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

CSU, Chico, Patrick McCaffrey, Ph.D


Chapter 12. Types of Dysarthria: Upper Motor Neuron Damage


Spastic Dysarthria

As noted above, spastic dysarthria results from damage to the pyramidal tract. Recall that most pyramidal tract cells originate in the cortex of the precentral gyrus of the frontal lobe which is also known as the motor strip. The pyramidal tract fibers that carry messages to the spinal nerves are called the corticospinal tract. The part of the pyramidal tract that sends information to the cranial nerves involved in speech and swallowing is called the corticobulbar tract. According to Dworkin (1991, p.188), spastic dysarthria is characterized by "Generalized hypertonicity, weakness, immobility, abnormal force physiology, and exaggerated reflexes of virtually all muscles of the speech mechanism produce obvious dysfunction of the articulation subsystem. Speech is slow-labored, and imprecise articulatory efforts, compounded by disturbances of respiration; resonation, and phonation often render speech unintelligible."

The spinal nerves receive only contralateral innervation from the corticospinal tract. This means that spinal nerves on the left side of the body receive input only from the motor strip of the right hemisphere while those on the right side of the body are innervated by the half of the motor strip located in the left hemisphere. Therefore, unilateral lesions of the pyramidal tract can cause paralysis of the muscles innervated by the spinal nerves on the opposite side of the body. For example, damage to the left half of the corticospinal tract could result in paralysis of the right side of the body.

On the other hand, almost all of the cranial nerves receive bilateral innervation from the fibers of the corticobulbar tract. This means that both the left and right members of most pairs of cranial nerves are innervated by the motor strips of the right and left hemispheres of the brain. This redundancy is a safety mechanism. If there is a unilateral lesion of the pyramidal tract, both sides of body areas connected to the cranial nerves will continue to receive messages about voluntary motor movement from the cortex. These messages may not be quite as strong as they were previously, but paralysis will not occur. This means that the muscles involved in speech would continue to function adequately in spite of damage to one side of the pyramidal tract. Therefore, only bilateral lesions of the pyramidal tract will cause a serious, spastic dysarthria. When such bilateral lesions occur, they produce a condition known as pseudo-bulbar palsy. This name is derived from the term bulbar palsy which is used to describe the symptoms of brain stem lesions.

(The two exceptions to the innervation scheme described above are the portion of CN XII that controls tongue protrusion and the part of CN VII that innervates the lower face. Like spinal nerves these two cranial nerve nuclei receive only contralateral innervation from the pyramidal tract. Thus, a unilateral pyramidal tract lesion could cause drooping of the lower part of the face or problems with tongue protrusion on the opposite side of the body. However, neither a unilateral weakness of the facial muscles nor the inability to protrude one half of the tongue are likely to cause a significant speech impairment. For this reason it can still be said that only bilateral pyramidal tract lesions seriously affect speech.)

It should be noted that although speech will not be much impaired by unilateral pyramidal tract damage, swallowing may be compromised by this type of lesion. Unilateral lesions are unlikely to cause a serious dysphagia, but they may produce abnormalities that require treatment. Remember that swallowing requires great precision as does speech and is therefore easily impaired by neurological damage. Many of my dysphagia patients over the years had been diagnosed with unilateral upper motor neuron lesions. Swallowing is less forgiving than speech of neurological damage.

The Extrapyramidal Tract

This system is involved in automatic motor movements, and in gross rather than fine motor movement. It works with the autonomic nervous system to help with posture and muscle tone and has more influence over midline structures than over those in the periphery. It regulates reflexes and maintains posture and tone (Duffy, 1995). Facial expression is one important communicative behavior that is mediated by the extrapyramidal tract. In contrast to the pyramidal tract, the extrapyramidal tract is an indirect, multisynaptic tract that is primarily inhibitory.

Components of the extrapyramidal tract include the basal ganglia, the red nucleus, the substantia nigra, the reticular formation, and the cerebellum. All of these structures send information to the lower motor neurons in an indirect, multi-synaptic fashion.

The basal ganglia acts to inhibit the release phenomenon, or the rapid firing of motor neurons. It is aided in this function by the substantia nigra of the midbrain. The muscles most often affected by this inhibitory function are those controlling the head, the hands, and the fingers.

