Why Multiple Sclerosis Causes Pyramidal Signs
Multiple sclerosis causes pyramidal signs because demyelinating lesions preferentially affect the periventricular white matter where corticospinal tract fibers are densely concentrated, disrupting motor signal transmission from the motor cortex to the spinal cord. 1
Anatomical Basis of Pyramidal Involvement
The corticospinal tracts are particularly vulnerable in MS due to their anatomical location and the disease's predilection for specific CNS regions:
Periventricular lesion distribution: MS lesions characteristically occur in periventricular white matter, directly contacting the lateral ventricles without intervening tissue, where corticospinal fibers course in close proximity 2
Dawson's fingers orientation: Lesions oriented perpendicular to the ventricles ("Dawson's fingers") are pathognomonic for MS and frequently intersect descending motor pathways 2
Spinal cord involvement: MS commonly affects the spinal cord with focal demyelinating lesions that directly damage corticospinal tracts as they descend, producing upper motor neuron signs below the level of the lesion 1, 3
Pathophysiological Mechanism
The pyramidal signs result from the specific pathology of MS lesions affecting motor pathways:
Demyelination disrupts saltatory conduction: Inflammatory demyelination strips the myelin sheath from axons, impairing rapid nerve impulse transmission along corticospinal fibers 4, 5
Axonal transection causes permanent deficits: Beyond demyelination, MS involves axonal transection and neuronal loss, leading to irreversible pyramidal dysfunction 4, 5
Chronic neurodegeneration: Progressive MS involves chronic diffuse neurodegeneration with activated microglial inflammation that continues damaging motor pathways independent of acute relapses 5
Clinical Manifestations
Pyramidal signs in MS typically present as:
Motor weakness: Unilateral or bilateral weakness affecting limbs, often asymmetric initially 4, 6
Spasticity: Increased muscle tone from loss of descending inhibitory control 6, 7
Hyperreflexia and Babinski sign: Exaggerated deep tendon reflexes and extensor plantar responses indicating upper motor neuron dysfunction 3, 6
Impaired coordination: Though often attributed to cerebellar involvement, corticospinal tract damage contributes to motor incoordination 4, 6
Important Clinical Distinctions
A critical caveat: Not all MS patients exhibit typical pyramidal signs throughout their disease course. In certain atypical presentations, particularly pure motor variants with acute motor axonal neuropathy (AMAN) subtype on electrophysiology, normal or even exaggerated reflexes might be observed—though this description comes from Guillain-Barré syndrome literature and should not be conflated with MS 1
The presence of pyramidal tract signs can help differentiate MS from mimics:
Bickerstaff brainstem encephalitis may present with impaired consciousness and pyramidal tract signs but typically includes anti-GQ1b antibodies 1
Neuromyelitis optica spectrum disorders show cloud-like enhancement patterns and longitudinally extensive spinal cord lesions rather than the typical periventricular MS pattern 1, 3
Imaging Correlation
MRI findings that correlate with pyramidal dysfunction include:
T2/FLAIR hyperintense lesions in periventricular white matter and corona radiata where corticospinal fibers concentrate 1, 2
Spinal cord lesions producing dermatomal sensory levels and pyramidal signs below the lesion level 3
Chronic T1 hypointense lesions ("black holes") indicating severe demyelination and axonal loss in motor pathways, correlating with permanent pyramidal deficits 1
The severity and persistence of pyramidal signs correlate with the extent of irreversible tissue damage rather than acute inflammatory activity alone 5