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Introduction
The
principle area of the brain that is examined by the coordination
exam is the cerebellum. The cerebellum is important for motor learning
and timing of motor activity. It fine-tunes the force of agonist
and antagonist muscle activity simultaneously and sequentially across
multiple joints to produce smooth flowing, goal directed movements.
Cerebellar dysfunction results in decomposition of movements and
under and over shooting of goal directed movements (dysmetria). Decomposition
of movement and dysmetria are the main elements of ataxia.
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Subdivisions of the Cerebellum
The cerebellum has 3 functional subdivisions, which function as feedback
and feed forward systems.
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Vestibulocerebellum
The first functional subdivision is the vestibulocerebellum. This consists of the connections
between the vestibular system and the flocculonodular lobe. Dysfunction
of this system results in nystagmus, truncal instability (titubation),
and truncal ataxia.
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Spinocerebellum
The 2nd subdivision is the spinocerebellum. This system consists
of the connections between the cutaneous and proprioceptive information
coming from the spinal cord to the vermis and paravermis regions with
corrective feedback predominantly to the muscles of truncal stability
and gait. Dysfunction of this system results in gait and truncal ataxia.
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Midline Ataxia
Clinically, the ataxic syndromes caused by vestibulocerebellar and spinocerebellar
disease are lumped together and are called midline or equilibratory
(gait) ataxias. The hallmarks of these midline ataxic syndromes
are truncal instability manifested by titubation (tremor of the trunk
in an anterior-posterior plane at 3-4 Hz) and gait ataxia (wide based,
irregular steps with lateral veering).
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Cerebrocerebellum
The 3rd subdivision of the cerebellum is the cerebrocerebellum.
This system consists of connections from the cerebral cortex (predominantly
motor) to the cerebellar hemispheres then back to the cerebral cortex.
This system is important in motor planning. Dysfunction of the cerebellar
hemispheres results in ataxia of speech (scanning dysarthria) and ataxia
of the extremities (appendicular ataxia). It is important to remember
that ataxia caused by disease of the cerebellar hemispheres will be ipsilateral
to the dysfunctional hemisphere. The findings of appendicular ataxia
are hypotonia, decomposition of movement, dysmetria, and difficulty with
rapid alternating movements (dysdiadochokinesia).
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Exam Tests
The following tests of the neuro exam can be divided according to which
system of the cerebellum is being examined:
Vestibulocerebellum
and spinocerebellum (midline):
- Station
- Walking
- Tandem gait
Cerebrocerebellum
(appendicular):
- Rapid alternating
movements
- Finger-to-nose
- Toe-to-finger
- Heel-to-shin
- Rebound and
check reflex
- Speech
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Traps
Although incoordination is primarily a symptom of cerebellar
disease, it can also be seen from other causes such as:
a. Sensory
dysfunction-loss of large myelinated sensory axons serving proprioception.
This causes a sensory ataxia. Key distinguishing feature for
this type of problem- a positive Romberg and loss of position
sense on the sensory exam.
b. Vestibular
dysfunction. Key distinguishing feature- vertigo.
c. Corticospinal
tract disease can cause incoordination of the distal extremities.
Key distinguishing feature- UMN signs.
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Pearls
Appendicular ataxia is maximum at the greatest extent and end
point of the motor act so make sure you have the patient get to the
full extent of the motor act when testing for ataxia.
Remember that
the Romberg test is a test for position sense and not of cerebellar
function.
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