69 BACK PAIN

# 70 BACK PAIN

👩‍⚕️ Back pain is one of the commonest surgical presentations and a magnet for SBA questions. The exam tests three things: can you separate mechanical from radicular from red flag pain, can you recognise a cauda equina before the patient ends up incontinent, and can you spot the classic pattern behind each diagnosis (young man with stiff spine, old lady with crush fracture, prostate cancer with sclerotic mets, etc.).

A framework for back pain

Group every back pain question into one of three buckets:

Bucket	Pattern	Examples
Mechanical	Worse on movement, better with rest. Localised. No neurology.	Muscle strain, facet arthropathy, disc degeneration
Radicular	Pain radiates in a dermatomal pattern. Often with weakness or paraesthesia.	Disc prolapse, sciatica, foraminal stenosis
Red flag	Night pain, weight loss, fever, neurology, age <20 or >55, trauma, steroid use, malignancy history	Cauda equina, malignancy, infection, fracture

➡ The single most important rule: any patient with back pain plus saddle anaesthesia, bilateral leg symptoms, or bladder/bowel dysfunction needs an MRI today, not next week.

The intervertebral disc

Each disc has two parts:

- Nucleus pulposus — gelatinous central core, remnant of the notochord, ~80% water in youth.

- Annulus fibrosus — concentric rings of fibrocartilage that contain the nucleus.

The disc is reinforced anteriorly by the anterior longitudinal ligament (broad and strong) and posteriorly by the posterior longitudinal ligament (narrow, especially in the lumbar region). This is why discs herniate posterolaterally — the PLL deflects the nucleus away from the midline.

➡ Disc prolapse mechanism: the annulus fibrosus tears or weakens with age and load → nucleus pulposus extrudes → compresses the traversing nerve root (one level below the disc) in the lateral recess.

Disc level vs nerve root

A common SBA trap. In the lumbar spine, a posterolateral disc prolapse compresses the root that exits one level below, because that root crosses the disc on its way to its own foramen.

Disc level	Root usually compressed
L3/L4	L4
L4/L5	L5
L5/S1	S1

(A far-lateral prolapse compresses the exiting root at the same level — examiners occasionally test this nuance.)

Sciatica root patterns

Root	Sensory loss	Weakness	Lost reflex
L4	Medial shin, medial foot	Knee extension, ankle dorsiflexion	Knee jerk
L5	Lateral calf, dorsum of foot, big toe	Big toe extension (EHL), foot dorsiflexion	None reliable
S1	Lateral foot, sole, little toe	Plantarflexion, toe-standing	Ankle jerk

➡ Can't extend the big toe → think L5. Can't stand on tiptoes → think S1. Lost ankle jerk → S1.

Cauda equina syndrome

A surgical emergency. The cauda equina is the bundle of lumbosacral nerve roots below the conus medullaris (which ends at L1/L2 in adults). A large central disc prolapse (or tumour, haematoma, abscess) compresses all the roots at once.

Red flag features — any one warrants emergency MRI:

- Saddle anaesthesia (perineum, buttocks, inner thighs — S2–S4 dermatomes)

- Urinary retention with overflow incontinence (late and ominous)

- Faecal incontinence or reduced anal tone

- Bilateral sciatica or leg weakness

- Loss of sexual function

Delay to decompression worsens outcomes — the cut-off most often quoted is 48 hours from onset of urinary symptoms. Examiners love a stem with bilateral leg pain and the patient who "can't quite feel the toilet paper."

> Pearl: Conus medullaris lesions give mixed UMN + LMN signs (lesion at L1). Cauda equina is pure LMN (lesion below the cord). Cord compression above L1 gives UMN signs — hyperreflexia, upgoing plantars, spasticity.

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Spinal stenosis

Narrowing of the central canal, lateral recess or foramen — usually from a combination of disc bulging, facet hypertrophy and ligamentum flavum thickening. Typically a patient over 60.

The defining symptom is neurogenic claudication: bilateral buttock/leg pain or heaviness on walking, relieved by spinal flexion.

➡ Flexion opens the canal; extension closes it. That's why patients describe:

- Relief leaning on a shopping trolley ("trolley sign")

- Walking uphill is easier than downhill

- Cycling is tolerated; walking is not

Neurogenic vs vascular claudication

Feature	Neurogenic	Vascular
Cause	Lumbar canal stenosis	Peripheral arterial disease
Pain onset	Variable; worse standing	Reproducible at fixed distance
Relief	Flexion (sitting, leaning forward) — minutes	Stopping alone — within ~1–2 min
Pulses	Normal	Reduced/absent
Walking uphill	Easier	Harder

Spondylolysis and spondylolisthesis

- Spondylolysis — a defect (often a stress fracture) of the pars interarticularis, usually L5. Common in young athletes doing repetitive hyperextension (cricket fast bowlers, gymnasts). Plain film "Scottie dog with a collar" sign.

