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In
epidemiologic studies examining natural history or risk factors
for TMD, the question arises as to whether cases with a co-morbid
pain condition be included or excluded from the study. Since epidemiologists
have generally studied conditions that are relatively rare compared
to pain conditions, they have often opted for exclusion of persons
with co-morbid conditions.
However, a
significant percentage of people with TMD pain have other chronic
pain conditions. Some of this co-morbidity may occur simply because
a number of pain conditions--notably headache and back pain--
are very prevalent and would be expected to occur at high rates
among people with TMD, just as in the general population.
Headache,
in particular, is frequently co-morbid with TMD pain. However,
rates of severe headache appear to be higher in persons with TMD
than in the population as a whole. Possibly headache and TMD share
common risk factors, or possibly they represent a spectrum of
related conditions (Benoliel
and Sharav, 1998).
In addition,
some individuals may be predisposed to experience pain in multiple
body sites. There is considerable evidence that persons with TMD
who have multiple additional pains are more psychologically distressed
(LeResche et al., 1987;
Dworkin
et al., 1990b), and at risk for poorer long-term outcomes
of their TMD pain (John
et al., 2003).
Thus, excluding
persons with other pain conditions from studies of TMD pain is
unlikely to be a satisfactory solution. It may, however, be necessary
to differentiate "pure" cases of TMD pain from those
with co-morbid pain conditions.
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