Dysarthria is a motor speech disorder resulting from neurological damage that impairs the muscle control used for speaking. This condition affects the physical execution of speech, including the strength, speed, range, and accuracy of movements required for breathing, voice production, resonance, and articulation. The difficulty is not in understanding language or formulating thoughts, but in the physical ability to produce clear, intelligible words. Damage to different parts of the nervous system produces distinct patterns of muscle impairment, allowing classification into several types based on acoustic characteristics.
Flaccid and Spastic Forms of Dysarthria
These two types represent damage to the distinct motor pathways that control muscle movement. Flaccid dysarthria arises from damage to the lower motor neurons (LMNs), which directly innervate the speech muscles. Injury to these peripheral nerves results in muscle weakness and hypotonia (reduced muscle tone), preventing the muscles from generating sufficient force for speech. This weakness is characterized by a continuous breathy voice quality, hypernasality due to a weak velum, and often short phrases due to poor respiratory support.
Spastic dysarthria, in contrast, results from bilateral damage to the upper motor neurons (UMNs). Damage to these pathways leads to muscle tightness (hypertonia) and overactive reflexes. The speech output is described as slow and effortful, with a characteristic strained or strangled voice quality that lacks pitch variation. The slow rate and reduced range of motion lead to imprecise consonant articulation and a monotonous delivery.
Ataxic Dysarthria
Ataxic dysarthria is caused by damage to the cerebellum, the brain structure responsible for coordinating movement. The primary impairment is a lack of muscle coordination (ataxia), which affects the timing, force, and accuracy of speech movements. This discoordination manifests as a speech pattern often described as sounding “drunk.”
A hallmark feature of ataxic dysarthria is “scanning speech,” characterized by a slow, deliberate rate with excess and equal stress placed on syllables. Rhythmic disruptions include irregular articulatory breakdowns and prolonged intervals between sounds and words. The overall effect is a jerky, poorly controlled rhythm, resulting in distorted vowels and excessive loudness variations.
Hypokinetic and Hyperkinetic Dysarthrias
Hypokinetic and hyperkinetic dysarthrias originate from pathology within the basal ganglia control circuit. This system modulates the amplitude, velocity, and initiation of movement, and dysfunction results in either a reduction or an excess of movement.
Hypokinetic dysarthria is most strongly associated with Parkinson’s disease and is characterized by a reduction in the range and speed of movement. Speech features include significantly reduced loudness (hypophonia), a monotone voice, and a rigid facial expression. The rate of speech often accelerates into short, rapid bursts, sometimes called rushes of speech, which blur articulation. This rapid, imprecise speech contributes significantly to poor intelligibility.
Hyperkinetic dysarthria is defined by the presence of abnormal, involuntary movements that disrupt speech. These movements, such as tremors, tics, or chorea (seen in conditions like Huntington’s disease), unpredictably interfere with the vocal folds, tongue, and jaw. The resulting speech profile is jerky and irregular, featuring sudden voice arrests, transient changes in loudness, and inappropriate silences. The overall effect is a variable rate and loudness, with prolonged sounds and intermittent articulatory distortions.
Mixed Dysarthria
Mixed dysarthria is diagnosed when neurological damage is diffuse, affecting two or more components of the motor system. This results in a combination of the perceptual characteristics of two or more pure dysarthria types.
Mixed dysarthria is the most common form of the disorder in clinical practice because many neurological diseases affect multiple areas of the nervous system simultaneously. For example, Amyotrophic Lateral Sclerosis (ALS) frequently causes a mixed flaccid-spastic dysarthria due to the degeneration of both upper and lower motor neurons. Multiple Sclerosis (MS) often presents as a mixed spastic-ataxic dysarthria, reflecting damage to the central motor pathways and the cerebellum. Traumatic brain injury (TBI) and stroke are also common causes of mixed presentations.