- Spondylolisthesis — anterior slip of one vertebra on the one below. May be isthmic (from bilateral pars defects), degenerative (older patients, usually L4 on L5), or congenital. Examination may reveal a palpable step in the lower lumbar spine.

Ankylosing spondylitis

A seronegative spondyloarthropathy. Classic stem: young man (late teens to 30s), insidious back stiffness worse in the morning and with rest, better with exercise, reduced lumbar movement and reduced chest expansion (<5 cm is abnormal).

- HLA-B27 positive in ~90%

- ESR/CRP raised; rheumatoid factor negative

- Associated with the "A" diseases: Anterior uveitis, Aortic regurgitation, Apical lung fibrosis, Achilles tendinitis, IBD, psoriasis

Radiographic signs

- Sacroiliitis — bilateral and symmetrical; the earliest finding

- Squaring of vertebral bodies (loss of anterior concavity)

- Syndesmophytes — thin vertical bony bridges between vertebrae

- Bamboo spine — late, from confluent syndesmophytes

- Dagger sign — single midline vertical line on AP film from ossification of the supraspinous/interspinous ligaments

- Trolley-track sign — three vertical lines (central dagger plus ossified facet capsules)

- Shiny corner (Romanus) lesions — sclerosis at vertebral body corners from enthesitis

> Pearl: A fused ankylotic spine is brittle. Minor trauma can cause an unstable fracture (often cervical). Treat every AS patient with back pain after a fall as a fracture until MRI says otherwise.

Osteoporotic compression fracture

Classical: postmenopausal woman, minor trauma (sometimes none), sudden thoracolumbar back pain, loss of height, kyphosis.

➡ Biochemistry in pure osteoporosis is normal — calcium, phosphate and ALP are all within range. Bone quantity is reduced, but the bone that remains is normally mineralised. This separates it from:

- Osteomalacia — low calcium, low phosphate, high ALP

- Paget's disease — markedly raised ALP with normal calcium/phosphate

- Myeloma/metastases — often raised calcium

Malignancy and the spine

Back pain at night, unrelieved by rest, with weight loss or a known primary, is malignancy until proven otherwise. Vertebral bodies are the commonest skeletal site of metastasis (rich red marrow, Batson's venous plexus).

Pattern	Causes
Lytic lesions	Multiple myeloma, breast, lung, renal, thyroid
Sclerotic (osteoblastic)	Prostate, treated breast, carcinoid, lymphoma (occasionally)
Mixed	Breast

➡ Prostate = sclerotic + raised PSA. Classic SBA pairing.

Multiple myeloma features: older patient, bone pain, anaemia, hypercalcaemia, renal failure, Bence Jones proteinuria, lytic "punched-out" lesions on skeletal survey, rouleaux on blood film.

Discitis and vertebral osteomyelitis

Suspect in: IV drug users, immunocompromised, diabetics, recent spinal procedure, bacteraemia. Pain is constant, often nocturnal, with fever and raised inflammatory markers. Staphylococcus aureus is the commonest organism; consider TB (Pott's disease) if there is chronic back pain with a paravertebral abscess or gibbus deformity. MRI with contrast is the investigation of choice.

Spinal cord blood supply

- One anterior spinal artery — supplies the anterior two-thirds of the cord (motor + spinothalamic).

- Two posterior spinal arteries — supply the posterior columns.

- Artery of Adamkiewicz — a large radicular artery, usually arising from a left posterior intercostal or lumbar artery between T9 and T12. It is the dominant feeder to the lower thoracic and lumbar anterior spinal cord. Injury (aortic surgery, aortic dissection, embolism, instrumentation) causes anterior cord syndrome: paraplegia, loss of pain/temperature below the lesion, preserved proprioception, and bladder dysfunction.

Imaging back pain

- MRI — gold standard for discs, cord, roots, marrow, infection, malignancy. First-line whenever neurology, red flags, or soft tissue pathology is suspected.

- CT — superior for bony detail (fractures, pars defects, bone destruction). Used when MRI contraindicated.

- Plain X-ray — limited; useful for alignment, gross fractures, AS changes, spondylolisthesis.

- Bone scan / PET-CT — disseminated metastatic disease.

[Image: MCQs banner]

Test yourself

A 25-year-old man gives a history of several attacks of low back pain after lifting, resolving spontaneously. He now has pain radiating down his left leg and foot and loss of lumbar lordosis. Diagnosis?

