The term cerebellar ataxia is used to indicate ataxia that is due to dysfunction of the cerebellum. This causes a variety of elementary neurological deficits, such as antagonist hypotonia, asynergy, dysmetria, dyschronometria, and dysdiadochokinesia.
How and where these abnormalities manifest themselves depends on which cerebellar structures have been damaged, and whether the lesion is bilateral or unilateral.
- Dysfunction of the vestibulocerebellum impairs the balance and the control of eye movements. This presents itself with postural instability, in which the person tends to separate his/her feet upon standing, in order to gain a wider base and to avoid bodily oscillations (especially forward-backward ones). The instability is therefore worsened when standing with the feet together, regardless of whether the eyes are open or closed. This is a negative Romberg's test, or more accurately, it denotes the individual's inability to carry out the test, because the individual feels unstable even with open eyes.
- Dysfunction of the spinocerebellum presents itself with a wide-based "drunken sailor" gait, characterised by uncertain starts and stops, lateral deviations, and unequal steps. This part of the cerebellum regulates body and limb movements.
- Dysfunction of the cerebrocerebellum presents with disturbances in carrying out voluntary, planned movements. These include:
- intention tremor (coarse trembling, accentuated over the execution of voluntary movements, possibly involving the head and eyes as well as the limbs and torso);
- peculiar writing abnormalities (large, unequal letters, irregular underlining);
- a peculiar pattern of dysarthria (slurred speech, sometimes characterised by explosive variations in voice intensity despite a regular rhythm).
The term sensory ataxia is employed to indicate ataxia due to loss of proprioception - the loss of sensitivity to the positions of joint and body parts.
This is generally caused by dysfunction of the dorsal columns of the spinal cord, because they carry proprioceptive information up to the brain.
In some cases, the cause of sensory ataxia may instead be dysfunction of the various parts of the brain which receive positional information, including the cerebellum, thalamus, and parietal lobes.
Sensory ataxia presents itself with an unsteady "stomping" gait with heavy heel strikes, as well as a postural instability that is usually worsened when the lack of proprioceptive input cannot be compensated for by visual input, such as in poorly lit environments.
Physicians can find evidence of sensory ataxia during physical examination by having the patient stand with his/her feet together and eyes shut. In affected patients, this will cause the instability to worsen markedly, producing wide oscillations and possibly a fall.
This is called a positive Romberg's test. Worsening of the finger-pointing test with the eyes closed is another feature of sensory ataxia. Also, when the patient is standing with arms and hands extended toward the physician, if the eyes are closed, the patient's finger will tend to "fall down" and then be restored to the horizontal extended position by sudden muscular contractions (the "ataxic hand").
The term vestibular ataxia is employed to indicate ataxia due to dysfunction of the vestibular system, which in acute and unilateral cases is associated with prominent vertigo, nausea and vomiting.
In slow-onset, chronic bilateral cases of vestibular dysfunction, these characteristic manifestations may be absent, and dysequilibrium may be the sole presentation.
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