Erythema chronicum migrans

Erythema chronicum migrans refers to the rash often (though not always) seen in the early stage of Lyme disease. It can appear anywhere from one day to one month after a tick bite. This rash does not represent an allergic reaction to the bite, but rather an actual skin infection with the Lyme bacteria, Borrelia burgodferi sensu lato. It is a pathognomonic sign: a physician-identified rash warrants an instant diagnosis of Lyme disease and immediate treatment without further testing, even by the strict criteria of the Centers for Disease Control. There is no other disease that can cause this type of rash.

It is also sometimes called erythema migrans (without the "chronicum") or "EM". However, this phrase is also used to describe geographic tongue.

History
In a 1909 meeting of the Swedish Society of Dermatology, Arvid Afzelius first presented research about an expanding, ring like lesion he had observed. Afzelius published his work 12 years later and speculated that the rash came from the bite of an Ixodes tick, meningitic symptoms and signs in a number of cases and that both sexes were affected. This rash is now known as erythema migrans, the skin rash found in early stage Lyme disease.

In the 1920s, French physicians Garin and Bujadoux described a patient with meningoencephalitis, painful sensory radiculitis, and erythema migrans following a tick bite, and they postulated the symptoms were due to a spirochetal infection. In the 1940s, German neurologist Alfred Bannwarth described several cases of chronic lymphocytic meningitis and polyradiculoneuritis, some of which were accompanied by erythematous skin lesions.

Description
The true incidence of the rash, erythema migrans, is disputed, with estimates ranging from less than 50% to over 80% of those infected. A systematic review by the Rational Clinical Examination of studies estimates that 80% of patients may have an expanding rash, erythema migrans (EM), at the site of the tick bite. Most patients with EM do not recall a deer tick bite.

The Rational Clinical Examination review found that the characteristic "bull's-eye" rash with central clearing is present in about 20% of endemic cases in the United States; whereas in Europe and the non-endemic United States 80% of rashes have central clearing. In endemic areas of the United States homogeneously red rashes are more frequent. The rash is classically 5 to 6.8 cm in diameter appearing as an annular homogenous erythema (59%), central erythema (30%), central clearing (9%), or central purpura (2%).

Sometimes, erythema migrans may be less than 5 cm in diameter. Multiple painless EM rashes may occur, indicating disseminated infection.

Pathological Findings
Image courtesy of Professor Peter Anderson DVM PhD and published with permission © PEIR, University of Alabama at Birmingham, Department of Pathology

External Links (Images)

 * Lyme Disease Association Rash Photo Album
 * Canadian Lyme Foundation - Erythema migrans (variety of rash presentations)
 * EUCALB Images: Medical Images
 * Lyme Disease Foundation Rash Picture Gallery
 * Lyme Disease Foundation Rash Picture Gallery

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