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There is currently a general consensus of agreement that dystonia is a disease of the basal ganglia, although dystonic symptoms have been observed in association with lesions in various different sites of the sensory and motor pathways.1 In particular, cervical intramedullary lesions have been reported as being a rare cause of focal hand dystonia,2 although in these cases the pathogenesis of the movement disorder remains unclear. To help clarify this point, we report the case of a patient who developed dystonic features of the right hand after a cervical whiplash injury.
A 44 year old man developed sensory alterations and impairment of strength in the right hand immediately after a whiplash injury. Neurological examination showed proprioceptive and tactile anaesthesia of the first three fingers of the right hand, mild hypasthenia on grasping, and adiadochokinesis of the right upper limb. Tendon reflexes, muscle tone, and plantar responses were normal, and thorough neurological examination of the upper left limb and lower limbs also yielded normal findings. Cervical magnetic resonance imaging (MRI) revealed a small right posterior C5-C6 lesion of the spinal cord (see fig 1). Brain MRI, nerve conduction studies, EMG, and transcranial magnetic stimulation were all normal. Two months later, the patient developed writhing movements of the first three fingers and a dystonic posture of the right hand, worsened by movement, and more evident when the eyes were closed. Ability to write, use a knife or fork, and hold a glass were moderately impaired, especially without visual guidance. Neurological examination at this time revealed slight cutaneous and proprioceptive hypaesthesia and paraesthesias of the first three fingers of the right hand, while grasping strength was normal and the EMG recording showed a pattern of co-contraction of the forearm flexor and extensor muscles. Median nerve sensory evoked potentials (SEPs) revealed normal peripheral and central conduction times, but spinal and cortical waves were larger in response to stimulation of the affected side. In particular, the cervical potential showed a right:left ratio of 1.2, while the right:left ratio for the cortical waves ranged from 1.8 to 2.4. The patient had no history of neuroleptic intake.
This is the first report of hand athetosis-dystonia after a cervical whiplash injury to the spinal cord. We are reasonably confident that the lesion was secondary to the trauma, rather than representing inflammation, because the symptoms appeared immediately after the whiplash injury and in view of the fact that MRI was performed within a couple of days and no signs of spinal cord oedema were detected. To date, only a few reports have described dystonic hand disorders in intramedullary cervical lesions, the vast majority being cases of syringomyelia, and occasionally spinal tumours, demyelinating lesions, and post-irradiation myelopathy.2 The patient’s involuntary movements may be interpreted as dystonia rather than pseudoathetosis, as they were also present when the eyes were open, and the simultaneous recording of agonist and antagonist forearm muscles showed synchronised motor unit activation, whereas the pathogenetic mechanism of pseudoathetosis is a loss of postural tone secondary to the proprioception deficit. Proprioception was only slightly impaired at the time of onset of the movement disorder and the SEP data suggested the presence of a larger sensory input to the patient’s central nervous system rather than deafferentation.
Various hypotheses have been adduced to explain hand dystonia secondary to cervical lesions. A dysfunction of spinal interneuronal circuits, resulting in a lack of reciprocal, recurrent spinal inhibition may be important in the pathogenesis of the symptoms, especially in centromedullary lesions such as syringomyelia. Supraspinal mechanisms, initiated by erroneous sensory inputs, might also lead to dystonia.
We believe that involvement of the somatosensory pathway was the main determinant of the hand dystonia in our patient. This hypothesis is based on a number of findings. Firstly, MRI showed a cervical lesion involving the right posterior column and sparing the centre of the spinal cord. Secondly, the patient developed paraesthesias of the fingers, possibly attributable to a focus of ephaptic activity in the posterior cord, which is a mechanism known to cause positive symptoms in white matter lesions.3 The most intriguing finding, however, is that the SEP cortical waves were markedly larger in response to stimulation of the median nerve on the dystonic side, and the spinal potential was slightly larger on the affected side. The asymmetry of SEP amplitudes was not attributable to different intensities of stimulation on the two sides, as the intensity of the peripheral shock was the same, as was the amplitude of the afferent volleys recorded at the elbow and at the Erb’s point. A larger N30 potential has previously been described in dystonic patients, but this finding has not been replicated in other studies.4 It may seem strange to find larger SEPs associated with a sensory deficit, but the presence of ephaptic spread in the lemniscal pathway may account for the larger afferent input to the cortex, as well as for the paraesthesias. We may postulate that this larger sensory input may have triggered an abnormal motor command, resulting in a movement disorder with features favouring dystonia over those seen with sensory deafferentation (that is, pseudoathetosis). This report may confirm the central role of abnormal sensory processing in the pathogenesis of dystonic symptoms.5
Competing interests: none declared.
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