lower motor neuron signs, since weakness can easily be mistaken for ataxia or the two signs may coexist A very useful test that requires little strength to be performed and can help differentiate weakness from ataxia is done by asking the patient to follow one of the creases of her thumb with her index finger Truncal and appendicular ataxia may coexist in the presence of large lesions or diffuse processes Eye movement abnormalities in cerebellar dysfunction may be more difficult to detect; nystagmus is often present, and saccades may under- or overshoot the target Also, speech can be affected, with irregular changes in volume and rate (scanning speech) Ataxia can result from lesions outside of the cerebellum, when the cerebellar afferent or efferent pathways are disrupted, for example by a lesion in the pons, or by lesions in the posterior column - medial lemniscal pathway, which result in positive Romberg sign and loss of joint position sense DIFFERENTIAL DIAGNOSIS AND ROLE OF IMAGING AND LABORATORY TESTS While several diseases can manifest with ataxia (Table 15.2 ), the most common cause of acute ataxia in children with normal mental status is acute postinfectious cerebellar ataxia This condition is usually observed in toddlers (60% of patients in large series were aged between and years) and is characterized by a prodrome of nonspecific viral illness followed after days or up to to weeks by ataxia The clinical course is usually benign, with full return to baseline within months in 70% of children This remains a clinical diagnosis, as imaging and CSF analysis are usually unremarkable; sometimes, especially in younger patients where the examination may not be entirely reliable, brain MRI and lumbar puncture (LP) should be considered to rule out other causes, such as acute disseminated encephalomyelitis (ADEM—see below), because of potential treatment implications In a child with ataxia and abnormal mental status , urgent brain imaging (plain CT head or, ideally, brain MRI if available without significant delay) is warranted, and LP should be considered Imaging, particularly MRI, allows the clinician to rule-out brain tumors, demyelinating conditions, cerebellitis, and stroke CSF pleocytosis (>5 cells/mm3) may be observed in different conditions and while bacterial meningitis is unlikely in an ataxic child without fever or meningismus, viral meningoencephalitis remains a possibility, especially if the pleocytosis is primarily lymphocytic