Dysarthria Health Article

Definition

Dysarthria is a speech diagnostic term that can be used to classify various types of neuromuscular speech disturbances. Dysarthria results from notable degrees of one or more abnormalities involving speech musculature, including weakness, paralysis, incoordination, sensory deprivation, exaggerated reflex patterns, uncontrollable movement activities, and excess or reduced tone. Generally speaking, the dysarthrias are considered motor speech disorders because speaking difficulties are largely due to breakdowns in movement control of one or more muscle groups that compose the speech mechanism. The name of each dysarthria subtype is partially derived from the basic characteristics of the overlying movement disturbances. Notably, normal speech production involves the integration and coordination of five primary physiological subsystems: respiration (breath support); phonation (voice production); articulation (pronunciation of words); resonation (nasal versus oral voice quality); and prosody (rate, rhythm, and inflection patterns of speech).

Description

The pioneering works of Darley, Aronson, and Brown in 1975 led to the general model of dysarthria classification that continues to be used to date. These clinical researchers from the Mayo Clinic studied individuals with different neurological disorders for the primary purpose of identifying and describing in detail the various speech problems that they exhibited. These analyses helped to formulate predictable subtypes of speech abnormalities in individuals with specific kinds of neuropathologies. Besides the six primary forms of dysarthria identified, a seventh type has been added to the differential diagnostic scheme in the past decade. The seven dysarthria subtypes are spastic, unilateral upper motor neuron, ataxic, hypokinetic, hyperkinetic, flaccid, and mixed.

Demographics

There are no known figures regarding the overall incidence of the various dysarthrias in the general population. Moreover, because numerous possible neuropathological conditions can result in dysarthria, it is unproductive to speculate about either the specific or overall demo-graphics of this multi-varied disorder.

Spastic dysarthria

Spastic dysarthria is caused by damage to the primary voluntary motor pathways, which originate in the frontal lobes of the brain and descend to the brainstem and spinal cord. These central tracts constitute the pyramidal or upper motor neuron (UMN) system. Virtually all individuals with spastic dysarthria present with a broad spectrum of speech disturbances, including:

• abnormally excessive nasal speech quality
• imprecise articulation behaviors such as slurred sound productions and periods of speech unintelligibility
• slow-labored rate of speech
• strained or strangled voice quality
• limited vocal pitch and loudness range and control
• incoordinated, shallow, forced, uncontrolled, and overall disruptive speech breathing patterns

Individuals with spastic dysarthria often suffer from co-occurring weakness and paralysis of all four limbs. This occurs because the nerve tracts that supply movement control to these structures run in close parallel to those that regulate muscles of the speech mechanism, thereby making them equally susceptible to damage. The specific combination and severity of these features tend to vary from person to person based on the extent of associated UMN damage. In general, people with spastic dysarthria struggle with these speech difficulties because of widespread involvement of the tongue, lip, jaw, soft palate, voice box, and respiratory musculature. Problems with emotional breakdowns, such as unprovoked crying and laughing, also occur in many cases, due to uncontrolled releases of primitive reflexes and behaviors normally regulated, in part, by a mature and healthy UMN system. Finally, swallowing difficulties, known as dysphagia, are not uncommon in this population, because of underlying weakness and paralysis of the tongue and throat wall muscles.

The most common causes of spastic dysarthria include spastic cerebral palsy, multiple sclerosis, amyotrophic lateral sclerosis (ALS, or Lou Gehrig's disease), multiple strokes, and closed head injuries (particularly those that cause damage to the brainstem where the UMN tracts converge on the way to nerves that directly connect with the various muscles of the head, neck, limbs, and girdle).

Unilateral upper motor neuron (UMN) dysarthria

Unilateral UMN dysarthria is caused by damage to either the left or right UMN tract, anywhere along its course to the brainstem and spinal cord. The individual with this diagnosis generally presents with mild to moderate weakness and paralysis of the lower face, tongue, arm, and leg on the side of the body opposite the damaged UMN tract. The hemiplegia may necessitate use of a cane or wheelchair, and the facial and tongue musculature disturbances usually only result in mild speech production and swallowing difficulties because the unimpaired opposite half of the lips and tongue often compensate well for this unilateral problem.

Speech breathing and inflection patterns, voice characteristics, and nasal resonance features are not typically abnormal in the individual with unilateral UMN dysarthria. However, it is not uncommon for this person to suffer from a significant language processing disorder (i.e., aphasia) and/or apraxia in which the brain damage also involves areas of the cortex that normally regulate motor programming and language formulation abilities.

The most common causes of this dysarthria subtype are cerebral vascular accidents (i.e., strokes) and mild-to-moderate head injuries.