What is the difference between akinesia and bradykinesia?

Difference Between Akinesia and Bradykinesia

Bradykinesia refers to slowness of ongoing movement, while akinesia refers to the failure of willed movement to occur or an excessively prolonged time to initiate movement 1.

Key Distinctions

Bradykinesia

• Definition: Slowness of movement that is already in progress
• Mechanism: Results from failure to energize muscles to the level necessary to complete a movement in standard time 1
• Pathophysiology: Linked to abnormal functioning within intrinsic basal ganglia circuitry for scaling movements to appropriate magnitude 2
• Cortical involvement: Associated with abnormal functioning of cortical mechanisms, particularly affecting movement execution 3
• Clinical manifestation: Movements are performed with normally timed EMG bursts but the amount of EMG activity is underscaled relative to desired movement parameters 4

Akinesia

• Definition: Complete failure or absence of willed movement
• Two mechanisms 1:
  1. Movement is so slow and small that it cannot be seen
  2. Time needed to initiate movement becomes excessively long (prolonged reaction time)
• Pathophysiology: Linked to hypoactivation of the supplementary motor area secondary to insufficient thalamocortical facilitation 2
• Cortical involvement: Associated with brainstem mechanisms rather than cortical mechanisms 3
• Clinical manifestation: Simple reaction time is delayed, while choice reaction time remains normal 1

Clinical Context in Parkinson's Disease

Both terms are cardinal features of Parkinson's disease but represent distinct motor execution disturbances 5. Bradykinesia is the most commonly used term in modern clinical practice, particularly since the 1980s with the adoption of standardized criteria sets 6.

Important Clinical Distinctions

In Parkinson's disease specifically:

• Bradykinesia and rigidity are classic findings that respond to dopaminergic therapy 5
• Akinesia may be so severe that patients must wait for a beat of rest tremor in the agonist muscle before initiating movement 1
• The dual anatomo-functional basis suggests hypokinesia (reduced amplitude) is mediated by brainstem mechanisms while bradykinesia is mediated by cortical mechanisms 3

Common Pitfall

The terms are often used interchangeably in clinical practice, but this is imprecise 6. Akinesia represents absence or severe delay in movement initiation, while bradykinesia represents slowness of ongoing movement execution. This distinction matters for understanding the underlying pathophysiology and potentially for targeted therapeutic interventions.

Extrapyramidal Contexts Beyond Parkinson's

In hepatic encephalopathy, extrapyramidal dysfunction may include "bradykinesia, hypokinesia, monotony and slowness of speech" as common findings 7, demonstrating that these terms apply beyond primary parkinsonian syndromes.

The current terminology lacks unity and fails to capture the nuances of movement execution disturbances 6. However, for practical clinical purposes, remember: akinesia = failure to initiate or absent movement; bradykinesia = slow execution of ongoing movement.