Dysmetria

Causes

Dysmetria results from lesions in the cerebellum or in the proprioceptive nerves that lead to the cerebellum, which coordinate visual, spatial, and other sensory information with motor control. Damage to proprioceptive nerves prevents the cerebellum from accurately judging limb or eye position.

These lesions are often caused by strokes, multiple sclerosis, amyotrophic lateral sclerosis, or tumors. Two types of cerebellar disorders produce dysmetria: midline cerebellar syndromes, which can cause ocular dysmetria (difficulty tracking objects and maintaining fixation), and hemispheric cerebellar syndromes, which cause typical motor dysmetria. Cerebellar motor syndrome, marked by gait impairments, disordered eye movements, tremor, swallowing difficulty, and poor articulation, also causes dysmetria, as does cerebellar cognitive affective syndrome.

Anatomy

The cerebellum, located inferior to the cerebrum, contributes to coordination and motor processes. Sensorimotor integration, crucial for performing motor tasks, occurs in the post parietal cortex and involves integrating sensory and proprioceptive information with visual data. Visual information about eye position is translated into spatial information, which the cerebellum uses to execute motor tasks; damage to connecting pathways can cause dysmetria.

Motor dysmetria from hemispheric syndromes manifests as dysrhythmic tapping of hands and feet and dysdiadochokinesis (impaired alternating movements). Damage to the cerebellum slows a person's ability to orient extremities in space. Motor control is a learning process occurring in the synapses of Purkinje dendrites. Some models propose the cerebellum as an array of adjustable pattern generators (APGs) that produce burst commands; others suggest it acquires an inverse model of the motor apparatus.

Recent electrophysiology research has identified modular structures in the spinal cord called motor primitives. In the APG model, modules control motor learning through a positive feedback loop involving inhibitory input from the cerebellar nucleus, motor cortical cells, and Purkinje cells, which receive learning information from parallel fibers of granule cells.

Diagnosis

Diagnosis of cerebellar disorders should be made by a qualified neurologist. A general practitioner or MS nurse may first perform a finger-to-nose test, where the patient touches the clinician's finger and then their own nose repeatedly to assess target judgment.

Other tests include a heel-to-shin test, where proximal overshoot indicates dysmetria, and drawing an imaginary circle with arms or legs without decomposition of movement. After a positive finger-to-nose test, an MRI is used to detect cerebellar damage. Cerebellar patients have difficulty adapting to unexpected changes in limb inertia, which can be used to confirm diagnosis, and they show abnormal responses to changes in damping, supporting the cerebellum's role in predictions.

Treatments

There is no cure for dysmetria itself, as it is a symptom of an underlying disorder. Isoniazid and clonazepam have been used to treat it, and Frenkel exercises are also employed. Researchers are testing rehearsal by eye movement: in one study, patients with saccadic dysmetria who overshot movements improved motor performance after visually rehearsing a path before walking.

For multiple sclerosis patients, deep brain stimulation (DBS) remains a possible treatment, though long-term effects are under review. Subjects who received DBS had no major relapse for six months and reported reduced disabling motor problems, with some stating their movement disorder resolved after surgery. However, these results are limited by the small number of subjects, and it is unknown if DBS is viable for all MS patients with motor control issues.

Did You Know?

• Cerebellar malformations extending to the brainstem can also present with dysmetria.
• Friedreich's ataxia is a relatively common cause of dysmetria.