Back pain

 Back pain is common — but as a med student, your job isn’t just to recognise it. It’s to understand it. That means knowing the underlying pathophysiology, linking it to the history, and spotting red flags.

Let’s discuss one of the most common clinical presentations — and what’s going on beneath the surface 👇 

🧍‍♂️ Case:

A 45-year-old man presents with lower back pain after lifting a heavy box at work yesterday.

He describes it as a dull ache in the lumbar region, worse with movement, better at rest. No leg pain, numbness, or red flags.

🤔 Diagnosis? Most likely a lumbar strain or mechanical back pain — but how does that actually happen?

🔬 Pathophysiology: Mechanical Back Pain

➡️ Muscle or ligament strain:

Sudden load or overuse can cause microtears in paraspinal muscles or supporting ligaments → leading to local inflammation and pain mediated by prostaglandins and cytokines.

Pain is usually localised, worsens with movement, and improves with rest. No neurological findings.

➡️ Intervertebral disc injury / disc herniation:

Lifting or twisting can increase pressure on lumbar discs → nucleus pulposus pushes through the annulus fibrosus → if it compresses a nearby nerve root → radicular pain (sciatica), sensory loss, or weakness.

Pain often radiates down the leg in a dermatomal distribution.

➡️ Facet joint dysfunction:

The small posterior joints of the spine (zygapophyseal joints) can become irritated or inflamed → causing localised pain, often worse with extension or rotation.

🧠 Bottom line: Mechanical back pain usually arises from dysfunction in muscles, discs, or joints. The exact structure affected drives the clinical pattern.

🧠 Differential Diagnosis by Mechanism of Injury

🔹 Sudden lifting / twisting

→ Think: muscle strain, disc herniation, facet joint irritation

🔹 Gradual onset, no injury

→ Consider: postural strain, chronic mechanical pain, axial spondyloarthritis

🔹 Recent trauma or fall

→ Especially in older adults → suspect vertebral compression fracture (don’t forget osteoporosis)

🔹 Fever, IVDU, recent infection

→ ⚠️ Red flag for discitis or epidural abscess

🔹 Cancer history, night pain, weight loss

→ ⚠️ Consider spinal metastases

🔹 Radiating pain, numbness, weakness

→ Think: radiculopathy due to nerve root compression (e.g. L4/L5, S1)

🔹 Saddle anaesthesia, urinary retention

→ ⚠️ Emergency: cauda equina syndrome

🚨 Cauda Equina Syndrome: Know It. Never Miss It.

Cauda equina syndrome occurs when there is compression of the bundle of lumbar and sacral nerve roots below the spinal cord (typically from L2 down).

Common causes include a large central disc herniation, tumour, trauma, or abscess.

🧠 Pathophysiology: These are peripheral nerves, and prolonged compression can lead to irreversible damage.

The hallmark features include:

•  Saddle anaesthesia (numbness in the perineum)
•  Urinary retention or incontinence
•  Bowel dysfunction
•  Bilateral leg weakness or numbness

👉 This is a neurosurgical emergency. Think fast, act fast — urgent MRI + decompression can preserve bladder and limb function.

📚 Pearl:

Back pain is a symptom, not a diagnosis.

Understanding the underlying structure and pathophysiology helps you differentiate a simple strain from a surgical emergency.

💬 Want to dive deeper into dermatomes, radiculopathy patterns, or neurophysiology of pain next? Let me know in the comments 👇

#BackPain #MedStudent #ClinicalReasoning #MedSchool #UOWMedicine