knowledge. Patients and control subjects were asked specifically
about all published risk factors for dystonia. Data were analyzed
by percentages calculated from the total number of responses and
the distribution of frequency. Statistical significance was esti-
mated on paired, two-tailed t tests of 2 values (Statview, version
V.1, Abacus Concepts, Inc., Berkeley, CA).
Diagnosis of spasmodic dysphonia was first made by a phy-
sician and a speech pathologist based on criteria in widespread
use and described in the literature.14,15 Patients with segmental
dystonias such as Meige’s disease were excluded from the study
and are not treated with botulinum toxin injection at the Hershey
Medical Center (Hershey, PA). After a careful review and discus-
sion with neurologists at our institution, we have not identified
cases in which benefit was added through the use of magnetic
resonance imaging; therefore, routine imaging of the head and
neck in cases of spasmodic dysphonia was not performed in these
patients.
DISCUSSION
In the current study, a 79% female predominance in
spasmodic dysphonia, as well as an association with child-
hood infectious disease, writer’s cramp, and essential
tremor, was identified. The average age was 56.7 years,
and median age, 56 years. A strong connection with child-
hood viral illness was also noted with 65% of patients
having had the measles or mumps. With respect to neu-
rological findings, there was an association with upper-
extremity essential tremor and, to a lesser degree, writer’s
cramp. In each patient, the manifestations of spasmodic
dysphonia remained focal. Fifty-one percent of patients
associated the onset of symptoms with an acute upper
respiratory tract illness or a major life stress. However,
this finding was well within confidence intervals of the
expected frequency of such occurrences within the general
population. There was no identifiable familial association
with spasmodic dysphonia. Geographical history does not
appear to be relevant to the etiology of spasmodic
dysphonia.
There are a number of inherent biases in examining
the patient population of a given practice area, including
referral, age, and sex bias. By examining trends between
similar studies, some of the biases may be ruled out. Onset
of symptoms in the middle decades of life has been noted
in studies of patients with spasmodic dysphonia, as well
as those of other focal types of dystonia.17,18 Another pre-
viously cited epidemiological finding is female predomi-
nance. This was noted initially by Izdebski et al.,18 which
they did not believe established “spastic dysphonia as a
predominantly female disorder.” This issue was re-
examined by Adler et al.,19 who reviewed their own data,
as well as five other published reports, and concluded that
female predominance is not a result of ascertainment bias.
They were able to find only one report of a higher male
ratio in spasmodic dysphonia.19 In a review of more than
900 patients with primary laryngeal dystonia, Blitzer et
al.20 recorded a 63% female predominance. Therefore, the
finding of female predominance is most likely an accurate
cross-section of the population of patients with spasmodic
dysphonia and is not related to ascertainment bias.
The reason for a predilection toward female sex in a
disorder currently thought to be neurological in origin is
unclear. The predominance of autoimmune diseases in
female patients is thought by some authors to be a phe-
nomenon of microchimerism.21 This theory holds that
persistent fetal cells in the mother’s circulation after preg-
nancy may be responsible for the autoimmune phenome-
non as a result of a mechanism similar to graft-versus-
host disease. A precedent in neurological disorders exists
in multiple sclerosis, which has features of other autoim-
mune disorders.22 This theory does not explain disease
occurrence in nonparous female patients; yet, the paral-
lels are compelling and certainly warrant further investi-
gation into this area.
RESULTS
Thirty-four male and 134 female patients were in-
cluded in the study. Included in the control group were 89
male and 97 female relatives. Age range at onset of spas-
modic dysphonia was 13 to 71 years with a mean age of 45
years. Mean age at the time of treatment was 56 years.
Fifty-five percent were born and lived for an average of 19
years in a rural area with a population of less than
150,000 individuals, then moved to an industrial area
with a population of more than 150,000. No relatives or
individuals living in the same area as the patients with a
diagnosis of spasmodic dysphonia were identified. The
average patient had completed 2 years of college.
Sixty-five percent of patients had previously had the
measles or mumps. This is statistically significantly dif-
ferent from the national average incidence of 17% under
the age of 25 years during the period between 1958 and
1962 (P ϭ .0001). Thirty percent of patients directly asso-
ciated onset of spasmodic dysphonia to an upper respira-
tory tract infection, and 21% to a major life stress (confi-
dence interval [CI], 15%–42%).
Eleven percent of patients had associated writer’s
cramp compared with 2% of individuals in the control
group (P ϭ .02). Associated features of other movement
disorders were observed. Twenty-six percent of patients
had an essential tremor of the upper extremities com-
pared with 4% of first-degree relatives (P ϭ .0001). De-
spite these findings, there was no significant incidence of
any other major medical or neurological condition or
symptomatology. All of the patients exhibited stable focal
disease without segmental spread over a mean interval of
11 years. The incidence of peripheral vascular concurrent
hypertension at 25% was similar to a 27% rate in the
father and a 43% rate in the mother. One patient de-
scribed a history of toxic exposure, and one an electrocu-
tion injury. There were no other cases of associated injury.
The Voice Handicap Index and Short-Form 36 sur-
veys were administered to this same group of patients
before and after treatment with botulinum toxin. Im-
provements in the patients’ perception of their functional,
physical, and emotional voice handicap were statistically
significant. In addition, treatment improved their social
functioning and their perception of their mental health.16
Examination of geographical data may give informa-
tion about possible environmental exposure during child-
hood that could potentially cause the disorder. Geograph-
ical data obtained indicates a rural prevalence during the
developmental years and an industrial or large-city envi-
ronment in adulthood. Most likely, this phenomenon is
Laryngoscope 112: February 2002
Schweinfurth et al.: Spasmodic Dysphonia
221