Human speech depends on three stages – thinking of the words to be spoken (conceptualization), recalling the manner in which these words are formed (formulation) and then speaking it (vocalization). The words that are spoken depends on multiple factors including the thought or emotion at the time, input from the other senses and the manner in which it is processed and the setting.
In order to vocalize, a person has to generate sound from the voice box (phonation) and then form words from the movement of the tongue, lips and mouth (articulation). When there is a problem with creating sound from the voice box then this is known as dysphonia. It may lead to a disturbance in voice quality, volume or pitch which often presents as a whisper or even hoarse voice. When the problem lies with forming words (articulation) then it is known as dysarthria. It usually presents as slurred, distorted speech or mumbling.
What is dysarthria?
Dysarthria is the medical term for difficulty in forming words (articulation) due to pathology affecting the muscles of speech. It is a broad term that includes difficulty producing sound from the voice box although this is discussed further under dysphonia. Dysarthria is largely a muscular problem, whether it is due to dysfunction within the nerves that supply these muscles, the muscles itself or other structures of the mouth that are responsible for the clarity of speech.
It is important to note that speech is possible, albeit impaired, without the use of the larynx to generate sound (phonation). This is known as alaryngeal speech and involves using using air trapped and forced from the esophagus (esophageal speech), mouth (buccal speech) or throat (pharyngeal speech). However, when the muscles of speech and/or the nerves supplying it are affected, it is not possible to speak clearly.
Various parts of the mouth and throat are responsible for articulation. Some of these areas can be moved by voluntary actions, others are under involuntary control and there may also be areas that are fixed. All these parts work together for the clear articulation of words. These areas are referred to as the places of articulation and further classified as active and passive. It broadly includes the following structures :
Dysarthria may stem from a disorder in any of these structures. Furthermore it may arise from the disorders in the central nervous system (motor cortex, sensory cortex, basal ganglia and cerebellum) or peripheral nerves innervating the muscles of speech.
Types of Dysarthria
There are several types of dysarthria which may vary depending on the site that is affected – muscles of speech, part of the nerve that makes contact with the muscle (motor end plate) or the central nervous system. The changes in speech and concomitant clinical features may provide a further indication of the type of dysarthria.
In this type of dysarthria, the disorder is located in the brain stem. It leads to a slurred speech with a nasal tone. Other features may include dysphagia (difficulty swallowing), diplopia (double vision) and ataxia (loss of bodily movements.
Here the disorder arises from the basal ganglia and the speech is slow and forced. Patients may also report twitching (involuntary muscle contractions).
In this type of dysarthria, the disorder lies with the motor end plate – the junction between the motor nerves and muscles. Speech is not clear and this may fluctuate to varying degrees. Furthermore the voice may be whispery and the patient is fatigued when speaking. Weakness of the face and neck muscles, drooping of the eyelid (ptosis) and double vision (diplopia).
In this case, the muscles of speech are affected. Speech is indistinct and there is a difficulty in forming words due to the impaired movement of the muscles. Patients also present with weakness of the neck, face and tongue.
The basal ganglia are also affected in this type of dysarthria. The speech apart from being unclear is also rapid and stuttering with a monotonous tone. Typical associated symptoms include slow shuffling gait, tremors and stiffness.
Here the disorder arises in the cerebellum. The speech is slurred and the timing impaired in a manner that gives a ‘sing-song’ quality of the voice. Other clinical features may include loss of motor control in the arms and legs, tremors of the head and limbs and nystagmus (rapid involuntary eye movements).
Also known as pseudo-bulbar dysarthria, it arises with disorders of the pyramidal tracts. Speech is mumbled and raspy and patients present with overactive reflexes and abnormal rapid tongue movements.
Causes of Dysarthria
A number of causes may be responsible for dysarthria and the patient’s history, rate of onset and other clinical features would be useful in diagnosis.
- Trauma to the head (traumatic brain injury), face or neck.
- Head and neck surgery
- Cerebral palsy
- Multiple sclerosis
- Myasthenia gravis
- Parkinson’s disease
- Wilson’s disease
- Lyme disease
- Muscular dystrophy
- Poorly fitting dentures
- Medication (side effects) like anti-epileptic drugs, depression and anti-anxiety drugs.
- Illicit substances (narcotics).
Signs and Symptoms
Dysarthria may vary in severity and nature. It can develop suddenly with causes like a stroke or more gradually in conditions like Parkinson’s disease. Dysarthria is most often described as slurred speech, mumbling or distorted speech. Overall the degree of change in speech depends on the cause and site affected but features may include :
- Hoarse voice or whispery tone
- Abnormal rhythm of the voice
- Uneven volume of the voice – loud to soft and then loud again
- Abnormally rapid or slow rate of speech
- Nasal tone to the voice
- Monotone voice
- Facial paralysis on one or both sides
- Difficulty moving the tongue
It is important to note that with dysarthria, the speech is constantly distorted and there are no periods of clear speech followed by distorted speech in one sitting.
Dysarthria. Pubmed Health (http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0004947/)
Colledge, N.R. et al (2010) Davidson’s Principles and Practices of Medicine. Churchill Livingstone.