Low Back Pain Approach
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Direct answer
Low back pain is usually mechanical and self-limiting, so the approach first excludes serious causes through red flags like cancer, infection and fracture. Cauda equina syndrome is the emergency to recognise, presenting with urinary retention, saddle anaesthesia and bowel dysfunction, and it needs urgent MRI and spinal surgical review. Imaging is reserved for red flags or progressive deficit, because routine imaging in uncomplicated back pain adds cost without better outcome.
What you must remember
- Red flags: Age over 50, cancer history, weight loss, fever, immunosuppression, intravenous drug use, steroid use and night pain each raise concern for serious pathology.
- Cauda equina syndrome: Urinary retention or overflow incontinence, faecal incontinence, saddle anaesthesia and reduced anal tone, often with bilateral sciatica.
- MRI first: Lumbar MRI is the investigation of choice for suspected cauda equina compression, infection or malignancy, while radiographs suit trauma and suspected fracture.
- Sciatica: Nerve root pain below the knee, most often L5 or S1 from disc herniation, with a positive straight leg raise.
- Crossed straight leg raise: Reproducing leg pain by lifting the opposite leg is highly specific for disc herniation.
- Reflex localisation: Reduced knee jerk points to L4, reduced ankle jerk to S1, and weak great toe extension to L5.
- Inflammatory pattern: Morning stiffness beyond half an hour, improvement with exercise and night pain suggest ankylosing spondylitis.
Sorting a serious presentation from a simple one
A 46-year-old lorry driver with two days of back pain, numbness between the legs, difficulty starting to pass urine and reduced perianal sensation shows the order of triage.
- Ask about sphincters first: Ask about bladder, bowel and sexual function before examining reflexes.
- Confirm retention: A large residual volume on bladder scan and weak anal tone strongly suggest cauda equina syndrome, which MRI then confirms.
- Arrange urgent imaging: An urgent lumbar MRI and same-day spinal surgical referral follow, because outcome depends on how soon decompression happens.
- Separate simple sciatica: A 52-year-old office worker with pain below one knee, a positive straight leg raise and normal bladder and bowel function has sciatica. Management is reassurance, staying active, simple analgesia and physiotherapy, with imaging deferred unless the pain fails to settle or a deficit develops.
- Screen for red flags: A 68-year-old with breast cancer, night pain and weight loss has red flags for metastasis, so the pathway goes to blood tests and MRI rather than a routine physiotherapy referral.
Where the exam tries to catch you
Examiners in back pain vignettes return to the same few traps.
- Trap: Painless urinary retention with back pain is treated as a simple infection. Correct: It is cauda equina until proven otherwise, and a large overflow bladder is a warning sign.
- Trap: Absent sphincter symptoms are taken to rule out the syndrome. Correct: Incomplete forms exist, so a single normal bladder scan does not exclude progression.
- Trap: Imaging is ordered for every case of non-specific back pain. Correct: Routine imaging does not improve early outcome, so it is reserved for red flags.
- Trap: Leg pain on walking is labelled as vascular claudication. Correct: Neurogenic claudication from lumbar stenosis eases on sitting, while vascular claudication is eased by rest and comes with pulse deficits.
Long-term opioids for chronic low back pain are not recommended as first line.
Frequently asked questions
Which investigation is urgent for suspected cauda equina syndrome?
Urgent MRI of the lumbar spine is required for suspected cauda equina syndrome, with same-day referral to a spinal surgeon. A bladder scan can document retention while the MRI is arranged, and the time to decompression strongly influences the outcome.
How does cauda equina syndrome differ from simple sciatica?
Cauda equina syndrome causes bladder or bowel dysfunction, saddle anaesthesia, reduced anal tone and often bilateral leg symptoms. Simple sciatica is typically unilateral and does not involve sphincter function, so any sphincter disturbance with back pain needs urgent assessment.
When is imaging indicated in non-specific low back pain?
Imaging is indicated when red flags are present, when a neurological deficit is progressive, or when infection, malignancy or fracture is suspected. In uncomplicated low back pain, routine imaging in the early weeks is not recommended because it does not improve recovery.
Which reflex changes suggest L5 or S1 root compression?
A reduced ankle jerk suggests S1 root involvement, while a reduced knee jerk points to L4. Weakness of great toe extension suggests L5, and a positive crossed straight leg raise supports disc herniation as the cause.