The neurotransmitters involved in the inhibitory function of the basal ganglia include dopamine, which is produced by the substancia nigra, acetylcholine, and GABA, which is a glutamate. Dopamine is an especially powerful inhibitor.

Extrapyramidal Projections to Lower Motor Neurons

The rubrospinal tract passes through the red nucleus. The cerebellum sends messages to the spinal nerves along this tract. Information flows from the superior cerebellar peduncle to the red nucleus and finally to the spinal nerves. This information is very important for somatic motor, or skeletal muscle control and the regulation of muscle tone, smoothness of movement and for posture.

The reticulospinal tract runs from the reticular nuclei of the pons and medulla to the spinal nerves. It is involved in somatic motor control like the rubrospinal tract and also plays an important role in the control of autonomic functions.

The tectospinal tract has points of origin throughout the brain stem, but especially in the midbrain area, and ends in the spinal nerves. It is involved in the control of neck muscles and also in visual and auditory reflexes. So, when you jump after hearing a noise or duck when you see something coming toward you, this tract helps to mediate these reactions.

The vestibulospinal tract runs from the vestibular nuclei located in the lower pons and medulla to the spinal nerves. It is involved in balance.

(Note that all of these tracts receive input from the cerebellum.)

Extra Pyramidal Diseases and Syndromes Affecting Communication/Swallowing

Lesions in the extrapyramidal tract cause various types of diskinesias or disorders of involuntary movement.

The problems mostly common anomalies affecting the extrapyramidal tract include degenerative diseases, metabolic encephalopathy, encephalitis, and tumors.

Parkinson's Disease, which is a degenerative disease, is probably the most frequently occurring illness that results from extrapyramidal tract lesions. It occurs when the dopaminergic neurons of the substantia nigra are destroyed. Its symptoms include:

Tremor

Festinating movements, especially a festinating gait, but these movements can also affect speech. (Festinating movements are movements which become increasingly rapid and uncontrolled).

Hypokinetic dysarthria

Slow movements

Tremor

Postural instability

Altered gait

Weak Voice

Mask-like facial expression

Freezing

This will be discussed in greater detail a little later.

Diseases associated specifically with lesions of the basal ganglia include Huntington's Chorea and Sydenham's Chorea. The word "chorea" comes from the Greek "khoros" which means to dance. Both of these diseases are associated with jerky, uncontrolled movements of the limbs. Sydenham's chorea was probably the cause of the malady that was known as St. Vitus' Dance during the middle ages. Huntington's Chorea is an inherited degenerative disease. These will be discussed further a little later.

Essential Tremor Syndrome, which is associated with spastic dysphonia may also be the result of basal ganglia lesions.

Lesions of the basal ganglia will also cause hyperkinetic dysarthria.

Note that not only is the definition of the extrapyramidal system controversial, but also many sources say that it is very difficult to make functional distinctions between the extrapyramidal and pyramidal systems. When upper motor neuron lesions occur, it is often difficult to determine which tract has been damaged.

Hyperkinetic Dysarthria

Hyperkinetic dysarthria results from damage to the extrapyramidal tract. It specifically results from lesions of the basal ganglia. The extrapyramidal system is involved in automatic motor movements and works with the autonomic nervous system to control posture and muscle tone. Components of the extrapyramidal system include the basal ganglia (caudate and lenticular nuclei), the red nucleus, the substantia nigra, the reticular formation and the cerebellum. Also included are the rubro spinal, reticulo spinal, and vestibulo spinal tracts. The latter runs from the vestibular nucleus in the pons to the spinal cord. The vestibular nucleus also connects to the semicircular canals in the ear and to the cerebellum for balance and orientation in space. Again, hyperkinetic dysarthria most often results from damage to the basal ganglia specifically. Dworkin (1991) describes hyperkinetic movements as quick, jerky and unsustained as with Huntington's chorea, or slow, writhing and sustained as observed in people with athetosis and dystonia. He describes those with essential tremor and myoclonus as having tremorous movements. As you can see there are several different types of speech movement abnormalities in people with hyperkinetic dysarthria. Dworkin's 1991 text is exclusively on remediation of motor speech disorders. While being mindful that there is a good deal of diversity among patients, Dworkin (1991, p.189), generally recommends the following type of sub-system exercises for hyperkinetic dysarthria: "1. Lingual, labial, and mandibular force physiology training. 2. Phonetic stimulation in various contexts." He also feels that this treatment in general can also apply to ataxic patients after they receive treatment of the respiratory and phonation sub-systems.

