The term cerebellar ataxia is employed to indicate ataxia 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 depend on which cerebellar structures are lesioned, and whether the lesion is bilateral or unilateral.
Dysfunction of the vestibulocerebellum impairs balance and control of eye movements. This presents with postural instability, in which the person tends to separate the feet on standing to gain a wider base and avoid oscillations (especially posterior-anterior ones); instability is therefore worsened when standing with the feet together (irrespective of whether the eyes are open or closed: this is a negative Romberg's test, or more accurately, denotes the inability to carry out the test as the individual is unstable even with open eyes).
Dysfunction of the spinocerebellum presents with a wide-based "drunken sailor" gait, characterised by uncertain start and stop, 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, including intention tremor (coarse trembling, accentuated over the excution of voluntary movements, possibly involving the head and eyes as well as the limbs and torso), peculiar writing abnormalities (large, unequal letters, irregular underlining), and a peculiar pattern of dysarthria (slurred speech, sometimes characterised by explosive variations in voice intensity despite a regular rhythm).
Sensory ataxia
The term sensory ataxia is employed to indicate ataxia due to loss of proprioception (sensitivity to joint and body part position), which generally depends on dysfunction of the dorsal columns of the spinal cord, since they carry proprioceptive information up to the brain; in some cases, the cause may instead be dysfunction of the various brain parts that receive that information, including the cerebellum, thalamus, and parietal lobes. Sensory ataxia presents with an unsteady "stomping" gait with heavy heel strikes, as well as postural instability that is characteristically worsened when the lack of proprioceptive input cannot be compensated by visual input, such as in poorly lit environments. Doctors can evidence this during physical examination by having the patient stand with his / her feet together and eyes shut, which will cause the patient's instability to markedly worsen, 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 examiner, if the eyes are closed, the patient's finger will tend to "fall down" and be restored to the horizontal extended position by sudden extensor contractions ("ataxic hand").
Vestibular ataxia
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.
