Electrophysiological observations made in the hands of a group of 16 rock-drillers were compared with 15 controls. Motor and sensory conduction velocities in the median and ulnar nerves together with the latency, duration, and the amplitude of the evoked action potentials were measured. The differences between the groupswere statistically significant mainly in latency, duration, and amplitude, especially of the sensory action potentials. Measurement of the conduction velocities, in general, proved to be less sensitive, and the only significant change observed was in the sensory conduction velocity in the median nerve when the first digit in the right hand was stimulated. The most interesting result was evidence of an increased prevalence of possible carpal tunnel syndrome in the exposed (44 % compared with 7 % in the control group). A similar set of data, but exclusively sensory and not standardised for age and sex, was obtained from 25 university students for comparison with the assigned groups. The results showed that apart from sensory duration the control group had values that were closest to the students while the vibration group had values furthest away.

SUMMARY Characteristics of the surface recorded F-response, including frequency, occurrence of identical responses, shape and amplitude have been investigated in the ulnar nerve of control subjects. During a train of 200 stimuli, F-response frequency varied between 50% and 93% in different subjects with a mean of 79%. The vast majority of responses (96.6%) occurred only once; of those responses which repeated, 89-5% occurred between 2-5 times, 9% between 6-10 times and only 1-5% 11 or more times. F-response shape was variable, the majority containing two or more negative peaks. F-response amplitudes tended to be a relatively small proportion of the compound M-response, with median F%M values ranging from 0-8% to 4%. The data suggest that a large proportion of surface recorded F-responses following supramaximal stimulation are composed of recurrent discharges derived from more than one motor unit.The basic neurophysiological mechanisms underlying the production of F-responses by antidromic invasion of the anterior horn cell body have been shown by the studies of Renshaw,' Eccles2 and Schiller and StAlberg3 to be highly complex. The generation of a recurrent discharge may be influenced by a variety of factors, the principal of which is the balance of excitatory and inhibitory post-synaptic potentials on individual anterior horn cells. These factors have a direct effect on the critical phase which is the time required for repolarisation of the axon hillock before reinvasion of this segment by the soma-dendritic spike.3In view of the uncertain conditions surrounding the production of the F-response, knowledge of its frequency in individual motor neurons and the motor neuron pool as a whole would be of value, particularly as these parameters may be altered by diseases which affect the upper and/or lower motor

SUMMARY During a train of 200 stimuli, F-response frequency, frequency of identical responses, and F-response shape were studied in the ulnar nerve of 17 patients with motor neuron disease (MND) and 16 patients with cervical spondylosis (CS). In MND patients, F-response frequency varied between 5% and 96% with a median of 39% which was significantly lower than controls (p < 0 001), and showed a significant rank correlation with the M-response amplitude (r = 0-62, p < 0.004). Identical responses occurred more frequently than in controls (median 308%, p < 0 001). F-response frequency was normal in CS patients (median 76-3%, range 35% to 97%), but the frequency of identical responses (median 6-3%) was higher than normal (p < 00 1). Reduced F-response frequency in MND was thought to reflect loss of lower motor neurons, while the presence of spacticity was probably the major factor underlying the increased frequency of identical responses in both disorders. F-response shape tended to be simpler in MND and rather more complex in CS patients than controls.Recent studies in normal human subjects have shown the production of F-responses in individual motor neurons to be an uncommon event,' 2 whereas in the motor neuron pool as a whole such recurrent responses occur relatively frequently.3 The production of these responses seems to depend mainly on the balance of excitatory and inhibitory activity at the appropriate spinal cord level.' In general, increased levels of excitability as seen in spasticity, lead to an increase in F-response production by responding motor neurons.' On the other hand frequency falls in the immediate postictal period following a cerebrosvascular accident during which muscle tone is decreased and the deep tendon reflexes are depressed.4It might also be expected that F-response frequency may be influenced by changes affecting the lower motor neuron, and for example, that a reduction in their number might lead to a decrease in F-response $Present address:

SUMMARY The case of a 40-year-old woman with increasing ataxia is described. Although the clinical presentation and evoked response studies raised the possibility of multiple sclerosis, further investigation revealed multiple cystic intracranial lesions. Surgical excision of one of the lesions relieved the patient's symptoms. Histological examination revealed that this was an enterogenous cyst. Although single cysts of this type have rarely been reported occurring in the posterior cranial fossa, the occurrence of multiple lesions, some in the supratentorial compartment, appears to be unique. Case reportA 40-year-old-lady presented with a four month history of progressive difficulty in walking. She had become progressively more unsteady on her feet and had also noticed increasing stiffness in her legs with a tendency to spontaneous jerking of her legs when resting. For two months prior to presentation she had been aware of weakness in her arms when carrying heavy objects. She had had some hesitancy of micturition but no urgency or incontinence. There were no ocular or sensory symptoms and apart from occasional episodes of migraine, there were no headaches suggesting raised intracranial pressure and no systemic symptoms. Prior to her presenting illness, in childhood, a small "cyst" had been excised from her scalp in the occipital region. Regrettably no further information was available regarding the nature of this lesion. There was a history of Huntington's disease in her maternal family but her mother had died aged 62 years without manifesting this condition. Our patient had two older and two younger siblings and three children, all of whom were perfectly well. Examination revealed no systemic abnormalities. Examination of the scalp revealed no evidence of a cutaneous sinus. There were no involuntary movements and higher cerebral function was intact. The visual fields, fundi and acuity were normal. Sustained first degree horizontal nystagmus was present on lateral gaze to both sides, the maximum amplitude was on gaze to the right. The rest of the cranial nerves were intact and Radiographs of the chest, skull, dorsal and lumbar spine were normal. Radiographs of the cervical spine revealed failure of fusion of the posterior arch of the atlas but no other abnormality. Pattern reversal visual evoked responses (VERs) were abnormal with symmetrical delay and dispersion of the major components. Somatosensory evoked responses from median nerve stimulation at the wrist were normal. Brainstem auditory evoked responses (BAERs), however, were abnormal, being significantly asymmetrical (right V-I 377ms, left V-I 4-6ms), this delay occurring between waves III and IV. An electroencephalogram (EEG) revealed a significant generalised excess of theta and delta activity, which was sharply contoured in the left temporal region and of highest amplitude in both frontal regions. Cranial computed tomographic (CT) scans (fig 1) revealed a large, low attenuation lesion (+ 2 to -139 Hounsefield units) in the region of the fourth ventricle ...

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