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:
- Lingual,
labial, and/or mandibular musculature
strengthening.
- Lingual,
labial, and/or mandibular force physiology
training.
- Phonetic
stimulation in various context
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.