Discitis

Discitis is an infection or inflammation of the intervertebral disc space. It is most common in young children, in whom the disc is vascularised and can be seeded directly from the blood, and in adults after spinal surgery or procedures. In children it is often called paediatric spondylodiscitis and it frequently involves the adjacent end plates.

It is rare, with a few cases in 100 000 children each year, in a bimodal age pattern: under 4 years (peak 6 months to 4 years) and in adolescents. The lumbar spine, particularly L3 to L4 and L4 to L5, is most often affected.

The organism is found in only 30 to 60%.

• Staphylococcus aureus is the commonest.
• Kingella kingae is frequent in children under 4 years.
• Other organisms are streptococci, Gram negative bacilli, and Salmonella in sickle cell disease.

After lumbar discectomy, discitis occurs in 0.5 to 3%, usually 2 to 8 weeks after surgery.

Most children recover fully, but late complications are disc space loss, vertebral fusion, kyphosis, chronic back pain, and rarely epidural abscess. Delay in diagnosis is common because symptoms are non-specific.

Pathophysiology.

In children, arteries pass from the end plates into the annulus and nucleus, and bacteraemia seeds the disc. In adults the disc is avascular, so infection arises from the adjacent end plate or from direct inoculation during surgery, discography or spinal injection. Low virulence organisms (Kingella kingae) cause subacute disease. Infection destroys the disc and end plates and may extend to the epidural and paraspinal spaces.

Children: refusal to walk, stand or sit, a limp, irritability, back pain or abdominal, hip or leg pain, a stiff back with a flexed posture, refusal to bend forward to pick up objects, and a low grade fever (about half are afebrile). Infants may refuse to crawl or sit. Neurological signs are rare.

Adults after surgery: severe back pain with muscle spasm and little fever, which begins weeks after the operation.

Examine the spine for tenderness over the spinous process, loss of lumbar lordosis, spasm, hamstring tightness, a positive straight leg raise, hip movement (to exclude hip infection), the abdomen, and neurological state.

• ESR above 20 mm/hour in about 90%, and raised CRP; the white cell count is often normal.
• Blood cultures before antibiotics (positive in 30 to 50%); PCR for Kingella kingae on blood or tissue, particularly in children under 4 years.
• MRI with gadolinium (the key test): disc space narrowing, T2 hyperintensity in the disc, end plate oedema and enhancement, and epidural or paraspinal collections; sedation or general anaesthesia may be needed in young children.
• Radiographs: disc space narrowing after 2 to 3 weeks and end plate irregularity after 2 to 4 weeks. Bone scan if MRI is unavailable.
• Image directed disc biopsy when blood cultures are negative and there is no response to empirical treatment, atypical features, or immunosuppression; tests for tuberculosis and Brucella when relevant.

Differential diagnosis.

• Vertebral osteomyelitis and septic arthritis of the hip or sacroiliac joint.
• Langerhans cell histiocytosis (vertebra plana), leukaemia, Ewing sarcoma and neuroblastoma.
• Scheuermann disease, juvenile idiopathic arthritis and psoas abscess.
• Appendicitis, pyelonephritis, transient disc calcification and aseptic (inflammatory) discitis.

The aims are to treat infection, relieve pain and protect the disc and spine. Surgery is rarely needed.

Antibiotic treatment in children (start once blood cultures are taken).

• Empirical intravenous treatment: cefazolin 33 mg/kg intravenously every 8 hours (maximum 2 g a dose), which covers Staphylococcus aureus and Kingella kingae (Kingella is resistant to clindamycin and vancomycin).
• Alternative in regions where Kingella is uncommon: flucloxacillin 50 mg/kg intravenously every 6 hours (maximum 2 g a dose).
• With MRSA risk: vancomycin 15 mg/kg intravenously every 6 hours (adjust to trough 10 to 15 mg/L; monitor creatinine), or clindamycin 10 mg/kg intravenously every 6 to 8 hours (maximum 600 mg a dose) if susceptible.
• Sickle cell disease (Salmonella): ceftriaxone 50 mg/kg intravenously once daily (maximum 2 g).
• Oral step down when the child is afebrile, the pain is better and CRP has fallen by half (usually after 3 to 7 days, up to 2 weeks): cefalexin 25 mg/kg orally every 6 hours (maximum 1 g a dose).
• Oral step down for MRSA or penicillin allergy: clindamycin 10 mg/kg orally every 6 to 8 hours (maximum 600 mg a dose).
• Total duration is 4 to 6 weeks in most. Shorter courses (2 to 3 weeks) may be enough for uncomplicated Kingella infection with a quick response.

Antibiotic treatment in adults (post-operative). Vancomycin 15 to 20 mg/kg intravenously every 8 to 12 hours (adjust to trough 15 to 20 mg/L and creatinine) with ceftriaxone 2 g intravenously every 24 hours until cultures define the organism. Then give pathogen directed therapy for 6 weeks (12 weeks if implants are present or abscess is undrained), with oral step down after 1 to 2 weeks to a well absorbed agent.

Rest and immobilisation. Rest and avoidance of heavy activity for 2 to 6 weeks; a thoracolumbosacral orthosis or a body cast for pain relief in young children; return to play when the pain and inflammatory markers settle.

Analgesia.

• Children: paracetamol 15 mg/kg orally every 4 to 6 hours (maximum 75 mg/kg daily and not exceeding 4 g daily).
• Children: ibuprofen 10 mg/kg orally every 6 to 8 hours with food (maximum 40 mg/kg daily and not exceeding 2400 mg daily) for up to 5 days. Avoid ibuprofen with kidney impairment, dehydration or active bleeding.
• Oral morphine 0.1 to 0.2 mg/kg every 4 hours as needed for severe pain for up to 2 to 3 days (maximum 10 mg a dose); monitor sedation and respiration.
• Adults: paracetamol 1 g orally every 6 hours (maximum 4 g daily); oral morphine 5 to 10 mg every 4 hours as needed for severe pain (start 2.5 to 5 mg with age over 75 or eGFR under 30 mL/min/1.73 m2); laxative.
• Corticosteroids are not used.

Surgery (rare). Biopsy when the organism is unknown; drainage of an abscess; debridement or stabilisation for neurological deficit, instability or failure of antibiotics.

Monitoring. Temperature, pain and walking daily at first; CRP weekly until normal (ESR may stay raised for weeks); full blood count, creatinine and liver function on prolonged courses; vancomycin levels. Repeat MRI only if there is no response in 48 to 72 hours or if complications are suspected. Radiographs at 6 weeks, 3, 6 and 12 months for disc height, kyphosis and spontaneous fusion.

Referral and escalation. Refer any child with a refusal to walk, back pain and raised inflammatory markers to paediatric orthopaedics and infectious disease the same day. Emergency referral for neurological deficit, sepsis, epidural abscess, or failure to improve within 48 to 72 hours.

Special groups.

• Infants and children under 4 years: Kingella kingae infection; use PCR; cefazolin is first line.
• Sickle cell disease: Salmonella; ceftriaxone and haematology advice.
• Immunosuppressed patients: biopsy early; consider tuberculosis and fungal infection.
• Adults after spinal surgery: look for implant infection; prolonged therapy.