Why does damage to the cortical motor (cerebral motor) areas cause hypotonia?

Why Cortical Motor Damage Causes Hypotonia

Acute damage to cortical motor areas, particularly the supplementary motor area (SMA), causes hypotonia because it disrupts descending supraspinal inhibitory pathways that normally suppress spinal reflex circuits, resulting in temporary loss of tonic drive to alpha motor neurons before compensatory mechanisms develop. 1

The Paradox of Upper Motor Neuron Injury

The classical teaching that upper motor neuron (UMN) lesions cause hypertonia and hyperreflexia is incomplete and misleading for acute presentations:

• Acute UMN injuries consistently produce hypotonia and hyporeflexia initially, not the expected hypertonia—this occurs after stroke, spinal shock, and cortical lesions 1, 2
• The traditional explanation attributing hyperreflexia to "loss of descending inhibition" fails to explain why acute lesions cause the opposite finding: hyporeflexia 1
• The supplementary motor area (SMA) plays a dominant role in maintaining baseline muscle tone through descending inhibitory control, and its acute disruption unmasks the hypotonic state 1

The Mechanism: Loss of Supraspinal Tonic Drive

The pathophysiology involves disruption of complex tonic drive networks:

• Multiple supraspinal centers (including SMA, motor cortex, brainstem nuclei) continuously modulate spinal motor neuron excitability to maintain baseline muscle tone 2, 3
• Cortical motor damage acutely removes this tonic facilitatory drive to alpha motor neurons, resulting in decreased muscle activation and hypotonia 1, 3
• The SMA specifically sends inhibitory projections through corticospinal tracts that normally suppress certain spinal reflex circuits—when damaged, the loss of this organized inhibition paradoxically reduces overall motor output 1
• Muscle tone requires continuous neural input; passive tissue properties alone cannot maintain normal tone without active neural regulation 3

Clinical Evidence: Supplementary Motor Area Syndrome

The clearest demonstration comes from postoperative SMA syndrome:

• Patients develop acute hypotonia, hyporeflexia, and motor deficit after SMA resection despite intact primary motor cortex connections to the spinal cord 1
• This proves that cortical areas beyond M1 are essential for maintaining normal tone through their descending influences 1
• The phenomenon generalizes to explain hypotonia after stroke, atonic seizures, and spinal shock 1

Distinguishing Central from Peripheral Hypotonia

Understanding the mechanism helps differentiate cortical (central) from lower motor neuron causes:

• Central hypotonia from cortical damage preserves or increases deep tendon reflexes (once the acute phase resolves), unlike peripheral causes that diminish reflexes 4, 5
• Creatine kinase remains normal or only mildly elevated in central hypotonia, whereas peripheral muscular causes show significant elevation 4, 5
• The presence of hypotonia with preserved reflexes rules out cerebral palsy in its typical spastic form, which presents with increased tone and hyperreflexia 5

The Temporal Evolution

The hypotonic phase is typically temporary:

• Acute cortical lesions initially cause hypotonia due to loss of tonic drive 1, 2
• Over time (days to weeks), compensatory mechanisms and altered spinal circuit excitability can lead to the development of spasticity 2
• The loss of supraspinal control mechanisms eventually gives rise to hypertonia as disinhibited spinal reflexes become hyperactive 2

Clinical Implications

For diagnostic evaluation:

• Do not assume all UMN lesions present with hypertonia—acute cortical damage characteristically causes hypotonia initially 1
• The presence of hypotonia does not exclude central nervous system pathology; timing and associated findings are critical 4, 6
• Hypotonia can be a symptom of over 500 different genetic disorders affecting central, peripheral, or combined pathways, requiring systematic diagnostic testing 6

The key insight is that normal muscle tone depends on continuous, organized supraspinal input—particularly from the SMA and related cortical motor areas—and acute disruption of these pathways removes the tonic drive necessary to maintain baseline muscle activation, resulting in hypotonia until compensatory mechanisms develop.