Ataxia is what cerebellar dysfunction looks like at the bedside. The word comes from Greek a-taxis — without order. The patient's body still works. The muscles fire. The strength is preserved. What fails is the precise temporal and spatial coordination of movements, so what should be smooth becomes wobbly, what should land on target overshoots, what should be a single fluid action breaks into discrete jerky steps.
The bedside tests for cerebellar function probe different aspects of coordination. Finger-to-nose tests dysmetria — the patient cannot accurately land their fingertip on the moving target of the clinician's finger, overshooting or undershooting in a way that does not improve with attention. Heel-to-shin tests truncal and lower-extremity coordination in the same way. Rapid alternating movements (alternating supination and pronation of the hand on the knee) test dysdiadochokinesia — the patient cannot smoothly switch between opposing motor patterns.
Romberg testing distinguishes cerebellar from sensory ataxia. Stand the patient with feet together. Cerebellar patients sway with eyes open and eyes closed equally — the coordination problem is not vision-dependent. Sensory-ataxia patients are stable with eyes open but fall with eyes closed, because they were relying on visual input to compensate for lost proprioception.
Cerebellar speech — dysarthria of a particular kind — is slurred, scanning, halting. The patient speaks as if each syllable were separately deliberated. Recordings of severe cerebellar dysarthria sound machine-like, with explosive stresses on unexpected syllables. It can be socially isolating even when intelligible.
The differential for new-onset cerebellar ataxia is broad: acute stroke, multiple sclerosis, paraneoplastic syndromes (especially anti-Yo with ovarian cancer), genetic ataxias, alcohol, toxic exposures, vitamin deficiencies, posterior fossa tumors. The clinical workup follows the time course — acute presentations point to stroke; subacute to autoimmune or paraneoplastic; chronic and progressive to genetic or toxic.
Hold the picture. The choreographer is damaged. The dancers still move, but no one is correcting the missteps. Every gesture lands slightly off the intended target, and the patient knows it.