A STUDY OF HEREDITARY ESSENTIAL TREMOR

Twenty index patients with hereditary essential tremor and their kindr eds were studied to define the phenotype of this condition. Ninety-thr ee first degree and 38 more distant relatives were examined; 53 defini te and 18 possible secondary cases were identified. The age of tremor onset was bimodally distributed with a median at similar to 15 years. Segregation analysis indicated autosomal dominant inheritance and pene trance was virtually complete by the age of 65 years. There were no ex amples of the disease skipping a generation. Men and women were affect ed in equal proportions. About 50% of cases were alcohol responsive. I n the majority of families alcohol responsiveness was either consisten tly present or did not occur but in 20% of kindreds definite heterogen eity of responsiveness was encountered within each family. The typical phenotype was a mild symmetrical postural tremor of the upper limbs. Tremor of the legs, head facial muscles, voice, jaw and tongue occurre d but never in isolation and rest, task specific (e.g. primary writing tremor) and primary orthostatic tremors were not found. Head tremor w as invariably mild and 75% was of a 'no-no' type. Dystonia (e.g. torti collis and writer's cramp) were not encountered, a finding which stron gly suggests that many previous studies of 'essential tremor' were con taminated by cases of idiopathic or hereditary torsion dystonia. No as sociation with Parkinson's disease was found but classical migraine oc curred in similar to 26% of cases and co-segregated with tremor. The s everity of arm tremor (assessed using a clinical rating scale and by s coring tremor in Archimedes spirals) and disability increased with adv ancing age and increasing tremor duration, but there was no correlatio n between age at tremor onset and either tremor severity or disability . Men and women were affected with equal severity. The sex of the affe cted parent had no influence on the severity of tremor or the degree o f disability experienced by an affected child. Disability commenced in the second decade and progressively increased. All the index patients and 59% of the definite secondary cases had tremor induced disabiliti es. Eighty-five percent of index patients and 38% of secondary cases a lso reported some degree of social handicap. Twenty-five percent of in dex patients and 12% of secondary cases had been compelled to change j obs of retire. Biological fitness was normal.