A 10-year-old boy with a red ring on his shoulder

A 10-year-old male presents to your clinic with a 3-day history of an asymptomatic expanding erythematous patch on his left shoulder. The patient states the rash began as a small red bump and has slowly expanded to its current size. He denies similar lesions elsewhere. His mother reports that he has had intermittent low-grade fevers and generally complains of feeling unwell.

On skin examination, you note a 6-cm target-like erythematous patch on his left shoulder. His exam is also notable for shotty, axillary lymphadenopathy. Upon further questioning, the patient reports that he went camping with his father about 10 days ago but does not recall any insect bites.

What is the most likely etiology?

Diagnosis: Erythema migrans (the cutaneous marker of Lyme disease)

Erythema migrans (EM), also known as erythema chronicum migrans, is the cutaneous manifestation associated with Lyme disease. Lyme disease is caused by Borrelia burgdorferi and is transmitted most commonly by the Ixodes tick. The white-tailed deer is the most common carrier of the tick. This tick is smaller than the more common wood tick and often goes unnoticed.

There are many systemic findings that can be associated with Lyme disease; however, the finding of EM is characteristic. The most common presentation is an annular or circular erythematous patch that expands outward from the site of the tick bite. The eruption can occur anywhere from 1 to 3 weeks after the initial tick bite.

Lyme disease is most common in the eastern United States, as well as Europe, during the summer months when patients are more likely to be exposed to ticks. There are three stages of disease based on the signs and symptoms. The first stage involves a single lesion of EM, with or without other systemic symptoms, such as fever, malaise, arthralgias and myalgias. Multiple lesions of EM constitute the second stage. Neurologic, cardiac or joint manifestations may also manifest during the second stage. The third stage of disease involves severe erosive arthritis, persistent neurologic symptoms, or acrodermatitis chronica atrophicans (ACA). ACA is a skin manifestation seen in late-persistent disease, mainly in Europe, and is defined by multiple enlarging edematous bluish-red plaques commonly found on the lower extremities that progress to atrophic, hyperpigmented plaques.

Classic EM has been reported in 90% of children with Lyme disease and begins as a red papule at the location of the actual tick bite. Erythema then begins to slowly expand centrifugally to form a ring or circle. Occasionally, parts of the ring may be vesicular. EM may measure up to 30 cm in size. The initial red papule may remain at the center of the lesion and may contain a central punctum, providing a clue to the diagnosis. Patients may develop several secondary, usually smaller, EM lesions away from the site of the tick bite that can be more transient in nature. EM is usually asymptomatic, lasts several weeks and resolves spontaneously despite treatment.

Differential diagnosis

The differential diagnosis includes other annular erythematous conditions such as erythema annulare centrifugum, tinea corporis, erythema marginatum, erythema infectiosum, urticaria, neonatal lupus erythematosus, subacute cutaneous lupus erythematosus and erythema multiforme. The initial red papule may be confused with insect bite hypersensitivity.

EM is a clinical diagnosis and can often be diagnosed with history and physical exam alone. Many patients do not recall a tick bite but often report a recent history of exposure to wooded or grassy areas. However, if there is a known tick exposure, the tick needs to be attached for 48 to 72 hours to transmit Borrelia effectively. Definitive diagnosis involves an initial screening test with an enzyme-linked immunosorbent assay (ELISA) or indirect fluorescent antibody (IFA) test to identify immunoglobulin M and IgG to B. burgdorferi. If those tests are positive or equivocal, a Western blot should be done to confirm the diagnosis. Polymerase chain reaction is also available.

Treatment for EM and Lyme disease depends on the stage of disease and associated findings. Doxycycline for 14 days is the recommended treatment for stage I disease. Longer courses (up to 28 days) are recommended in patients with arthritis or heart block. Parenteral therapy with IV ceftriaxone is recommended for patients with Lyme meningitis or disseminated disease. In children aged younger than 8 years, alternatives to doxycycline such as amoxicillin or cefuroxime can be used.

Prevention is best

Prevention with protective clothing and tick repellent are paramount to avoiding Lyme disease. Patients who cannot avoid wooded or grassy areas should wear hats, tuck their pants into their socks and perform self-examinations after spending time outdoors.

Physicians should be able to recognize EM, as well as the other associated systemic findings of Lyme disease, to assure early diagnosis and provide prompt treatment to prevent the late, more serious sequelae of the disease.

For more information:

• Bhate C. J Am Acad Dermatol. 2011;64:619-636.
• Bhate C. J Am Acad Dermatol. 2011;64:639-653.
• Paller AS, Mancini AJ. Hurwitz Clinical Pediatric Dermatology, 3rd Edition. Philadelphia, Pa.: Elsevier; 2006:388-390.

Marissa J. Perman, MD, is a pediatric dermatology fellow at The Children’s Hospital of Philadelphia. Disclosure: Dr. Perman reports no relevant financial disclosures.