Adult-onset primary torsion dystonia (AOPTD) is autosomal dominant with reduced penetrance. Sensory abnormalities in unaffected relatives may indicate nonmanifesting gene carriage—an ideal endophenotype being present in all patients and 50% of first-degree relatives. The temporal discrimination threshold (TDT) is the shortest time interval at which two stimuli are detected to be asynchronous. We aimed to compare the utility of visual, tactile and mixed TDT tasks in AOPTD patients and first degree relatives. TDTs were examined in 42 AOPTD patients (29 with cervical dystonia, 13 with writer's cramp), 48 first degree relatives and 51 controls using visual (two LED lights), tactile (nonpainful electrical stimuli) and mixed (one LED, one electrical) stimuli. The mean TDT in controls <50 years was 24.45 ms and >50 years was 30.87 ms. The upper limit of normal was defined as control mean +2.5 SD. In AOPTD patients, the visual task detected abnormalities in 35/42 (83%), the tactile task in 35/42 (83%) and the mixed task in 30/42 (58%); the mixed task was statistically less sensitive than the other two (p=0.001). The abnormality rates in unaffected relatives using the visual, tactile and mixed tasks were 23/48 (48%), 20/48 (42%) and 22/48 (46%) respectively. Specificity was 100% for all 3 tasks. The tasks performed similarly in CD and WC patients. The visual and tactile tasks were found to be more sensitive than the mixed, which could be omitted in practical applications of the endophenotype.
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