Conditions that may cause hyperkinetic dysarthria include essential tremor syndrome, chorea, athetosis, and dystonia and tardive dyskinsia. All result in the suprimposition of automatic movements upon volitional movements.

    1. Essential tremor syndrome is also known as heredofamilial tremor. Usually, various parts of the body, including the jaw, head and hands are affected, displaying oscillatory movements superimposed upon voluntary and involuntary movements. When only the laryngeal musculature is affected, the condition is called organic voice tremor. In this case, phonation is abnormal but resonance and articulation are not impaired. The voice disorder resulting from organic voice tremor may involve alternations of pitch and loudness at regular intervals. It may also involve irregular, complete stoppage of voicing. When the later symptom occurs, organic voice tremor may resemble spastic dysphonia. However, there is no strong concensus as to whether the two disorders are actually related.

    2. Chorea is a "disorder characterized by irregular, spasmodic, involuntary movements of the limbs or facial muscles" (Love & Webb, 1992, p. 289). "Choreic movements are rapid and coordinated but purposeless." These "fidgets" can cause dysarthria by impairing the coordination of respiration with speech and interrupting the movements of the articulators during speech (Love & Webb, 1992, p. 149). There are two types of chorea, Sydenham's Chorea and Huntington's Chorea.

    3. Sydenham's chorea is a non-hereditary form of the disorder that occurs during childhood following infections like strep throat, rheumatic fever or scarlet fever. The symptoms of this disorder will resolve spontaneously within six months of onset. (It is believed that this is the disease that was known as St. Vitus' Dance during the Middle Ages.)

    4. According to Fitz Gerald (1996), Huntington's chorea is transmitted genetically (chromosome 4 ). Because the disorder is transmitted by a single, dominant gene, a patient's children have a fifty percent chance of developing it themselves. This form of chorea is progressive and fatal, causing the loss of neurons in the cortex as well as in the caudate nucleus and globus pallidus. The onset of Huntington's chorea typically occurs in the 50's although it may begin much earlier, even in childhood.

    5. Athetosis is a disorder that causes slow writhing movements of the entire body but especially of the arms, face and tongue. These movements can affect speech by interrupting the action of the articulators and of the respiratory system. Athetosis most often results from lesions of the putamen. The caudate nucleus may also be involved. It is a rare disorder that usually occurs as a form of cerebral palsy brought on by perinatal anoxia. An especially rare form of athetosis begins during adolescence and follows a progressive course. According to Fitz Gerald (1996), spontaneous movements are considered an escape phenomenon caused by damage to the striatum and/or subthalamic nuclei. This silences the globus pallidus permitting the ventral lateral (VA) nucleus of the thalamus to fire spontaneously.

    6. Dystonia causes slow jerky movements which are most likely to occur in the trunk, neck and proximal parts of the limbs. These may be exacerbated by voluntary movements, like those involved in speech (Love & Webb, 1992). Rosenfeld (1991) feels that it results from a combination of dopaminegric and cholinergic overactivity in the basal ganglia. When oralfacial muscles are involved the condition is called focal mouth dystonia (Duffy, 1995).

    Some have speculated that spastic and spasmodic dysphonia, a condition "characterized by effortful, strained voice quality with voice arrests" (Love & Webb, 1992), may be a localized form of dystonia. Not much evidence supports this view, however. Although it has been believed that the intermittent aphonia seen in cases of spastic dysphonia is the result of hyperadduction or spasm of the vocal folds. Indirect laryngoscopy generally shows that the folds continue to move normally during these episodes (Love & Webb, 1992). It should also be noted that some cases of spastic dysphonia appear to be psychogenic in origin and are thus not related to any hyperkinetic movement disorder.

    As its etiology has not yet been identified, the treatment of spastic dysphonia remains controversial. In the past, the recurrent laryngeal nerve was cut to ameliorate symptoms. While this operation improves voice for a time, symptoms generally reappear. At this time, injections of
    botulinum-A toxin (Botox) seem to be the most promising treatment for spastic dysphonia.

