Causes of Bilateral Spastic Paraparesis
Vitamin B12 deficiency (Option C) is a well-established cause of bilateral spastic paraparesis, presenting with subacute combined degeneration of the spinal cord affecting the corticospinal tracts bilaterally. 1, 2, 3
Analysis of Each Option
Option C: Vitamin B12 Deficiency - CORRECT
Vitamin B12 deficiency causes subacute combined degeneration affecting the corticospinal tracts bilaterally, resulting in spastic paraparesis with upper motor neuron signs including hyperreflexia, spasticity, and extensor plantar responses (Babinski sign). 1, 2
The mechanism involves demyelination of the lateral and posterior columns of the spinal cord, with the corticospinal tract involvement producing the characteristic bilateral spastic weakness of the lower extremities. 2, 3
Clinical presentation includes progressive bilateral lower limb weakness, sensory loss, and sphincter dysfunction, as documented in case reports showing patients presenting with sudden onset bilateral lower limb paraparesis that responded to B12 replacement. 2
Spinal MRI should be performed to exclude other causes when evaluating myelopathy, as recommended for differential diagnosis of hepatic myelopathy which similarly requires exclusion of vitamin B12 deficiency. 1
Option A: Cerebellar Disease - INCORRECT
Cerebellar lesions produce ataxia, dysmetria, and intention tremor—not spastic paraparesis. The cerebellum does not contain upper motor neurons or corticospinal tracts. 1
Cerebellar dysfunction presents with hypotonia and hyporeflexia, which is the opposite of the hyperreflexia and spasticity seen in corticospinal tract lesions. 1
Option B: Peripheral Neuropathy - INCORRECT
Peripheral neuropathies affect lower motor neurons and produce flaccid weakness with hyporeflexia or areflexia, not the spasticity and hyperreflexia characteristic of upper motor neuron lesions. 1
Guillain-Barré syndrome, a classic peripheral neuropathy, presents with areflexia and ascending flaccid paralysis, demonstrating that peripheral nerve disorders do not cause spastic paraparesis. 1
Option D: Parkinson Disease - INCORRECT
Parkinson disease is a basal ganglia disorder causing bradykinesia, rigidity, and tremor—not spastic paraparesis. The rigidity in Parkinson's is "lead-pipe" or "cogwheel," distinct from spasticity. 1
Parkinson disease does not involve corticospinal tract dysfunction and therefore does not produce upper motor neuron signs like hyperreflexia or Babinski sign. 1
Additional Causes to Consider
Bilateral cerebral lesions affecting the motor cortex or internal capsule can produce bilateral spastic paraparesis, particularly with parasagittal lesions affecting the leg area of the motor homunculus. 1
Spinal cord compression from any cause (tumor, abscess, Pott's disease) affecting the thoracic cord bilaterally produces spastic paraparesis below the level of the lesion. 3, 4
Multiple sclerosis and other demyelinating diseases commonly present with bilateral spastic paraparesis due to demyelination of corticospinal tracts. 3
HTLV-1 infection causes tropical spastic paraparesis, a progressive myelopathy endemic in certain regions. 3
Hereditary spastic paraplegias represent a group of genetic disorders causing progressive bilateral lower extremity spasticity. 5, 6