease, GM1 gangliosidosis, infantile Refsum's disease, propionic acidaemia, ataxia telangiectasia, Bardet-Biedl syndrome, vermis astrocytoma, vermis cyst, carotid fibromuscular hypoplasia, Cornelia de Lange syndrome, and microphthalmos. Perinatal and postnatal problems were found in 15% including perinatal hypoxia, meningitis, periventricular leucomalacia, athetoid cerebral palsy, perinatal septicaemia and anaemia, herpes encephalitis, and epilepsy. Only 27% were idiopathic.

CONCLUSION

Quick phase failure is a constant feature of OMA, whereas abnormal head movements were detected in only about half, depending on the underlying diagnosis. This oculomotor sign is better described as an intermittent saccade failure rather than as a true apraxia. It indicates central nervous system involvement, has wide clinical associations, but it is not a diagnosis.

Delayed visual maturation in infants: a disorder of figure-ground separation?

Delayed visual maturation (DVM) is characterised by visual unresponsiveness in early infancy, which subsequently improves spontaneously to normal levels. We studied the optokinetic response and recorded pattern reversal VEPs in six infants with DVM (aged 2-4 months) when they were at the stage of complete visual unresponsiveness. Although no saccades or visual tracking with the eyes or head could be elicited to visual objects, a normal full-field rapid buildup OKN response occurred when viewing biocularly or during monocular stimulation in the temporo-nasal direction of the viewing eye. Almost no monocular OKN could be elicited in the naso-temporal direction, which was significantly poorer than normal age-matched infants. No OKN quick phases were missed, and there were no other signs of "ocular motor apraxia." VEPs were normal in amplitude and latency for age. It appears, therefore, that infants with DVM are delayed in orienting to local regions of the visual field, but can respond to full-field motion. The presence of normal OKN quick-phases and slow-phases suggests normal brain stem function, and the presence of normal pattern VEPs suggests a normal retino-geniculo-striate pathway. These oculomotor and electrophysiological findings suggest delayed development of extra-striate cortical structures, possibly involving either an abnormality in figure-ground segregation or in attentional pathways.