Undifferentiated Back Pain — Written Clinical Reasoning

SAQ 1 — Integrated Diagnostic Approach to Inflammatory Back Pain (20 marks, 30 minutes)

Prompt: Outline your integrated diagnostic approach to Mr Tran's presentation, addressing: (a) the triage framework and the prioritised differential; (b) the clinical features that discriminate inflammatory from mechanical back pain and the ASAS classification criteria for axial spondyloarthritis; (c) the role of the HLA-B27, the X-ray and the MRI in the diagnostic work-up; (d) the extra-articular associations and the surveillance you would undertake; (e) the pharmacological management, including the first-line therapy and the escalation pathway; and (f) the common exam trap in this patient. ^[1]

Model Answer

(a) Triage framework and prioritised differential (3 marks): ^[1]

The triage framework sorts every back pain patient into non-specific (mechanical) back pain, radicular (nerve root) pain, or back pain with red flags (serious pathology) ^[1]. Mr Tran does not fit the mechanical bin (his pain improves with exercise, not with rest; he has morning stiffness over an hour), he does not have radicular pain (no dermatomal radiation, no neurological deficit), and he does not have the infective or malignant red flags (no fever, no weight loss, no night sweats, no progressive unremitting pain). He does, however, have the classic inflammatory back pain criteria, with the extra-articular features of psoriasis and uveitis. The prioritised differential is therefore: first, axial spondyloarthritis (ankylosing spondylitis), strongly supported by the clinical picture, the raised inflammatory markers, the HLA-B27 and the bilateral sacroiliitis; second, the spondyloarthritis associated with psoriasis or inflammatory bowel disease (to be screened for); third, a mechanical contributor (possible, but not the primary driver).

(b) Discrimination of inflammatory from mechanical back pain and the ASAS criteria (4 marks): ^[1]

The ASAS inflammatory back pain criteria are my framework, and Mr Tran meets at least four of the five: age of onset under 40 (he is 34), insidious onset, improvement with exercise, no improvement with rest, and night pain improving on rising. He also has morning stiffness over an hour, which is the inflammatory signature. The discriminating features from mechanical pain are the opposite response to activity (inflammatory improves, mechanical worsens), the night pain that improves on rising (mechanical pain is typically relieved by lying down), and the morning stiffness over 30 minutes. Four out of five of these criteria have a sensitivity of over 70 per cent and a specificity of over 80 per cent for axial SpA ^[6].

The ASAS classification criteria for axial spondyloarthritis apply to the patient with chronic back pain (over 3 months) and onset under 45 years. There are two arms: the imaging arm (sacroiliitis on imaging — radiography or MRI — plus at least one SpA feature) and the clinical arm (HLA-B27 positive plus at least two SpA features). Mr Tran meets the imaging arm — he has bilateral grade 2 sacroiliitis on the pelvic X-ray (the imaging) plus psoriasis (the SpA feature). The SpA features include inflammatory back pain, arthritis, enthesitis (heel), uveitis, dactylitis, psoriasis, Crohn disease or ulcerative colitis, a good response to NSAIDs, a family history of SpA, and an elevated CRP ^[6].

(c) The role of the HLA-B27, the X-ray and the MRI (3 marks): ^[1]

The HLA-B27 is neither necessary nor sufficient for the diagnosis. It is present in about 90 per cent of patients with ankylosing spondylitis but also in about 8 per cent of the general population, most of whom never develop the disease. Mr Tran's positive HLA-B27 is supportive but not diagnostic; he already meets the ASAS imaging arm, so the HLA-B27 is confirmatory. A negative HLA-B27 would not have excluded the diagnosis. ^[1]

The pelvic X-ray confirms the structural sacroiliitis — the bilateral grade 2 change — which is sufficient for the modified New York criteria for ankylosing spondylitis. The limitation of the X-ray is that the radiographic sacroiliitis takes years to develop, which is the source of the average 6 to 10 year diagnostic delay. ^[1]

The MRI of the sacroiliac joints is the modality that detects the early active inflammation (the bone marrow oedema) before the radiographic change, and it is the investigation that closes the diagnostic delay in the patient with the clinical picture but the normal X-ray. In Mr Tran, the MRI is not needed to make the diagnosis (the X-ray and the clinical picture are sufficient), but I would arrange it to assess the degree of active inflammation, which guides the escalation to a biologic. ^[1]

(d) The extra-articular associations and the surveillance (4 marks): ^[1]

Axial spondyloarthritis is a systemic disease, not just a back pain, and the surveillance spans multiple systems. The anterior uveitis (the commonest extra-articular feature, in about a third of patients) — I ask about eye pain, redness and photophobia, and I link him to ophthalmology for the early treatment of recurrence. The cardiovascular associations (aortic regurgitation from aortic root dilatation, and the conduction disease) — I arrange a baseline echocardiogram and an ECG. The pulmonary associations (apical pulmonary fibrosis and the restrictive chest wall disease from the costovertebral rigidity) — I monitor the chest expansion and the spirometry; his chest expansion is already reduced at 4 cm. The osteoporosis (the inflammation and the immobility increase the fracture risk) — I monitor the bone density. The inflammatory bowel disease — I ask about the chronic diarrhoea, the abdominal pain and the rectal bleeding, and I screen if any of these are present ^[6].

(e) The pharmacological management and the escalation pathway (4 marks): ^[1]

The first-line pharmacological therapy is a full-dose NSAID at the maximum tolerated dose for at least two to four weeks — naproxen 500 mg twice daily, or celecoxib 200 mg twice daily, with gastroprotection (a proton pump inhibitor) for the at-risk patient. The response to NSAIDs is itself a SpA feature in the classification criteria, and about half to two-thirds of patients respond adequately ^[6].

If the NSAID is insufficient, or if there are contraindications (renal impairment, peptic ulcer, cardiovascular disease), or if the disease is high-activity (a raised CRP, the active sacroiliitis on the MRI, the reduced function), I escalate to a biologic under rheumatology guidance — a tumour necrosis factor inhibitor (adalimumab 40 mg subcutaneously every two weeks, etanercept 50 mg weekly, or infliximab) or an interleukin-17 inhibitor (secukinumab), guided by the NICE NG65 and the international treat-to-target recommendations. I avoid the systemic corticosteroid for the axial disease (it is largely ineffective) and I reserve the intra-articular corticosteroid for the peripheral joint or the sacroiliac joint injection in selected cases. The non-pharmacological management — the structured physiotherapy, the daily spinal mobility and postural exercise, and the smoking cessation — is the other cornerstone ^[6].

(f) The common exam trap (2 marks): ^[1]

The trap is the diagnostic delay — the registrar who attributes the insidious-onset back pain of a young man with psoriasis and uveitis to a "lumbar strain" or a "muscular pain" and does not ask the inflammatory back pain criteria questions, misses the diagnosis for years, during which the untreated inflammation causes the irreversible structural damage and the spinal rigidity. The corollary is that the registrar who asks the inflammatory back pain criteria, examines for the extra-articular features (the skin, the eyes, the entheses), and measures the modified Schober and the chest expansion has made the diagnosis that day ^[6]. The second trap is treating the back pain with an opioid or an epidural injection without addressing the underlying inflammation — the symptom is suppressed but the disease progresses.