CMT Prevalence in Denmark May Be 16% Higher Than Estimates

The true prevalence of Charcot-Marie-Tooth disease (CMT) in Denmark may be higher than previously thought, according to a recent Danish study in which researchers applied diagnostic criteria for CMT to individuals with nerve damage and no clear underlying cause.

Based on their results, the scientists estimated an approximately 16% increase in Denmark’s true CMT rate.

The study, “A Search for Undiagnosed Charcot-Marie-Tooth Disease Among Patients Registered with Unspecified Polyneuropathy in the Danish National Patient Registry,” was published in the journal Clinical Epidemiology.

CMT is a heterogeneous disorder, meaning that it can arise from diverse causes. This makes accurate diagnoses complicated, as patients with mild or atypical symptoms can be misdiagnosed — or go undiagnosed for varying lengths of time.

Denmark’s established CMT rate is 22.5 cases per 100,000 people.

Given that CMT is the most common inherited disorder of the peripheral nervous system, and that prevalence estimates vary considerably, researchers at Aarhus University — the largest university in the country — began to suspect that this was likely a minimum estimate.

In pursuit of a potentially more accurate count, the investigators applied the diagnostic criteria for CMT to all patients in the Danish National Patients Registry (DNPR) diagnosed with unspecified polyneuropathy or UP. UP is damage to the nerves of the skin, muscles, and other organs, with no clear underlying cause.

Patients younger than age 40, with diagnoses from 1977 to 2012, were included in the review.

Although the researchers say that the validity of a CMT diagnosis in the DNPR is high, patients may not be registered with such a diagnosis if they were reclassified to CMT following a different, earlier diagnosis, further skewing the estimation of CMT prevalence.

A total of 31,422 people in the registry received a UP diagnosis during the study period, of whom 516 were also diagnosed with CMT and three with the related Refsum’s disease.

From among the remaining 30,903 cases, 940 cases involved patients with at least one primary UP diagnosis prior to age 40 and no diagnoses related to alcohol, diabetes, or a specific neuropathy.

By randomly sampling 20 patients for every five calendar years from this group, the researchers identified 95 cases, 79 of which had records complete enough to review. Of those, 17 (21.5%) fulfilled the CMT criteria.

Applying this rate to the 940 other possible CMT cases resulted in approximately 200 patients with potentially undiagnosed CMT in Denmark. This corresponds to a prevalence of 26.1 CMT cases per 100,000 people, a 16% increase.

“This supports the hypothesis,” the investigators concluded, “that many patients with CMT are undiagnosed. As we expected, the estimates of prevalence and incidence of CMT in Denmark, as reported in an earlier study, are likely to be underestimated.”