• ((Central prolapse of lumbar intervertebral disc::Central prolapse gives bilateral symptoms and risks cauda equina.))
• ((Spondylolisthesis of L4 and L5 vertebrae::Causes chronic mechanical pain and a palpable step, not acute radiculopathy.))
• ((Posterolateral prolapse of lumbar intervertebral disc::☑️ PLL deflects nucleus laterally — compresses one root, giving unilateral sciatica.))
• ((Stress fracture of vertebral body::Localised bony pain in athletes; no radicular pattern.))
• ((Spinal stenosis::Older patients with bilateral neurogenic claudication relieved by flexion.))

👩‍⚕️ Posterolateral prolapses at L4/5 and L5/S1 compress the traversing L5 or S1 root respectively.

A patient has a prolapsed intervertebral disc. Which structural deficiency is most responsible?

• ((Nucleus pulposus::This is the material that herniates, not the structure that fails.))
• ((Annulus fibrosus::☑️ Tearing of the outer fibrous ring allows the nucleus pulposus to extrude.))
• ((Posterior longitudinal ligament::Narrow in the lumbar spine — explains posterolateral direction, not the failure itself.))
• ((Ligamentum flavum::Hypertrophy causes spinal stenosis, not disc prolapse.))

A 58-year-old woman complains of leg pain on walking that improves when she walks uphill or pushes a trolley. Underlying cause?

• ((Meningocele::Congenital neural tube defect — presents in infancy.))
• ((Vertebral disc prolapse::Causes radicular pain not relieved by flexion.))
• ((Spinal stenosis::☑️ Neurogenic claudication — flexion opens the canal, hence trolley sign and uphill walking.))
• ((Spondylolisthesis::May contribute, but the flexion-relief pattern defines stenosis.))

👩‍⚕️ Vascular claudication is relieved by stopping; neurogenic by bending forward.

A 25-year-old sportsman complains of stiffness and persistent backache with restricted movements in all directions and chest expansion of 5 cm. Diagnosis?

• ((Ankylosing spondylitis::☑️ Young man, global stiffness, reduced chest expansion, HLA-B27 association.))
• ((Intervertebral disc degeneration and prolapse::Does not cause global stiffness or reduced chest expansion.))
• ((Multiple myeloma::Older patients, lytic lesions, anaemia, raised calcium.))
• ((Spina bifida occulta::Usually an asymptomatic incidental finding.))
• ((Spondylolisthesis::Localised pain with palpable step; not global stiffness.))

A 32-year-old man presents with global spinal stiffness and restricted movement in all regions. No trauma. Most likely cause?

• ((Spondylolisthesis::Localised mechanical pain, not global stiffness.))
• ((Spondylolysis::Pars defect — localised pain in young athletes.))
• ((Spinal canal stenosis::Older patients with neurogenic claudication.))
• ((Ankylosing spondylitis::☑️ HLA-B27 seronegative spondyloarthropathy of young men.))

👩‍⚕️ Bamboo spine, dagger sign and bilateral sacroiliitis are the radiographic giveaways.

Patient with chronic low back pain worse on extension. Imaging shows facet joint involvement. Diagnosis?

• ((Disc prolapse::Worse on flexion with radiculopathy.))
• ((Spinal stenosis::Relieved by flexion (trolley sign), not isolated extension pain.))
• ((Facet arthropathy::☑️ Degenerative zygapophyseal joint disease — pain worsens on extension and rotation.))
• ((Sacroiliitis::Inflammatory pain worse at night, improves with activity.))

A 75-year-old woman presents with acute back pain after a minor fall. Imaging reveals an osteoporotic crush fracture. Most likely electrolyte finding?

• ((Hypercalcaemia::Suggests malignancy or hyperparathyroidism, not osteoporosis.))
• ((Hyperphosphataemia::Not a feature of osteoporosis.))
• ((Hypocalcaemia::Suggests osteomalacia.))
• ((Hyponatraemia::Unrelated to bone metabolism.))
• ((Normocalcaemia::☑️ Osteoporosis has normal calcium, phosphate and ALP — quantity reduced, quality normal.))

👩‍⚕️ Raised ALP in an elderly back-pain patient = think Paget's, fracture healing, or metastases — not pure osteoporosis.

A 72-year-old man presents with progressive back pain. MRI shows multiple sclerotic vertebral lesions. Diagnosis?

• ((Multiple myeloma::Causes lytic "punched-out" lesions and Bence Jones proteinuria.))
• ((Metastatic carcinoma of the lung::Lung mets are usually lytic.))
• ((Metastatic carcinoma of the prostate::☑️ Prostate metastases are classically osteoblastic/sclerotic.))
• ((Metastatic carcinoma of the breast::Mixed lesions; not classically sclerotic.))

