Abstract

A 28 year old right handed man presented with a ten year history of difficulty in writing. He had been prescribed metoclopramide for an episode of vomiting one week prior to this and had been admitted for an acute dystonic reaction with oculogyric crisis. He had difficulty gripping the pen, his writing was untidy and he pressed excessively on the paper. Over the next year, involuntary movements developed, including posturing of the right arm and right foot particularly on action, and fine tasks, such as doing up buttons, were impaired. Examination showed dystonic posturing of the right foot on running, writer's cramp and finger/nose ataxia on the right. Magnetic resonance imaging revealed a large arteriovenous malformation (AVM) in the right cerebellar hemisphere extending into the pons. Formal arteriography confirmed a large high flow AVM with a large vein draining in the torcular with a preceding stenosis.

Dystonia is a hyperkinetic movement disorder characterised by involuntary sustained muscle spasms and posturing. Secondary dystonia has many causes including structural lesions of the basal ganglia, particularly the putamen. This case illustrates right sided dystonia and upper limb ataxia ipsilateral to a large cerebellar AVM and adds to the emerging literature for the role of the cerebellum in dystonia. Stimulation of deep cerebellar regions in humans can result in tonic posturing of the neck and limbs. Brainstem lesions causing dystonia have been reported and have implicated cerebellar pathways in the pathophysiology. Diffusion tensor imaging has indicated that in DYT1 and DYT6 mutations there are dorsal pontine microstructural abnormalities in the superior cerebellar peduncle, implicating cerebello–thalamo–cortical pathways in dystonia pathogenesis. A study of patients with primary focal dystonia indicated delayed eyeblink conditioning compared to controls which is an experimental paradigm dependent on the cerebellum. This provides indirect neurophysiological evidence of olivo–cerebellar pathway changes in these patients. However, in primary dystonia there is absence of cerebellar clinical examination signs which may indicate a compensatory role of the cerebellum. It is unclear whether all manifestations of dystonia involve both basal ganglia and cerebellar circuitry, or whether there is a spectrum ranging from pure cerebellar dystonia, overlapping basal ganglia and cerebellar dystonia, and pure basal ganglia dystonia. There is evidence in from many experiments to support a role for the cerebellum in the pathogenesis of dystonia. Our case adds to the literature. The key area that needs clarification is whether the cerebellar outflow abnormality causing dystonia is stimulatory or inhibitory role.

Figure 1: T2 weighted Magnetic Resonance Imaging showing a large arteriovenous malformation in the right cerebellar hemisphere extending into the pons.