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Ataxia is defined as the inability to generate a normal voluntary movement trajectory that cannot be attributed to weakness or involuntary muscle activity.1 The word derives from Greek and means without order, referring to disorganized, poorly coordinated, or clumsy movments.2 The disorder may be caused by dysfunction of the cerebellum or its immediate projections, and is traditionally referred to as cerebellar ataxia. In such cases, ataxia may involve appendicular (e.g., limb) or axial (e.g., truncal) control, often affecting balance and gait. Cerebellar ataxia is estimated to affect 26 in 100,000 children worldwide.3 Injuries to the proprioceptive system may also present with clumsy, disorganized movements, known as sensory ataxia. Finally, vestibular injury may affect posture and balance, which much be distinguished from ataxia. This chapter focuses on symptoms, signs, and etiologies of cerebellar ataxia, with an emphasis on infectious and para/postinfectious causes.


Cerebellar ataxia is characterized by disturbances of rhythm, rate, and force of voluntary movements. In addition, there is increasing recognition that cognitive and emotional processing are affected by cerebellar injury.3 Key historical elements that guide the clinical evaluation include the pace of symptom onset (sudden or gradual), duration of ataxia (hours, days, or months), course of illness (static, episodic, slowly or rapidly progressive), and associated symptoms.

Gait abnormalities are the most common presenting symptom. Toddlers may refuse to walk, while older patients may have frequent falls. Often ataxic patients are described as wobbly, uncoordinated, or intoxicated-appearing. Patients typically have a widened stance and may depend on walls or furniture to prevent falls. Sudden changes of pace or direction exacerbate ataxia.

Patients may report difficulty with fine-motor tasks, such as reaching for objects, eating, or writing. The examination may disclose dysmetria (incoordination when reaching for a discrete target, such as the tip of the nose or a finger puppet, with overshooting or undershooting) and dysdiadochokinesia (difficulty in performing alternating, rhythmic tasks, such as tapping different fingers). Movements can have an irregular trajectory (decomposition), may require extreme slowing to maintain accuracy, and/or have a tremulous nature (intention tremor).

Speech can be affected, with abnormal variability in melody, pitch, and speed (scanning dysarthria). Eye movements may demonstrate nystagmus. Cognitive testing may reveal subtle abnormalities in executive and visuospatial processing, linguistic difficulties, and impairments in emotional modulation (e.g., blunted affect).4 Muscle tone can be decreased, and reflexes can have a pendular nature.

Because the cerebellum resides in the tight quarters of the posterior fossa, clinicians must maintain a high index of suspicion for increased intracranial pressure whenever cerebellar dysfunction is found on exam. Features that may suggest increased intracranial pressure include nocturnal or early-morning awakening with headache; pain that is occipital in location or that worsens after lying horizontally; bending; or with cough, sneeze, or Valsalva maneuver. Lethargy, somnolence, or ...

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