Differential Diagnosis of Low Back Pain: A Physiotherapist's Guide
Low back pain is the leading cause of disability worldwide and one of the most common presentations in physiotherapy practice. The vast majority of cases are benign and self-limiting, but a small proportion represent serious underlying pathology that requires urgent investigation or referral. This guide walks through a systematic approach to differentiating the causes of low back pain, from the most common mechanical presentations to the red flags that must not be missed.
The Diagnostic Framework: Start with the Subjective
The subjective assessment is where most of your differential diagnosis is formed. Before touching the patient, you should already have a working hypothesis based on:
Area and distribution of pain — Localised lumbar pain suggests mechanical or discogenic pathology. Pain radiating below the knee in a dermatomal pattern suggests nerve root involvement (radiculopathy). Bilateral leg symptoms, saddle anaesthesia, or bladder/bowel dysfunction are red flags requiring immediate referral.
Onset and mechanism — Acute onset with a specific mechanism (lifting, twisting) is consistent with disc injury or facet joint strain. Insidious onset in an older patient raises the possibility of spinal stenosis, osteoporotic fracture, or malignancy.
Behaviour of pain — Mechanical pain is typically worse with movement and relieved by rest. Inflammatory pain (ankylosing spondylitis, sacroiliitis) is worse in the morning and improves with activity. Constant, unremitting pain that is not influenced by position or movement is a red flag for serious pathology.
Common Mechanical Causes
Non-specific Low Back Pain The most common presentation — approximately 85% of low back pain has no identifiable structural cause. It is typically activity-related, improves with movement, and resolves within 6–12 weeks. Management focuses on education, reassurance, and active rehabilitation.
Lumbar Disc Herniation Disc herniation with nerve root compression presents with dermatomal leg pain (sciatica), often worse with sitting, coughing, or sneezing. Positive straight leg raise (SLR) at <60° is a sensitive indicator. L4/L5 and L5/S1 levels are most commonly affected.
Facet Joint Syndrome Facet joint pain is typically unilateral, localised to the lumbar region, and worsened by extension and rotation. It is often worse in the morning and with prolonged standing. There is no neurological deficit.
Sacroiliac Joint Dysfunction SI joint pain presents as unilateral buttock pain, sometimes with referral to the posterior thigh. It is typically worsened by single-leg activities (climbing stairs, getting out of a car). A cluster of three or more positive SI joint provocation tests (FABER, Gaenslen's, thigh thrust, compression, distraction) has good diagnostic accuracy.
Neurological Presentations
Lumbar Radiculopathy Radiculopathy results from nerve root compression, most commonly due to disc herniation or foraminal stenosis. Key features include dermatomal pain, paraesthesia, and neurological deficit (weakness, reduced reflexes). The level of involvement can be identified by the dermatomal and myotomal pattern:
| Level | Dermatomal Pain | Weakness | Reflex |
|---|---|---|---|
| L3/L4 | Anterior thigh, medial leg | Knee extension | Knee jerk |
| L4/L5 | Lateral leg, dorsum of foot | Ankle dorsiflexion, EHL | None reliable |
| L5/S1 | Posterior thigh, lateral foot | Ankle plantarflexion | Ankle jerk |
Lumbar Spinal Stenosis Spinal stenosis presents with neurogenic claudication — bilateral leg pain, heaviness, or paraesthesia brought on by walking or standing and relieved by sitting or forward flexion. It is more common in older patients (>60 years) and is associated with a wide-based gait. The bicycle test (symptoms relieved by cycling but reproduced by walking) helps distinguish neurogenic from vascular claudication.
Serious Pathology: Red Flags You Must Not Miss
A small but important proportion of low back pain presentations represent serious underlying pathology. The following red flags require urgent investigation or referral:
Cauda Equina Syndrome — Bilateral leg weakness or numbness, saddle anaesthesia (perineum, inner thighs), and bladder or bowel dysfunction. This is a surgical emergency. Any patient with these features requires same-day emergency referral.
Spinal Malignancy — History of cancer (especially breast, lung, prostate, kidney, thyroid), unexplained weight loss, night sweats, age >50 with no previous history of LBP, constant progressive pain unrelated to movement, pain at rest or at night.
Vertebral Fracture — History of osteoporosis, prolonged corticosteroid use, significant trauma, age >70. Sudden onset of severe thoracolumbar pain in an older patient should raise suspicion.
Spinal Infection — Fever, recent infection (UTI, skin infection, dental procedure), IV drug use, immunosuppression, constant severe pain. Discitis and epidural abscess are rare but serious.
Inflammatory Spondyloarthropathy — Age <45, insidious onset, morning stiffness >1 hour, improves with exercise, associated with psoriasis, IBD, or uveitis. Refer for rheumatology assessment.
Objective Assessment: Key Tests for Differentiation
After your subjective assessment, use the objective examination to confirm or refute your working hypothesis:
- Straight Leg Raise (SLR) — Sensitivity 91% for disc herniation with radiculopathy; specificity 26%. A positive SLR at <60° is highly sensitive but not specific.
- Slump Test — More sensitive than SLR for detecting neural tension; useful when SLR is negative but radiculopathy is suspected.
- Neurological examination — Myotomes, dermatomes, and reflexes to identify the level of nerve root involvement.
- SI joint provocation cluster — Three or more positive tests from: FABER, Gaenslen's, thigh thrust, compression, distraction.
- Extension loading — Centralisation of symptoms with repeated extension suggests discogenic pathology (McKenzie approach).
Document your findings clearly and update your differential diagnosis after each component of the assessment.