Spastic Paraparesis in PACES: Signs, Differential and Viva

How it presents in PACES

In the neurology station you meet a patient with weak, stiff legs and a scissoring or dragging spastic gait. It is worth saying up front: spastic paraparesis is a syndrome, not a diagnosis. Your task is to demonstrate bilateral upper motor neurone (UMN) signs in the legs, localise the lesion to the spinal cord until proven otherwise, and offer a sensible differential driven by the sensory pattern.

The classic opening picture is increased tone in the legs, brisk reflexes with clonus, and a pyramidal pattern of weakness. A useful early discriminator is that in a cord or pyramidal lesion the stiffness tends to outstrip the actual weakness, and the patient reports more trouble than their examination alone would suggest — a feature most striking in cervical myelopathy. Report markers of chronicity as you go: distal wasting, a walking aid or wheelchair, a catheter, or trophic changes, since these speak to both duration and complications.

Signs to look for

Run through this focused checklist — the discriminators here are what separate a clean localisation from a vague description:

• Bilateral UMN signs in the legs: increased (clasp-knife, velocity-dependent) tone, hyperreflexia, ankle clonus and upgoing plantars.
• A pyramidal pattern of weakness — the leg flexors give way before the extensors, so hip flexion, knee flexion and ankle dorsiflexion are hit first. Calling this 'global' weakness is a recognised mark-loser.
• A scissoring, stiff, dragging gait — offer to watch the patient walk if safe.
• Distinguish paraparesis (legs only) from quadri-/tetraparesis (arms involved implies a lesion above C4), and actively hunt for a sensory level.
• Test two sensory modalities — pinprick AND dorsal columns (vibration/proprioception). Skipping dorsal columns is how SACD gets missed.
• Examine the back and neck for scars, deformity or a tuft of hair; check upper-limb reflexes for a cervical level; and offer a PR examination for anal tone and saddle anaesthesia.
• In the arms, look for cervical myelopathy signs: Hoffmann's, inverted supinator jerk, finger-escape and the myelopathy hand.
• Ask about Lhermitte's — an electric tingle down the spine on neck flexion (MS, cervical myelopathy, SACD).

Confirming the diagnosis and differential

The examiner's framework is to split the causes by the sensory pattern — build your differential the same way and you will sound structured rather than scattergun.

Normal sensation: think hereditary spastic paraplegia (HSP), motor neurone disease (which adds LMN signs and fasciculations), a parasagittal/cortical lesion such as a meningioma, small-vessel subcortical disease, or longstanding cerebral palsy. A clear sensory level points to a transverse cord lesion — compressive (cervical/thoracic spondylotic myelopathy, the treatable one, plus metastases, meningioma, abscess, trauma, TB), transverse myelitis, or cord infarct. Patchy bilateral sensory loss with upper-limb myelopathic signs points to cervical myelopathy.

A dissociated level (pain and temperature lost, vibration and proprioception preserved) points to anterior spinal artery infarction. Isolated dorsal-column loss suggests subacute combined degeneration (B12, copper or vitamin E), tabes or MS. Cape-like dissociated loss with wasted hands suggests syringomyelia. Learn the 'spastic paraparesis plus' additions too: plus cerebellar signs raises MS, Friedreich's ataxia or a craniospinal-junction lesion; plus a peripheral neuropathy raises Friedreich's or HSP variants.

One combination is a perennial favourite: spastic paraparesis with upgoing plantars but absent ankle jerks (mixed UMN and LMN). The list is SACD, tabes, Friedreich's, MND, a conus/cauda lesion, and — the commonest in real life — dual pathology, classically cervical myelopathy plus diabetic peripheral neuropathy. Do not forget the infective bucket either: HTLV-1 tropical spastic paraparesis, HIV vacuolar myelopathy and syphilis are easily overlooked.

The viva: what examiners ask

Investigations, classification and management are where the conversation goes. Structure your answer so the first thing out of your mouth is exclusion of a treatable cause.

• Investigations, first line: an urgent dedicated MRI of the whole spine (± brain) with contrast to actively exclude cord compression — the one step you cannot skip. Plain spine films are only a stopgap.
• Bloods: B12 (with methylmalonic acid/homocysteine), copper and caeruloplasmin, vitamin E, VDRL/TPHA, HTLV-1, HIV, inflammatory/autoimmune markers, and glucose/HbA1c for dual pathology.
• CSF for cells, protein, oligoclonal bands and viral PCR; neurophysiology (NCS/EMG, VEPs) to define mixed UMN/LMN pictures or demyelination.
• Genetic testing (SPG/spastin panel) for HSP — but only once acquired and treatable causes are excluded, since HSP is a diagnosis of exclusion.
• Management — treat the cause: surgical decompression for compressive/cervical myelopathy; B12 or copper replacement for SACD; disease-modifying therapy or pulsed steroids for MS; antivirals/antiretrovirals for infective myelitis; secondary prevention and cord perfusion for anterior spinal artery infarction.
• Symptomatic care: spasticity with baclofen (± gabapentin, tizanidine, botulinum toxin); bladder with anticholinergics or intermittent self-catheterisation; plus physiotherapy, orthotics, walking aids and genetic counselling for hereditary causes.

Common pitfalls and how to score

Most lost marks here are predictable. Guard against them deliberately.

• Not stating you would exclude cord compression first. Open every spastic paraparesis discussion with urgent MRI whole spine — omitting it is treated as unsatisfactory.
• Not testing dorsal columns, which lets SACD masquerade as a 'pure' spastic paraparesis.
• Over-committing to HSP before you have addressed cervical myelopathy — the treatable partner. Split them on upper-limb myelopathic signs and sensory pattern, then confirm on MRI.
• Calling pyramidal leg weakness 'global' — describe the flexor-weaker-than-extensor pattern instead.
• Forgetting the dual-pathology and infective buckets (HTLV-1, HIV, syphilis).
• Excess plantar attempts or rough examination — a patient-welfare penalty. Elicit the sign once, cleanly, and move on.