Ataxia definition
The term ataxia refers to poorly coordinated movements.
It is a disorder of motor coordination that affects the amplitude and direction of movements with altered gait and associated disequilibrium.
Usually results from lesions of the cerebellum and/or cerebellar connections,
but in addition to cerebellar ataxia there are also cases of sensory or vestibular ataxia caused by lesions of spinal proprioceptive pathways or the vestibular system,
respectively.
The different types of ataxia have specific clinical features (table 1),
depending on the nature and location of the underlying lesion:
Types of Ataxia:
Cerebellar ataxia manifestations commonly include a wide – based unsteady gait as well as poor coordination or clumsy movements of the extremities (dyssynergy).
Patients stand with their legs farther apart than normal and sway or fall in attempts to stand with the feet together.
Mild gait ataxia may be exaggerated when the patient attempt tandem – walking in a straight line.
Cerebellar ataxia is frequently associated to slowness of movements and dysdiadochokinesia (impaired ability to perform rapid,
alternating movements).
Abnormal motor trajectory or placement during active movement (dysmetria with hypometria or hypermetria) and muscle hypotonia (decreased resistance to passive movement) are frequent.
Muscle reflexes usually have a pendular pattern.
Involvement of the dentate nucleus or its efferent fibers causes intention tremor with increasing amplitude of the oscillations when is approaching the target.
There can be poor coordination of speech presenting as dysarthria (slurred speech or scanning speech,
sometimes characterized by explosive variations in voice intensity despite a regular rhythm) and abnormality in ocular motility presenting as saccadic ocular dysmetria and horizontal nystagmus.
Relatively isolated trunk ataxia with abnormal stance and gait and axial or head tremor (titubation) is usually associated to disorders that involve the cerebellar midline or vermis.
Extremities ataxia is usually seen in disorders that affect the cerebellar hemispheres.
With unilateral cerebellar hemisphere lesions an ipsilateral disturbance of gait,
posture and movement are observed.
Sensory ataxia is less frequent than cerebellar ataxia and is due to loss of proprioception.
It could be central (due to lesions of the somatosensory cortex,
the thalamocortical pathways,
the thalamus,
and medial lemniscus pathway),
from disorders of the posterior columns of medulla (lesions of the afferent somatosensory pathways in the dorsal portion of the spinal cord - fasciculus gracilis and fasciculus cuneatus) or peripheral (caused by disease processes affecting the peripheral sensory nerves).
Clinically,
it manifests with proprioceptive deficit (loss of position sense and inability to detect vibrations),
an unsteady “stomping” gait,
and loss of balance and significant worsening of clumsiness with eye closure.
There is a positive Romberg’s sign (loss of balance in a patient standing up when close the eyes). The Romberg’s sign is caused because the patient can still maintain balance by using vision and vestibular function.
Vision is able to compensate for the loss of position sense to a great degree and thus minimizes sensory ataxia.
Additionally,
a phenomenon of “pseudoathetosis” can be seen in affected extremities in sensory ataxia.
Pseudoathetosis is an abnormal writhing movements caused by a failure of joints position sense or proprioception,
usually observed in fingers.
Analogous to Romberg’s sign,
pseudoathetosis is most pronounced with eye closure.
Vestibular ataxia is rare and may be classified as a specific subtype of sensory ataxia.
In vestibular ataxia there are serious stance and gait difficulties (vestibular disequilibrium),
but there aren’t alterations in coordination of extremities or in speech.
With a unilateral vestibular lesion a “flank walking” in the direction of the altered side is observed.
Additionally,
vestibular ataxia could be associated with nystagmus and vertigo with prominent dizziness,
nausea and vomiting,
particularly in acute cases.
Sometimes vestibular ataxia is associated with hearing loss.
Anatomic Review:
The smooth and precise execution of a movement requires a properly functioning “regulatory system”.
The regulatory system (cerebellum) must integrate proprioceptive input from conscious and unconscious proprioceptive pathways,
along with further input from the vestibular and visual systems,
and the use of this data to optimize and coordinate each phase of the movement and plan its force and amplitude.
It includes the pathways responsible for carrying sensorial proprioceptive information to the parietal cortex (conscious proprioceptive pathway) and cerebellum (unconscious proprioceptive pathway).
1.
Conscious Proprioceptive Pathway: (Fig.
1,
2 and 3)
This pathway carries information from exteroceptors (tactile sense,
stereognosis and vibration) and proprioceptors (position sense) through peripheral nerves and the dorsal root of spinal nerves (spinal ganglion cells - first sensory neuron).
Then it travels by way of the posterior columns of spinal cord to the nucleus gracilis and nucleus cuneatus of the medulla,
without any intervening relay in the spinal cord.
These medullary nuclei contains the second sensory neuron,
whose axons decussate to the contralateral side of the stem brain and ascend in the medial lemniscus,
which travels to the ventral posterolateral nucleus of the thalamus.
From there,
the third – order neurons project to the somatosensory (parietal) cortex.
