Spinal Stenosis MRI: Diagnosis Guide

Spinal stenosis — narrowing of the spinal canal or the neural foramina (the tunnels where nerves exit the spine) — is one of the most common causes of back and leg pain in adults over 50. It affects around 11% of the older UK population and is the leading reason for spinal surgery in people aged 65 and over. MRI is the definitive investigation for diagnosing spinal stenosis, showing exactly where the narrowing is, how severe it is, and which nerves are affected.

If walking has become increasingly difficult because your legs feel heavy, weak, or numb after a few minutes, and sitting down relieves the symptoms, spinal stenosis is a strong possibility — and an MRI will confirm it.

What Is Spinal Stenosis?

The spinal canal is the bony tunnel that protects the spinal cord and nerve roots. Several structures can narrow this space over time:

• Disc bulging or herniation — degenerative disc disease causes discs to bulge into the canal.
• Facet joint hypertrophy — the small joints at the back of the spine thicken with arthritis.
• Ligamentum flavum thickening — this ligament, which runs along the back of the spinal canal, thickens with age. Normal thickness is 2-4mm; in stenosis it can be 6-8mm or more.
• Spondylolisthesis — one vertebra slipping forward on another, narrowing the canal at that level.
• Bone spurs (osteophytes) — bony projections from the vertebral bodies or facet joints.

All of these changes are part of normal ageing, and many people have some degree of canal narrowing without symptoms. Stenosis becomes a problem when the narrowing is enough to compress nerves.

Types of Spinal Stenosis

• Central stenosis — narrowing of the main spinal canal. Compresses the cauda equina (bundle of nerve roots below L1-L2). Causes neurogenic claudication — leg pain, heaviness, and weakness with walking, relieved by sitting or leaning forward.
• Lateral recess stenosis — narrowing of the space where nerve roots sit before exiting through the foramen. Compresses the traversing nerve root.
• Foraminal stenosis — narrowing of the bony tunnel where the nerve root exits the spine. Compresses the exiting nerve root. Can cause radiculopathy (sciatica-type leg pain).

Neurogenic Claudication — the Classic Symptom

The hallmark symptom of lumbar spinal stenosis is neurogenic claudication:

• Leg pain, heaviness, numbness, or weakness that comes on with walking or standing
• Symptoms typically affect both legs (though can be asymmetric)
• Relieved by sitting, leaning forward, or bending (these positions open the spinal canal slightly)
• People with spinal stenosis often find they can walk further when pushing a shopping trolley (because they lean forward)
• Walking distance gradually decreases over months to years

This must be distinguished from vascular claudication (poor blood supply to the legs), which also causes leg pain with walking. Vascular claudication is relieved by simply standing still; neurogenic claudication requires sitting or bending forward.

What Spinal Stenosis Looks Like on MRI

The radiologist assesses stenosis at every lumbar level. Key findings include:

• Spinal canal cross-sectional area — measured on axial images. Normal canal area is roughly 150-200mm². Moderate stenosis: 75-100mm². Severe stenosis: below 75mm².
• AP (anterior-posterior) diameter — measured on sagittal images. Normal is over 12mm. Below 10mm is considered stenotic.
• Nerve root compression — visible bunching or loss of CSF (bright fluid) around the nerve roots. The Schizas classification grades central stenosis from A (normal) to D (severe, with no visible CSF and nerve roots indistinguishable).
• Contributing structures — the radiologist documents what is causing the narrowing: disc, facet hypertrophy, ligamentum flavum thickening, spondylolisthesis, or a combination.
• Number of levels affected — single-level vs multilevel stenosis. This affects surgical planning.
• Foraminal stenosis — graded by how much fat signal remains around the nerve root in the foramen. Grade 0 (normal) to grade 3 (no fat, nerve root compressed).

MRI Sequences

• T2-weighted sagittal — the best overview. CSF appears bright, making areas of compression clearly visible as "waisting" or loss of the bright fluid signal.
• T2-weighted axial — cross-sectional view at each level, showing the canal area and nerve root position.
• T1-weighted — good anatomical detail, especially for foraminal stenosis.
• STIR — highlights any active inflammation or oedema.

What MRI Cannot Show

• Dynamic stenosis — MRI is done lying down. The spinal canal is actually slightly wider when lying down than when standing or walking. Upright MRI or CT myelogram in the standing position may be needed if standard MRI does not match the clinical picture.
• Symptom severity — some people with severe stenosis on MRI have mild symptoms, while others with moderate stenosis are severely disabled. Clinical correlation is essential.
• Vascular claudication — MRI of the spine does not assess the blood vessels in the legs. If vascular claudication is suspected, ankle-brachial pressure index (ABPI) or arterial Doppler ultrasound is needed.

Alternative Diagnostic Approaches

• CT scan — better for bony detail. Shows osteophytes and facet joint changes well. Less good for soft tissue (disc, ligaments, nerve roots). Sometimes used alongside MRI for surgical planning.
• X-ray — shows alignment, spondylolisthesis, and disc height loss. Flexion-extension X-rays assess dynamic instability. Cannot show the canal directly.
• CT myelogram — CT after injection of contrast into the spinal canal via lumbar puncture. Shows nerve root compression directly. Used when MRI is not possible (e.g. pacemaker) or when MRI findings are unclear.
• Nerve conduction studies (NCS/EMG) — can help distinguish neurogenic claudication from peripheral neuropathy.

Treatment Pathways

• Conservative management (first line) — physiotherapy (core strengthening, flexion-based exercises, cycling), activity modification (using a walking aid if helpful), pain medication (NSAIDs, neuropathic painkillers), and weight management.
• Epidural steroid injection — can provide temporary relief (weeks to months) for both central stenosis and foraminal stenosis. Useful as a bridge to surgery or for patients who cannot have surgery.
• Surgical decompression (laminectomy) — the standard surgery for spinal stenosis. The lamina (bony arch at the back of the vertebra) and thickened ligamentum flavum are removed to create more space for the nerves. Can be done at one level or multiple levels. Success rates: around 70-80% of patients have significant improvement in walking distance and leg symptoms. Usually involves an overnight stay.
• Laminectomy with fusion — decompression combined with spinal fusion (joining two or more vertebrae together with screws and rods). Needed when there is associated instability (spondylolisthesis) or when decompression alone would destabilise the spine.
• Minimally invasive decompression — newer techniques (interspinous process devices, endoscopic decompression) aim to achieve decompression through smaller incisions. Evidence is still evolving.

UK Statistics

• Lumbar spinal stenosis affects approximately 11% of the older UK population
• It is the most common indication for spinal surgery in people over 65
• Around 5,000-8,000 lumbar decompression operations are performed in England annually
• Peak age of diagnosis is 60-70 years
• Walking distance typically decreases gradually over years without treatment

Frequently Asked Questions

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