    7. Tardive Dyskinesia is a condition which results from longterm use of anti-psychotic drugs called phenothiazines. These drugs are sometimes used to treat schizophrenia. This movement disorder causes "choreiform.and peculiar rythmical movement" (Love & Webb, 1992), note that these are especially likely to affect the tongue and lips. Involuntary movements of the tongue in and out of the mouth, which are known as fly-catcher's movements, are characteristic of tardive dyskinesia. It should be noted that the symptoms of this form of hyperkinesia do not always resolve when the use of phenothiazines is discontinued.

Hypokinetic Dysarthria

Hypokinetic dysarthria is usually the result of Parkinson's Disease (discussed earlier). The disorder causes the degeneration of the substantia nigra, which is one component of the extrapyramidal system. The substantia nigra supplies dopamine to the cells of the basal ganglia, allowing it to inhibit the release phenomenon or the rapid firing of motor neurons. Thus, involuntary movements occur as a result of the shortage of dopamine caused when the substantia nigra is damaged. Dworkin (1991) recommends the same type and sequence of sub-system exercises as for hyperkinetic dysarthria described above. According to Dworkin (1991) "widespread rigidity and associated pareisis of virtually all muscles of the speech mechanism justify treatments to improve resonance, phonation, articulation, and indeed prosody." It is currently believed that Parkinson's Disease results from genetically based abnormalities of the mitochondria, passed on through the female. This trait appears to be X-linked. According to Fitzgerald (1996), the disease affects about 1% of people over fifty years of age.

Parkinsonism, or Parkinson's-like symptoms, can also cause hypokinetic dysarthria. Parkinsonism can result from carbon monoxide poisoning, arteriosclerosis, the use of tranquilizers like Haldol, or from repeated blows to the head. (Mohammad Ali has developed Parkinsonism due to injuries he received during his career as a boxer). Another Parkinson-like symptom , tardive dyskinesia (fly catcher syndrome) often occurs as a side effect of psychoactive medication.

Parkinson's Disease and Parkinsonism are often treated with L-dopa. Levadopa is metabolized to dopamine, a neurotransmitter, during passage through the blood-brain barrier (Fitz Gerald, 1996).

The symptoms of Parkinson's disease include:

    1. Pill-rolling tremor - patients move their thumbs and forefingers together as if rolling a small pill between them. This movement occurs at rest, but subsides during voluntary movement and sleep.

    2. Masked facies - Parkinson's patients usually display little facial expression, so their faces are described as "mask-like."

    3. Festinating movements - Movements become more rapid and less coordinated over time. Festinations may affect both gait and speech. For example, a Parkinson's patient may begin to walk successfully but, as he continues, his steps will become faster and increasingly uncontrolled. As a patient continues to talk, his speech will become increasingly unintelligible.

    4. Rigidity - If the limbs of a Parkinson's patient are moved passively, the muscles will often contract involuntarily, causing rigidity. This rigidity may be constant or intermittent. Intermittent rigidity is called cogwheel rigidity.

    5. Bradykinesia - This is the "reduced speed of movement of a muscle through its range" (Love & Webb, 1992, p. 146). Not to be confused with tardive dyskenesia that can occur as a side effect of psychoactive drugs.

    6. Hypokinesia - This is "the reduced amplitude of muscle movement" (Love & Webb, 1992, p. 146).

    7. Micrographia - Is the "tendency for handwriting to be very small in the height of the letters and to get progressively smaller as the person continues to write" (Love and Webb, 1992, p. 146).

    8. Dementia - Between 30 and 40 percent of Parkinson's patients suffer from dementia.

Ataxic Dysarthria

Ataxic dysarthria results from cerebellar lesions. Such damage could be caused by stroke, trauma or by neurological disorders like muscular sclerosis. The cerebellum plays an important role in the coordination of motor movement due to its integration of sensory and motor information. Due to its connections with the vestibular system, it also affects equilibrium. Therefore, cerebellar lesions result in incoordination and the disruption of smooth movements (tremor) as well as problems with gait and balance. After cerebellar damage, patients are slow to initiate movements. Their movements often under or overshoot targets. For example, if an ataxic patient is trying to pick up a glass of water, she may either fail to move far enough toward the glass or may reach beyond it.


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Other courses in the Neuroscience on the Web series:
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