Department of Rheumatology and 1 Department of Radiology, Guy's and St Thomas' Hospital, London, UK
Correspondence to: B. Kirkham, Department of Rheumatology, 4th Floor, Thomas Guy House, St Thomas Street, London SE1 9RT, UK. E-mail: bruce.kirkham{at}gstt.sthames.nhs.uk
SIR, Sciatica is a common presentation but the underlying cause may be sacroiliitis, which differs in prognosis and treatment options. We present two cases of sacroiliitis presenting as sciatica. The first case is a 28-yr-old man who for 18 months had recurrent episodes of left buttock pain radiating down to the lower calf, typical of sciatic pain. Straight leg raising and femoral nerve stretch tests were negative and MRI of the lumbar spine showed no evidence of disc herniation. MRI of the pelvis demonstrated marked inflammation of the left sacroiliac joint. Symptoms settled gradually with ibuprofen and conservative management. He presented 15 months later with similar classical sciatica symptoms, but this time on the right side. MRI revealed right-sided sacroiliitis and the previously involved left sacroiliac joint was now normal (Fig. 1). Diagnosis of a reactive sacroiliitis was made, and symptoms resolved with non-steroidal anti-inflammatory (NSAID) therapy.
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Sacroiliitis causing symptoms of sciatica have been reported in a handful of cases [1, 2]. But how can sacroiliitis cause symptoms mimicking sciatica? Traditionally, sciatica was thought to be due to mechanical nerve root compression caused by intervertebral disc herniation. However, it is now evident that radicular pain is not purely mechanical, and that nerve root inflammation and subsequent nerve injury is important. In fact, up to 76% of people without symptoms of back pain or sciatica have disc herniation on MRI, and not all patients with sciatica have disc herniation on MRI [3]. Inflammatory mediators, including pro-inflammatory cytokines released from the nucleus pulposus, may contribute to the production of radicular pain, tumour necrosis factor (TNF-
) being one of the most significant [4]. Indeed, there has been a recent proposal for the use of TNF-
blockade in the treatment of sciatica due to disc herniation [5].
There are two potential mechanisms by which sacroiliitis could generate sciatic symptoms of pain in the lower leg. They are (i) referred pain and (ii) inflammatory mediator release from the sacroiliac joint directly affecting adjacent neural structures. Pain referral patterns have been mapped in healthy volunteers who received a sacroiliac joint injection with contrast material as a stimulus. Pain was most commonly felt just inferior to the posterior superior iliac spine, but in some patients radiated further down the leg [6]. To investigate the communications of the sacroiliac joint, mapping of extravasated contrast medium from sacroiliac joint injections was performed using CT. Almost one-third of sacroiliac joints injected revealed communication to surrounding neural structures: ventral extravasation to the lumbosacral plexus, and dorsal leakage to the area of the first sacral nerve root and from the superior capsular recess to the area of the fifth lumbar nerve root [7]. There appears, therefore, to be a direct anatomical link between the sacroiliac joint and surrounding neural structures which may produce radicular pain. In the second case, straight leg raising test was positive, suggesting some irritation of the sciatic nerve. However, there were no clinical neurological deficits found in either case.
These cases show that sacroiliitis should be considered in patients who present with sciatica. Pain secondary to a large disc herniation frequently continues at rest as well as on movement. In the second case, pain was reduced later in the day, a rare finding with disc herniation. To confirm the diagnosis, X-ray of the sacroiliac joints is cheapest and most readily available, but is an insensitive technique, particularly in early stages. MRI detects subchondral bone marrow changes in both the active and inactive stages with 95% sensitivity (compared with 48% for scintigraphy and 19% for plain radiography) [8]. We therefore recommend MRI as the next investigation if plain radiography is normal and suspicion of sacroiliitis is high.
Management options differ for sacroiliitis and intervertebral disc herniation. With cases of sacroiliitis not responding to conservative treatment and NSAIDs, temporary improvement may be gained from periarticular corticosteroid injection. However, benefit usually lasts for not more than 3 months, and by 6 months the symptoms have usually returned to pretreatment levels [9]. High levels of TNF- are found in sacroiliac joint biopsy specimens, and infliximab and etanercept have been shown to improve disease activity, function, and quality of life in ankylosing spondylitis [10]. Therefore, leg pain suggestive of sciatica can be caused by sacroiliitis, which may lead to a different underlying diagnosis, prognosis, and array of therapy options.
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The authors have declared no conflicts of interest.
References
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