An Intravenous Drug User with Back Pain

A known intravenous drug user of no fixed abode presents with very severe back pain, fever, and left leg weakness. He has been woken at night several times because of the pain and is having difficulty walking. On examination he has tenderness over the lower lumbar spine and has weakness of left knee extension and foot dorsiflexion.

1. What is the most likely diagnosis?

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This patient is very likely to have a diagnosis of discitis. Discitis is an infection of the intervertebral disc space that can have potentially catastrophic consequences including sepsis and epidural abscess formation.

The typical clinical features of discitis include:

Back or neck pain (>90%)
Pain often awakes patient from sleep
Fever (60-70%)
Neurological deficits (up to 50%)
Children sometimes refuse to walk

Risk factors for developing discitis include:

Spinal surgery (occurs in 1-2% post-operatively)
Immunodeficiency
Intravenous drug use
Age less than 8 years
Diabetes mellitus
Malignancy

2. What is the most likely causative organism?

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The most common causative organism is usually Staphylococcus aureus.

Other organisms, such as Streptococcus viridans and Pseudomonas aeruginosa, may also occur in intravenous drug users and the immunocompromised and should be considered as a possible cause.

Other possible organisms include Gram-negative organisms such as Escherichia coli, and Mycobacterium tuberculosis (Pott’s disease).

3. How can the diagnosis be confirmed?

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MRI is the imaging modality of choice and has very high sensitivity and specificity. The whole spine should be imaged, as discitis frequently occurs at multiple levels.

Plain radiographs are insensitive to the early changes of discitis, and normal appearances can be maintained for 2-4 weeks. CT scanning is also relatively insensitive.

4. What treatment should this patient receive?

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Treatment is with admission for intravenous antibiotics. Three sets of blood cultures, as well as a full set of bloods including a CRP, should be sent to the lab prior to commencing the antibiotics.

A typical antibiotic regime for discitis would be:

IV flucloxacillin 2 g 6 hourly 1st line if no penicillin allergy
IV vancomycin if hospital acquired infection, high-risk of MRSA or documented pencillin allergy
+/- gentamicin IV if possibility of Gram negative infection (age >65, immunosuppressed or intravenous drug user)
IV piperacillin-tazobactam 4.5 g 8 hourly alone if acute kidney injury and Gram-negative cover required

Header image used on licence from Shutterstock

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