We studied the EMG potentials evoked in the bilateral first dorsal interosseus muscle by electromagnetic stimulation of the corticomotoneuronal descending system in 10 Parkinson's disease patients and in 10 age- and sex-matched normal controls. We selected patients who did not have tremor but had predominant rigidity with asymmetric body involvement. On the rigid side of the PD patients, the threshold to cortical stimulation was lower than on the contralateral side or than normal values. On average, patients had normal central conduction times, but their motor evoked potentials (MEPs) on the rigid side were larger than those of controls when the cortical stimulus was at rest or during slight tonic contraction of the target muscle. In the latter condition, a silent period shorter than that of controls followed MEPs, whereas the peripheral silent period following ulnar nerve stimulation at the wrist was prolonged. Alpha motor neuron excitability, tested by the F-wave method, was enhanced on the rigid side at rest. In rigidity, spinal motor nuclei may be more responsive than normal to descending inputs from motor cortex, or the entire corticomotoneuron system may prove hyperexcitable under given conditions.

No abstract

Cataract has been considered for a long time one of the major indicators of the presence of the mutated myotonic dystrophy (DM) gene in asymptomatic relatives of DM patients. However, some recent studies show that not all cases of cataract typical of DM are associated with the disease even in members of DM families. In order to determine the frequency of lens opacities characteristic of DM in the general population and to evaluate the specificity of lens anomalies for detection of the DM premutation, we screened a sample of 1,400 random individuals for the presence of’myotonic cataract’. Ten individuals were found with the typical lens opacities and no neuro-muscular signs of DM; molecular analysis of the DM mutation showed that they all carried two normal alleles. Our data allow to conclude that bilateral cortical iridescent and posterior cortical lens opacities cannot be considered a marker of the presence of the DM premutation in the general population.

A new case of regression of the CTG copy number in the myotonic dystrophy allele was observed in a 7 year old boy. His affected father had an expanded allele of about 100 repeats in his lymphocyte DNA while the child showed a 60 repeat allele, of the same size as that present in the grandfather. Analysis of the father's sperm DNA allowed us to detect an expanded fragment of approximately the same size (62 repeats) as that present in the child's and grandfather's lymphocytes. This fragment was not detectable in the father's lymphocytes. Thus the regression is constitutive in the child, being already present in his father's germline. It is therefore likely that the regressed allele is present in all the tissues of the child, allowing a favourable prognosis.