Reply

Geraghty and colleagues [3] recently reported a subgroup of patients with essential tremor (ET) who later developed Parkinson’s disease (PD). In agreement with certain previous studies El, 2 , 4 , 8 ] but not with others [5-71, they concluded that some patients with essential tremor are genetically at risk for Parkinson’s disease, the prevalence of parkinsonism in their population of patients with ET being 24 times greater than expected. Approaching the same question in a different manner, we recently obtained detailed family histories emphasizing the presence of tremor and features of parkinsonism in 159 patients with idiopathic PD. We compared the data from these patients with that for 104 ageand sex-matched controls who did not have P D and with that of 90 patients with various forms of idiopathic dystonia, a condition recognized to have an association with ET. Patients and controls were excluded if they were adopted or knew little about their families. Immediate family members were excluded if they had died early in life before an age at which tremor might have developed. For this same reason, the incidence of tremor in a parent and ssbling group was calculated separately from that for all immediate family members, as children may have been too young to develop tremor. This effect must be considered when comparing the dystonia group with the other two groups because these patients, and therefore their parents and siblings, were younger than those in the P D and control groups. Of the PD group, 17.0% had at least one immediate family member with tremor that was unassociated with other features of parkinsonism or another obvious cause. This figure was significantly (p < 0.01) greater than the 5.6% found in the control group. O f dystonics, 10% had a similar family history that was not markedly different from that of the other groups. The frequency of tremor in immediate family members (all immediate family memberdparents and siblings only) was also significantly higher in the PD group (2.7%/3.7%) compared with the controls (0.8%/l.l%) while the frequency in the patients with dystonia (l.? %/ 2.1%) did not differ from either of the other groups. Patients who were a younger age at the onset of PD (under 40) and those with tremor as the first symptom of their P D were more likely to have a positive family history of tremor, but these associations were not statistically significant. The frequency of a positive family history of PD and the prevalence of P D in first-degree relatives was the same in the three groups. In most cases a parent was the only family member affected by tremor. Although only 2 patients (1.3%) with P D had a history of long-standing ET before the development of parkinsonian features, 6 (including these two) (3.8%) had several relatives affected by tremor in a fashion suggestive of autosomal dominant transmission. Four (4.4%) of the dystonic patients but none of the controls had similar family histories. We did not find the striking association between ET and PD reported by Geraghty and colleagues {3] which would have predicted a higher incidence (up to five times that reported here) of ET predating the development of PD. However, our results d o confirm that some individuals with a family history of tremor may be genetically predisposed to the development of PD. The nature of the final precipitant(s), whether environmental or otherwise, remains to be determined. The difference in the rates of associated ET plus P D between our study, that of Geraghty’s group [3], and other previous reports may relate to the nature of the population studied, just as the prevalence of uncomplicated ET varies markedly in different locations. It would be of great interest to know the prevalence rate of ET in the Houston region in comparison with the other locations with both high [4 ,8] and low [6, 7) incidences for the association of ET with PD.