The Neuroscience on the Web Series:
CMSD 642 Neuropathologies of Swallowing and Speech

CSU, Chico, Patrick McCaffrey, Ph. D

Chapter 13. Types of Dysarthria: Lower Motor Neuron Damage

Lower Motor Neuron Damage

There are two types of lower motor neuron lesions: bulbar lesions and peripheral lesions.

Bulbar lesions occur on the nuclei of cranial nerves, in the brain stem. These lesions cause a condition known as bulbar palsy, which is likely to result in a very severe form of dysarthria. As the cranial nerves lie very close to one another in the brainstem, a lesion will usually damage more than one pair of nuclei. Thus, a number of muscles may be affected bilaterally. Bulbar lesions can be caused by stroke, trauma or disease.

Peripheral lesions damage the axons of the cranial nerves after they have left the neuraxis and may cause what is known as a peripheral dysarthria. Typically this condition is not as serious as bulbar palsy since muscles on only one side of the body will be denervated. Also, it is possible that only the axon of one cranial nerve will be damaged. In both bulbar palsy and peripheral dysarthria, the muscles on only one side of the body will be affected cranial nerves cease to receive any input from the central nervous system, causing them to become flaccid or limp.

Myasthenia Gravis can also cause a lower motor neuron dysarthria, because the muscles most affected are those supplied by cranial nerves. This disease impairs the ability of nerve fibers to synapse with the muscles by reducing the number of acetylcholine (ACh) receptors at the neuromuscular junction. It causes weakness in muscles throughout the body, especially those involved in speech and swallowing. This weakness is progressive. In the morning or after rest, the patient's speech may sound clear. However, as he becomes fatigued, the slurring of his speech will increase. According to Fitz Gerald (1996), it is caused by the immune system producing antibodies to the ACh receptor. Dworkin (1991, p. 189) recommends the following sequence of articulation sub-system treatment: for flaccid dysarthria:

  1. Lingual, labial, and/or mandibular musculature strengthening.
  2. Lingual, labial, and/or mandibular force physiology training.
  3. Phonetic stimulation in various context

Mixed Dysarthrias

Mixed dysarthrias occur when both upper and lower motor neurons are damaged. This typically occurs as the result of disease. Amyotrophic lateral sclerosis (Lou Gehrig's Disease) is the disease that most frequently caused mixed forms of dysarthria. This condition, which usually first affects people when they are in their fifties, causes the degeneration of both upper and lower motor neurons. Its etiology is unknown. It has a very poor prognosis.



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Other courses in the Neuroscience on the Web series:
CMSD 620 Neuroanatomy | CMSD 636, Neuropathologies of Language and Cognition

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