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Video Report Ocular motor apraxia Thein N Win, David E Laws (1),(2)Department of Ophthalmology Singleton Hospital, Swansea, UK
Correspondence: Dr Thein N Win Date of acceptance: 1st July 2006 |
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Ocular motor apraxia is an uncommon ocular motility finding, which may be present in a normal child or be symptomatic of more widespread neurological dysfunction. A video case is presented which demonstrates the typical, yet subtle, signs in a young patient. The condition is characterized by defective initiation of horizontal saccades and subsequent head thrusting, which is an adaptive manoeuvre. By rotating the head sharply, the patient utilizes the vestibulo-ocular reflex to forcibly deviate the eyes in the contralateral direction, enabling the patient to foveate on an eccentrically placed target. |
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[View Video: Fast connection] Note: This video is best viewed in Quicktime Case Report A 15 month old boy was referred by his general practitioner for a ‘squint'. His mother had concerns that the child would appear
not to see things intermittently and at 3 months, had noticed ‘wobbly' head movements. The patient was born at full term,
via a normal vertex delivery, weighing 7lb and 8oz. He was investigated at 6 weeks for failure to thrive, associated with
a urinary tract infection. This was successfully treated with antibiotics and weight began to improve by 12 weeks. Ultrasound
of the urinary tract, micturating cystogram and dimercaptosuccinic acid (DMSA) scan were normal. There were no other concerns
regarding developmental milestones, hearing or speech. Family history revealed squint in the paternal grandfather and myopia
in the maternal grandmother only. On examination, the patient's weight was on the 75th centile. There were no dysmorphic features. Tone, gait and reflexes were normal. Chest, heart and abdominal examination were unremarkable. Vision was 0.3 logMAR with both eyes open (Cardiff Acuity Tests), with a low hyperopic correction within normal limits for age. The patient demonstrated convergence to approximately 8cm. Frisby testing was not possible. Examination of the anterior segments and fundus of both eyes was unremarkable. Pupillary reactions were also normal. Examination of ocular movements showed intact elevation and depression. However, lateral versions were absent. Doll's head reflex was normal. Similarly, the patient was able to initiate saccades vertically, but not horizontally. Vertical refixation movements were observed when tested with the optokinetic drum. None were seen horizontally. Most characteristically was the horizontal ballistic head movements, observed when attempting to initiate horizontal saccades. The patient was diagnosed with ocular motor apraxia (OMA) based on these findings and was referred to a paediatric neurologist for further investigations to elucidate a possible cause (Clip 1) (Clip 2). Discussion OMA was first described in four children by Cogan in 1952[1]. It may be idiopathic, where there is absence of any other clinical entity, or associated with other neurological conditions. The inheritance of congenital forms remains debatable, typically sporadic, but also with suggestions of both dominant and recessive patterns [2,3]. The exact site of the causative lesion is speculative although abnormalities in the cerebellar vermis and agenesis of the corpus callosum have been reported to be the more common radiological defects[4]. However, neuroimaging may also be entirely normal[3,4]. The condition is characterized by defective initiation of horizontal saccades and subsequent head thrusting, which is an adaptive manoeuvre. However, the extent of head thrusting is variable and may not be present in all cases. Intermittent failure of the quick phase during optokinetic nystagmus is a more reliable sign[5]. By rotating the head sharply, the patient utilizes the vestibulo-ocular reflex to forcibly deviate the eyes in the contralateral direction, enabling the patient to foveate on an eccentrically placed target. The head is then able to return to a more comfortable central position, the target remaining fixated, with the aid of intact smooth pursuit eye movements. Onset of OMA is variable and will depend on any underlying condition. A normal child will begin to follow a target with its eyes from 6 weeks. Full head control should develop by 20 weeks and therefore the condition may not be manifest before then. However, onset may be later in children with other developmental abnormalities. With time, the head thrusting may diminish, although this may simply represent improved adaptive techniques as opposed to recovery of saccade initiation. Synkinetic blinking may be the only sign of OMA in the older patient, which acts to avoid target fixation during head rotation. OMA may also be associated with developmental delay, a wide-based gait, clumsiness, hypotonia and mild mental retardation5. Reading problems can lead to educational difficulties later in life. The range of conditions that have been associated with OMA is wide and ever increasing. They include neurodegenerative conditions, such as Gaucher's disease type 2 and 3, Refsum's disease, GM1 gangliosidosis, ataxia telangiectasia, Huntingdon's disease and Wilson's disease. Congenital malformations involving the aforementioned cerebellar vermis include Joubert and Dandy-Walker syndromes. Perinatal insult such as hypoxia and hydrocephalus, as well as cerebral palsy should be inquired about. Acquired causes include herpes encephalitis, bilateral fronto-parietal lesions and posterior fossa tumours. It is therefore prudent that OMA be viewed as a symptom, rather than a diagnosis, until any underlying condition is excluded. VEP/ERG recordings may help to distinguish between idiopathic cases and those associated with more widespread neurological abnormalities, where there is a higher proportion of abnormal readings[6]. It is also important to distinguish OMA from gaze palsy, hemianopia, slow saccades and poor eccentric gaze holding, although doing so may be particularly challenging in the younger patient. Conflict of Interest Statement The authors do not have any proprietary or commercial interests.
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