A patient presents with back pain and sclerotic spinal changes. Which is true?

• ((Hypercalcaemia::More associated with lytic lesions (myeloma, breast).))
• ((Increased PSA::☑️ Prostate cancer is the classic cause of sclerotic bone metastases.))
• ((Raised AFP::Hepatocellular and yolk sac tumour marker — unrelated.))
• ((Bence Jones proteinuria::Found in myeloma, which causes lytic lesions.))

👩‍⚕️ Prostate, treated breast and carcinoid are the three classic causes of sclerotic mets.

A patient develops sudden urinary retention and lower-limb paresis after a spinal procedure. Which artery's compromise best explains this?

• ((Anterior spinal artery::Correct territory but the Adamkiewicz is its dominant lower-cord feeder.))
• ((Artery of Adamkiewicz::☑️ Great anterior radiculomedullary artery — infarction causes anterior cord syndrome with paraplegia and bladder dysfunction.))
• ((Lumbar artery::Segmental supply — unlikely to produce this pattern alone.))
• ((Vertebral artery::Supplies upper cervical cord and brainstem.))

👩‍⚕️ Anterior cord syndrome spares proprioception and vibration (dorsal columns intact).

A patient presents with neck pain and loss of sensation in the C8 dermatome. Best imaging for compressive pathology?

• ((X-ray::Visualises only bone — poor for cord and roots.))
• ((CT::Excellent for bone but limited soft-tissue contrast.))
• ((MRI::☑️ Gold standard for cord, discs, nerve roots and marrow.))
• ((PET-CT::Used for metabolic/oncological staging, not first-line for compression.))

A 70-year-old woman with breast cancer reports new severe back pain, leg weakness and difficulty initiating micturition. Examination shows brisk knee jerks and upgoing plantars. Next step?

• ((Plain lumbar X-ray::Will not reliably demonstrate cord compression.))
• ((Outpatient MRI in 2 weeks::Delay risks irreversible cord injury.))
• ((Urgent whole-spine MRI and high-dose dexamethasone::☑️ Suspected metastatic cord compression — image and steroid within hours.))
• ((Bone scan::Useful for staging but not the emergency investigation.))
• ((Lumbar puncture::Contraindicated with suspected cord compression.))

👩‍⚕️ UMN signs (hyperreflexia, upgoing plantars) localise the lesion above L1 — this is cord compression, not cauda equina.

A 35-year-old man with low back pain reports new bilateral leg weakness, numbness around the perineum and inability to pass urine. Most appropriate next step?

• ((Reassure and discharge with analgesia::Negligent — these are cauda equina red flags.))
• ((Outpatient physiotherapy::Inappropriate with neurological deficit.))
• ((Emergency MRI lumbosacral spine::☑️ Saddle anaesthesia plus urinary retention demands same-day imaging and decompression.))
• ((Lumbar X-ray::Will not show the disc or cord.))
• ((CT abdomen::Not the right modality for cauda equina.))

Revision summary

➡ Disc prolapse: annulus fibrosus fails → posterolateral herniation → compresses traversing root one level below (L4/5 disc → L5; L5/S1 disc → S1).

➡ Root patterns: L4 = knee jerk + medial shin; L5 = big toe extension + dorsum of foot; S1 = ankle jerk + lateral foot + plantarflexion.

➡ Cauda equina red flags: saddle anaesthesia, urinary retention, faecal incontinence, bilateral leg weakness — emergency MRI.

➡ Cord compression (above L1) = UMN signs (hyperreflexia, upgoing plantars). Cauda equina = pure LMN.

➡ Spinal stenosis: neurogenic claudication relieved by flexion (trolley sign, walking uphill). Ligamentum flavum hypertrophy is a key contributor.

➡ Ankylosing spondylitis: young man, HLA-B27, reduced chest expansion, bilateral sacroiliitis, bamboo spine, dagger sign.

➡ Spondylolysis = pars interarticularis defect (Scottie dog collar). Spondylolisthesis = vertebral slip (palpable step).

➡ Osteoporotic fracture: elderly woman, minor trauma, normal calcium/phosphate/ALP.

➡ Sclerotic mets = prostate (raised PSA). Lytic = myeloma, breast, lung, renal, thyroid.

➡ Myeloma: anaemia, hypercalcaemia, renal failure, Bence Jones, lytic lesions, rouleaux.

➡ Discitis/osteomyelitis: Staph aureus most common; TB causes Pott's disease.

➡ Artery of Adamkiewicz: dominant feeder to lower anterior cord; infarction = paraplegia + bladder dysfunction + preserved proprioception (anterior cord syndrome).

➡ MRI is the gold standard for spinal cord, discs, roots, marrow and infection.