An 11-Year-Old Boy’s Ruptured Popliteal Cyst: What’s the Cause?

An 11-year-old boy presented to his pediatrician in an urban setting with left leg erythema, limping, lethargy, and fever. The day before, the boy had begun a regimen of cephalexin for presumed cellulitis after his parents texted the photo shown here to the pediatrician. The photo showed the boy’s red calf and popliteal area following a case of poison ivy contact dermatitis on his left leg.

Cephalexin was continued for the presumed cellulitis, and the redness improved, although the boy still found it painful to walk. The next day, he was lethargic, was still limping, and had a temperature of 38.5°C, and so he was admitted to the hospital for intravenous clindamycin to treat the presumed cellulitis. The boy’s knee joint appeared benign on physical examination.

Results of an inpatient complete blood count showed a normal white blood cell (WBC) count and an elevated erythrocyte sedimentation rate of 38 mm/h. One day into the hospital admission, his knee pain and swelling increased and an effusion developed. The redness of the calf had nearly resolved, but the popliteal area still was markedly red. Arthrocentesis of the knee was performed, and results of synovial fluid analysis showed a minimal WBC count of 3,056 cells. Samples were sent for Gram stain and culture tests and a Lyme titer.

Ultrasonography revealed a ruptured popliteal cyst in the boy’s left knee with fluid tracking down to the calf.

What do you suspect is the underlying cause of the boy’s popliteal cyst rupture?

A. Cellulitis

B. Juvenile idiopathic arthritis

C. Lyme disease

D. Septic arthritis

Answer: Lyme Disease

An 11-year-old boy presented to his pediatrician in an urban setting with left leg erythema, limping, lethargy, and fever. The day before, the boy had begun a regimen of cephalexin for presumed cellulitis after his parents texted the photo shown here to the pediatrician. The photo showed the boy’s red calf and popliteal area following a case of poison ivy contact dermatitis on his left leg.

Cephalexin was continued for the presumed cellulitis, and the redness improved, although the boy still found it painful to walk. The next day, he was lethargic, was still limping, and had a temperature of 38.5°C, and so he was admitted to the hospital for intravenous clindamycin to treat the presumed cellulitis. The boy’s knee joint appeared benign on physical examination.

Results of an inpatient complete blood count showed a normal white blood cell (WBC) count and an elevated erythrocyte sedimentation rate of 38 mm/h. A septic joint was not suspected because of the normal WBC count and the benign appearance of the knee. One day into the hospital admission, his knee pain and swelling increased and an effusion developed. The redness of the calf had nearly resolved, but the popliteal area still was markedly red. Arthrocentesis of the knee was performed, and results of synovial fluid analysis showed a minimal WBC count of 3,056 cells. Samples were sent for Gram stain and culture tests and a Lyme titer.

Ultrasonography revealed a ruptured popliteal cyst in the boy’s left knee with fluid tracking down to the calf. The inflammation in the leg was attributed to the synovial fluid collecting in the calf following the rupture of the cyst, although a diagnosis of cellulitis still was being considered, and so intravenous clindamycin was continued. A rheumatologist who was consulted to address the possibility of juvenile idiopathic arthritis recommended administering naproxen, which improved the boy’s pain and swelling.

The patient was discharged on day 4 and was instructed to continue oral clindamycin for 7 to 10 days. After discharge, laboratory test results returned; while results of Gram stain and culture testing were negative for bacteria, enzyme immunoassay results showed immunoglobulin G (IgG) antibodies to Borrelia burgdorferi, the tick-borne spirochete that causes Lyme disease, to be elevated at 3.39 LI (reference range, 0.00-0.79 LI). Confirmatory Western blot testing revealed the presence of the following Lyme IgG bands: 93, 66, 58, 45, 41, 39, 30, 28, 23, and 18.

The patient was notified of the positive Lyme disease test results 1 day after discharge, and a regimen of doxycycline, 75 mg twice daily for 21 days, was initiated. At a follow-up visit 2 days after discharge, the boy’s left knee still was larger than the right one, and his energy level still was only approximately 50% normal.

Lyme disease is the most common vector-borne illness in the United States. The incidence of Lyme disease among U.S. children has increased, especially in endemic regions such as the Northeast. The clinical presentation of Lyme disease includes rash, headache, lethargy, musculoskeletal pain, arthralgia, and neurologic symptoms. Patients often do not recall a tick bite. While monarthritis of the knee is a common finding in cases of pediatric Lyme disease, few cases of pediatric popliteal cysts associated with Lyme arthritis have been reported in the literature.^1,2

Genevieve A. Fasano is a medical student at Drexel University College of Medicine in Philadelphia, Pennsylvania.

Alexis S. Lieberman, MD, is a pediatrician at Fairmount Pediatrics and Adolescent Medicine in Philadelphia, Pennsylvania.

References

1. Magee TH, Segal LS, Ostrov B, Groh B, Vanderhave KL. Lyme disease presenting as popliteal cyst in children. J Pediatr Orthop. 2006;26(6):725-727.

2. Steelman TJ, Wagner SC, Jex JW. Popliteal cyst with positive antinuclear antibodies as the presentation of Lyme disease. Pediatr Infect Dis J. 2015;34(5):548.