Cerebellum

The cerebellum consists of two lateral hemispheres, divided into anterior and posterior lobes by the primary fissure. The hemispheres are joined by the vermis in the midline. The cerebellum connects to the cerebral cortex and brainstem via three paired sets of peduncles, the superior, middle and inferior cerebellar peduncles.

Cerebral hemispheres and vermis

There are three functional parts of the cerebellum. These are the archaecerebellum, the palaeocerebellum and the neocerebellum. The archaeocerebellum is formed by the flocculus, nodule and lingula of the vermis. It has only vestibular connections, which travel in the inferior cerebellar peduncle. The archaecerebellum controls balance. Dysfunction leads to truncal ataxia characterised by a drunken gait, with swaying of the trunk and titubation when sitting, standing or walking. Typically there is neither alteration in fine movements nor nystagmus. Reflexes are normal and there is no tremor. The anterior lobes of the lateral hemispheres, together with uvula and pyramid of the vermis form the palaeocerebellum. The palaeocerebellum connects to the spinocerebellar tracts and is involved with postural reflexes. Dysfunction leads to postural imbalance and increased reflexes. The posterior lobes of the lateral hemispheres form the neocerebellum, which connects to the cerebral hemispheres, basal nuclei and the pontine nuclei, via the middle cerebral peduncle. It is involved in the coordination of fine, voluntary movements. Dysfunction leads to nystagmus, intention tremor, dysdiadochokinesis, hypotonia and decreased or pendular reflexes. The vermis and surrounding nuclei of the cerebellum form the roof of the 4th ventricle. An expanding lesion may obstruct the cerebrospinal fluid (CSF) flow, with resultant hydrocephalus and truncal ataxia.

Cerebellar peduncles and connections

The cerebellum receives inputs from the vestibular and peripheral nervous systems. Outputs, which coordinate muscle movements by modifying tone and contraction, travel in three sets of peduncles (superior, middle, and inferior) between each cerebellar hemisphere and the brainstem.

Acute cerebellar ataxia

Acute cerebellar ataxia is the most common diagnosis in acute ataxia in children, particularly between 2 and 7 years of age. It is a diagnosis of exclusion, after consideration of more sinister causes such as tumours. An autoimmune aetiology is likely, with autoantibodies demonstrated in acute cerebellar ataxia following infections with varicella,^1,2 Epstein–Barr virus (EBV),³ mycoplasma and human parvovirus B19.⁴ The clinical presentation is of a prodromal illness, frequently non-specific, with or without an exanthema, 5–10 days prior to the onset of acute ataxia, though the timing may show considerable variation. If a specific aetiology is present it is most commonly varicella,⁵ though a number of other viruses have been implicated (Table 8.5.2).^6–22 Acute cerebellar ataxia usually presents with sudden onset of severe gait ataxia, though a small number of cases have an insidious onset. Most have dysarthric speech. Mild horizontal nystagmus occurs in 50% of cases. Findings of intention tremor, dysdiadochokinesis, hypotonia and decreased or pendular reflexes are seen in two-thirds of cases, but are less pronounced than the gait disturbance. Truncal ataxia is uncommon. Unlike acute disseminated encephalomyelitis (ADEM) or multiple sclerosis, there are no focal neurological signs.

Investigations are aimed at excluding an alternative diagnosis, if the diagnosis is unclear. The computerised tomography (CT) scan is normal in acute cerebellar ataxia; however, magnetic resonance imaging (MRI) may be abnormal. In one series, inflammatory changes were seen in the cerebellum of one of nine children.^23,24 There is a slight elevation of CSF cell count by 4–50 cells per microlitre in 32%, though occasionally (in 8%) the elevation is higher. There may also be slightly elevated protein 410–900 mg L^–1.²⁵

Acute cerebellar ataxia usually begins to improve within a few days, but full recovery may take from 10 days to 2 months. Patients who have a slower recovery are still likely to recover fully. In one series, 91% recovered fully from their ataxia, including all those with varicella, EBV or post-vaccination, but 8% had sustained learning problems. Varicella-associated ataxia recovered quicker than non-varicella.²⁴

Poisoning

Ataxia is not uncommonly a presenting symptom of ingestion or excessive therapeutic drug levels, particularly anticonvulsants.

Accidental poisoning occurs most commonly in 1 to 4-year-old children. A wide variety of compounds are implicated, including alcohols, substances of abuse and essential oils, as well as medications (Table 8.5.3). In addition to ataxia, these children may have nystagmus, altered mental status and vomiting. This is different to acute cerebellar ataxia where consciousness is unimpaired.

Anticonvulsants

Phenytoin toxicity with serum levels of >20–30 mcg mL^–1 may produce signs of ataxia, nystagmus on lateral gaze and drowsiness. Onset of symptoms following an acute ingestion is usually within 1–2 hours and may persist for 4–5 days. At >30 mcg mL^–1, the ataxia and drowsiness become more marked and the nystagmus vertical.^26–28

Carbamazepine toxicity may also lead to ataxia. There are usually associated findings of drowsiness and nystagmus. There may be progression to seizures and coma, particularly if the level is >100 μmol L^–1.²⁹

Benzodiazepines

Ataxia may be the sole presenting feature of benzodiazepine ingestion in children. In one series, it was an isolated finding in one-fifth of the cases who demonstrated ataxia. Other findings included lethargy (57%), GCS <15 (35%) and respiratory depression (9%).³⁰