2.
Unconscious Proprioceptive Pathway: (Fig.
4 and 5)
Unconscious proprioceptive pathway includes four fiber tracts that run up the spinal cord to the cerebellum.
These are: 1.
the ventral spinocerebellar tract,
2.
the dorsal spinocerebellar tract,
3.
the rostral spinocerebellar tract,
and 4.
the cuneocerebellar tract.
The dorsal and ventral spinocerebellar tractsrun up the spinal cord near the dorsolateral and ventrolateral surfaces respectively.
Both tracts terminate in the vermis.
The dorsal spinocerebellar tract rises ipsilaterally and enters the cerebellum through the inferior cerebellar peduncle.
The ventral spinocerebellar tractdecussates to the contralateral ventral cord,
and ascends to the cerebellum entering through the superior cerebellar peduncle.
The rostral spinocerebellar tractarises from the cervical portion of the cord,
rises ipsilaterally and enters the cerebellum through the superior and the inferior cerebellar peduncles.
The cuneocerebellar tractreaches the accessory cuneate nucleus (immediately above the cuneate nucleus) through the cuneate fasciculus and synapse with the second-order fibers of the cuneocerebellar tract.
These fibers ascend to the cerebellum via the inferior cerebellar peduncle.
- Vestibular System: (Fig.
6)
The vestibular receptors are located in the semicircular canals,
utricle and saccule of the inner ear.
They are mechanoreceptors that detect changes in the motion and position of the head.
This information is transmitted by cranial nerve VIII to the vestibular nuclei (in the dorsal pontomedullary junction).
Cerebellum participates in optimizing the amplitude,
speed,
and precision of voluntary movement and simultaneously in regulating the motor control of balance and adapt muscle tone.
It also plays a role in the regulation of gaze-related movements of the eyes and in ensuring the smooth complementary functioning of agonist and antagonist muscle groups.
To perform these coordinating tasks,
the cerebellum requires information from different parts of the nervous system:
- Cerebral cortex (sensorial and motor cortical areas)informationfor the initiation and planning of voluntary movement travels in the corticopontocerebellar pathway,
by way of the middle cerebellar peduncle,
to the Neocerebellum or Pontocerebellum (located in the cerebellar hemispheres and dentate nucleus).
(Fig.
7)
The lateral cerebellar cortex receives its major inputs from the contralateral pontine nuclei through the middle cerebellar peduncle.
The pontine nuclei receive inputs from the ipsilateral cerebral cortex.
Another important afferent pathway to the neocerebellum is the olivocerebellarpathway from the contralateral principal olivary nucleus.
The major outputs of the pontocerebellum are,
corticonuclear fibers from the hemisphere cerebellar cortex to the dentate nucleus and from this to the contralateral red nucleus (dentatorubral fibers),
and to the VL thalamus (dentatothalamic fibers).
From the VL nucleus,
fibers project extensively to numerous regions of the cerebral pre-motor and motor cortex. Dentate nucleus also sends fibers to the contralateral principal olivary nucleus (dentato-olivary fibers) and,
in lesser extent,
to contralateral pontine and reticular formation nuclei.
This part of the cerebellum is mainly responsible for the fine control - very precise movements,
particularly of the limbs and of the motor apparatus of speech.
Injury to the pontocerebellum (cerebellar hemispheres) can result in severe disruption of movement.
The ipsilateral side is affected because pontocerebellar efferents project to the contralateral motor cortex,
which in turn projects caudally in corticospinal fibers that decussate in the pyramids.
- Information regarding joint position and muscle tone from peripheral proprioceptors travels,
by way of the ventral and dorsal spinocerebellar tracts and the cuneocerebellar tract,
through the inferior and superior cerebellar peduncles to the Paleocerebellum or Spinocerebellum (superior vermis,
part of inferior vermis and Globose and Emboliform nuclei).
The spinocerebellum integrates the unconscious proprioceptive information and projects it to the cerebral cortex via the VL nucleus of the thalamus.
There are also efferent connections to the reticular formation,
vestibular nuclei and red nucleus (cerebellorubral fibers).
This part of the cerebellum is mainly responsible for the smooth and synergistic functioning of the muscles when the individual stands or walks and of postural tone.
(Fig.
4 and 5)
- Impulses from the vestibular system travel by way of the inferior cerebellar peduncle to the Archicerebellum or Vestibulocerebellum (nodulus,
flocculus,
adjacent vermian cerebellar cortex and the fastigial nucleus).
(Fig.
7)
This part of cerebellum also receives impulses from visual system (lateral geniculate body). The vestibulocerebellum sends efferent connections to the fastigial nucleus and from this to the vestibular nuclei and reticular formation.
From the vestibular nuclei fibers descend as the medial and lateral vestibulospinal tracts.
The vermis projects also to the contralateral VL thalamus,
and from there to the trunk areas of the motor cortex.
This part of the cerebellum is implied in keep balance during standing and walking.