Isolated septic arthritis: meningococcal infection

Meningococcal infection should be considered in patients with atraumatic knee effusion, not least so as to prevent secondary cases of infection.

CASE HISTORY

A 19-year-old first-year college student attended the accident and emergency department after three days of pain in the left knee. There was no history of joint trauma, headache or photophobia and she was not sexually active. There had been no recent febrile illness in her family or flatmates. On examination her temperature was 38 °C and she had an erythematous rash on the extensor surfaces of her legs. The left knee was tender and swollen with a moderate effusion and reduced range of motion. The peripheral blood white cell count was 15.8×10⁹/μL and C-reactive protein was 27 mg/L. About 75 mL of purulent fluid was aspirated from her left knee. Treatment was started with intravenous flucloxacillin and benzylpenicillin and an arthroscopic knee washout was performed. She responded well clinically and was afebrile within 24 hours.

Microscopic examination of the Gram-stained smear revealed numerous polymorphonuclear neutrophils with intracellular Gram-negative diplococci. Blood culture and knee aspirate taken before antibiotic therapy were sterile. Infection with Neisseria meningitidis serogroup C was established by polymerase chain reaction performed on the knee aspirate by the National Meningococcal Reference Laboratory.

Intravenous benzylpenicillin was continued for two weeks and the patient received oral rifampicin 600 mg twice daily for two days to clear any potential nasal carriage. Close college contacts were given rifampicin and immunized against N. meningitidis serogroup C.

COMMENT

Although named for the association with meningitis, invasive meningococcal disease is primarily a bloodstream infection with seeding to other sites. ^{1, 2, 3} The classic purpuric rash is easily overlooked when systemic features are mild or when, as in this case, localized joint infection dominates the clinical picture.

Without antibiotic treatment and drainage, meningococcal infection can rapidly destroy a joint. Intravenous antibiotics are required for a minimum of two weeks. ⁵ Arthroscopy is often the preferred method of irrigation for infections involving the knee or shoulder. Arthritis is a recognized manifestation in 2-10% of meningococcal infections. ⁶ The most frequent presentation of the arthritis is during the recovery period, when large joints are affected by a sterile effusion. This is a reactive arthritis related to the intense immune response. ⁷ Direct bacterial invasion of the synovium can occur during the acute infection (as in this case) or in the course of chronic meningococcaemia. True primary meningococcal arthritis without other clinical features typically affects large joints such as the knee. ^8,9

The distinction between gonococcal arthritis (Neisseria gonorrhoeae) and meningococcal arthritis may be difficult. On microscopic examination of joint aspirate N. meningitidis and N. gonorrhoeae are morphologically indistinguishable and cultures may be negative, especially if antimicrobial agents have been given. Polymerase chain reaction, which does not require organisms to be viable, ¹⁰ can provide a specific diagnosis in such cases. Penicillin is the treatment of choice for meningococcal disease, resistant strains of N. meningitidis being very rare. ¹¹

Meningococcal and gonococcal infection should be considered in the differential diagnosis of isolated monoarthritis. Early diagnosis is important not only for the patient but also to allow public health interventions. First-year students living in dormitories are at special risk. At the time of this case, immunization against N. meningitidis serogroup C was not routine in Ireland. A national immunization programme is now in place.