What Causes Spasticity in Multiple Sclerosis
Spasticity in MS is caused by demyelination and axonal damage in the central nervous system, particularly affecting the corticospinal tracts and motor pathways, which disrupts normal inhibitory control of muscle tone and leads to velocity-dependent increases in muscle tone and exaggerated reflexes.
Pathophysiological Mechanisms
Structural Damage
The primary cause of MS spasticity involves two key pathological processes 1, 2:
- Myelin degradation: Loss of the myelin sheath around nerve fibers disrupts normal nerve conduction
- Axonal damage: Direct injury to nerve fibers (axons) that begins in early disease stages and accumulates over time
- Demyelinating lesions: These occur in critical motor control areas including the thalamus, lenticular nucleus, globus pallidus, and internal capsule 3
More than 80% of MS patients experience spasticity at some point during their disease course 1, 2.
Dynamic Inflammatory Components
Beyond structural damage, ongoing neuroinflammation creates dynamic changes in motor circuit function 1:
- Inflammatory mediators: Cytokines, prostaglandins, and reactive oxygen species directly affect neuronal circuits
- Increased axonal sensitivity: Demyelinated axons become hyperexcitable, leading to abnormal signal transmission
- Stress hormones: These contribute to fluctuations in muscle tone
This inflammatory component explains why spasticity severity can vary day-to-day in MS patients, unlike static spasticity from spinal cord injury.
Clinical Manifestation
Spasticity in MS is defined as velocity-dependent hyperactivity of tonic stretch reflexes 4. The condition manifests as:
- Increased muscle tone that worsens with rapid movement
- Exaggerated reflexes
- Muscle spasms and cramps
- Pain and functional impairment
- Risk of contracture development
Disease Progression Impact
The pathophysiology differs between MS subtypes 5:
- Relapsing-remitting MS (85% of cases): Nerve conduction is affected during acute inflammatory relapses but may improve during remission phases
- Primary-progressive MS (15% of cases): Characterized by progressive neurological damage without distinct relapses
Over time, repeated relapses cause extensive myelin scarring with progressive loss of neuronal function, leading to more severe and persistent spasticity.
Key Clinical Considerations
Important caveat: The autoimmune nature of MS means spasticity is complicated by ongoing neuronal network damage, creating permanent changes in the clinical picture that distinguish it from spasticity in non-progressive conditions 1. Additionally, some disease-modifying therapies used for MS immunotherapy may paradoxically worsen spasticity, while antispasticity medications can potentially alter immune function 1.
The brainstem plays a central role in mediating spasticity and related symptoms (sleep, bladder function, pain), which explains why these symptoms often cluster together in what has been termed "spasticity-plus syndrome" 6.