Speech is one of the most complex behaviours developed by humans. In order for oral production to be performed normally it is necessary to control breathing and the muscles of the larynx, pharynx, palate, tongue and lips.
Dysarthria can be defined as a change in speech articulation due to lesions in the central nervous system as well as diseases of the nerves or muscles responsible for speech.
These impairments in articulation are manifested by omission, substitution, addition or distortion of one or more sounds, affecting the intelligibility of verbal expression. Moreover, these impairments are coupled with the difficulty in moving the mouth parts to perform any activity as sucking or blowing. The most extreme and severe dysarthria case would be anarthria or the inability to properly articulate the sounds speech (phonemes).
The assessment of dysarthria must be meticulous, since similar symptoms can result from very different disorders, both the speech assessment and the alteration of the muscles involved in breathing must be included, the larynx, soft palate, tongue, lips and jaw. In patients with dysarthria, movements of the speech muscles may be impaired with respect to strength, directionality, amplitude, time or resistance.
Dysarthria can cause paralysis, weakness and lack of coordination of the oral musculature, and can be identified by six different types based on the location of the lesion in different areas of the Central Nervous System:
- Dysarthria due to lesions in the extrapyramidal system: consisting of gray subcortical nuclei responsible for regulating muscle tone necessary for maintaining and changing posture, it organizes the movements associated with walking and facilitates automation in voluntary acts that require dexterity. Lesions in the extrapyramidal system can cause two types of dysarthria: 1) Hypokinetic: characterized by slow, restricted and stiff movements, repetitive movements in the speech muscles, a weak voice with defective articulation and lack of inflection, short sentences, lack of flexibility and control of the laryngeal muscles, tonal monotony and variability in the articulatory rate (Parkinson’s desease) and 2) Hyperkinetic: characterized by the possible impairment, successively or simultaneously, of all basic motor functions (breathing, phonation, resonance and articulation), it is impossible to predict their occurrence over time. Among the most significant disorders are:
- Ataxic dysarthria: this is due to lesions in the cerebellum, the organ that regulates the strength, speed, duration and direction of movements caused in other motor systems, causing hypotonia of the affected muscles, direction impairment, duration and intensity of movements, as well as slow and inappropriate force, possible irregular eye movements, impaired phonation with a voice that is raspy, monotonous and that has little change in intensity, distorted speech articulation and impairment to voice prosody, with an emphasis on certain syllables.
- Spastic dysarthria: this is due to damage to direct or indirect activation pathways from the cortex to the brain stem and spinal cord, which can be caused by stroke, head injury, multiple sclerosis, encephalitis, extensive tumours or degenerative diseases. The lesions to these upper motor neuromas cause weakness and spasticity on one side of the body, in the distal limb muscles, tongue and lips, resistance to passive movement of a muscle group, exaggerated muscle stretch reflexes, presence of pathological reflexes such as raising the big toe away from the other toes to stimulate the foot (Babinski sign), impaired breathing and phonation, low monotone and hoarse voice, articulation difficulties and slow speech, production of short sentences with tonal or breathing interruptions, and impaired emotional control with the sudden onset of excessive crying or laughing without any reason.
- Flaccid dysarthria: It is due to damage to the neurons of the cranial nerves or spinal cord caused by infections, cerebrovascular accidents and degenerative processes or congenital diseases. Also the cranial or spinal nerves can be affected by trauma, tumours, toxins, inflammatory processes and autoimmune, metabolic or deficiency diseases. Damage to any component of this lower motor unit involves the impairment of the voluntary, automatic and reflex movement, producing flaccidity and paralysis with decreased muscle reflexes, atrophy of the muscle fibres, weakness when exercising the eye and neck muscles, possible breathing impairment, impairment of the tongue and palate movement, decrease in the gag reflex, difficulty in swallowing, weakness of the vocal cords, and the palate and larynx, a hoarse voice with low intensity, hypernasality and distorted articulation of the consonants.
- Choreas: Disease of the nervous system characterized by involuntary movements and irregularities, slow or fast, of one or more of the muscles. The muscle tone is low and there are impairments in the coordination. These patients have slurred speech, vocal distortion, use short sentences, irregular speech and prosody impairment.
- Athetosis: is a neurological disorder characterized by involuntary and slow movements in the joint, frequent breathing and phonation problems, a raspy voice, slurred and monotone speech.
- Tremor: This is a rhythmic form of abnormal movement that sometimes causes interruptions to the emission of the voice.
- Dystonia: This is an abnormal, involuntary movement, that is slow and sustained, and includes changes in prosody, decreased pitch, audible breaths and voice tremors.
5. Mixed dysarthria: This is the most complex form of dysarthria, in which dysfunction of speech is the result of the combination of the characteristics of the motor systems involved, as in the case of tumours, inflammation, trauma, stroke, degenerative or demyelinating diseases (multiple sclerosis).
The speech therapy will be aimed at correcting defects in the articulatory production of words, either as a result of omission, replacement or distortion of sounds, to improve speech articulation and modify symptoms that significantly influence both the intelligibility of speech and the communicative faculty of the person.
To modify the symptoms presented by a patient it is necessary to intervene in several fundamental fields, such as posture, muscle tone and strength, respiratory mechanics, laryngeal deficiencies during phonation, resonance or excessive hypernasality, as well as areas covering defects in the articulation and impairment of prosody.