Ataxia in the child

A child with progressive incoordination: unsteady gait, dysmetria, intention tremor, dysarthria, or loss of previously fluent motor skills. "Ataxia" is a syndrome, not a disease, and the bedside question is whether the lesion is in the cerebellum, the sensory pathways (dorsal columns / peripheral nerves), or the vestibular system. The diagnostic work begins by separating acute / acquired ataxia (postinfectious cerebellitis, ingestion, posterior fossa tumor, opsoclonus-myoclonus) from progressive / genetic ataxia, which is what this leaf is about.

Three splits do most of the work:

1. Acquired versus genetic. Acute onset, fever, vomiting, headache, or a posterior fossa finding on imaging means acquired until proven otherwise. Slow progression over months to years, family history, or a multisystem signature (cardiomyopathy, immunodeficiency, telangiectasias) shifts you to genetic.
2. Cerebellar versus sensory versus vestibular. Pure cerebellar ataxia has dysmetria, intention tremor, nystagmus, and a wide-based gait independent of vision. Sensory ataxia worsens with eyes closed (positive Romberg) and has absent vibration / position sense; this is the Friedreich ataxia pattern.
3. Inheritance and treatability. Always front-load the treatable causes: vitamin E deficiency, cerebrotendinous xanthomatosis, abetalipoproteinemia, biotinidase deficiency, coenzyme Q10 deficiency, and refsum disease can all be partially or fully arrested with replacement or substrate restriction. Miss them at the patient's cost.

Must-not-miss treatable causes

• Ataxia with vitamin E deficiency (AVED, TTPA): mimics Friedreich exactly, including kyphoscoliosis and areflexia; vitamin E replacement halts and partially reverses neurologic decline.
• Cerebrotendinous xanthomatosis (CYP27A1): chronic diarrhea, juvenile cataracts, tendon xanthomas, then ataxia and cognitive decline. Chenodeoxycholic acid is disease-modifying.
• Abetalipoproteinemia (MTTP): acanthocytes on smear, fat malabsorption, retinitis pigmentosa, fat-soluble vitamin deficiencies. High-dose vitamin E plus low-fat diet.
• Biotinidase deficiency (BTD): seizures, alopecia, dermatitis, optic atrophy, ataxia. On most newborn screens; biotin replacement is curative.
• Refsum disease (PHYH): elevated phytanic acid, retinitis pigmentosa, anosmia, peripheral neuropathy. Phytanic acid restriction.
• Coenzyme Q10 deficiency: treatable with high-dose CoQ10; consider in ataxia with myopathy or nephrosis.

Autosomal recessive (childhood onset)

• Friedreich ataxia (FXN, GAA repeat expansion): the most common AR ataxia in children of European descent. Progressive gait ataxia before age 25, areflexia in the legs, extensor plantars, pes cavus, scoliosis, hypertrophic cardiomyopathy, and diabetes. Sensory pattern (dorsal columns, dorsal root ganglia) with brisk reflexes is paradoxical and a giveaway.
• Ataxia-telangiectasia (ATM): ataxia begins as a toddler, ocular telangiectasias appear later (often years after ataxia). IgA / IgG2 deficiency, recurrent sinopulmonary infections, leukemia/lymphoma risk, radiosensitivity. Elevated alpha-fetoprotein is the cheap, high-yield screening test.
• Ataxia with oculomotor apraxia (AOA1 APTX, AOA2 SETX): ataxia plus head-thrust to compensate for slow saccades. AOA2 also raises AFP (so AT is not the only one). AOA1 has low albumin; AOA2 has elevated AFP.

Mitochondrial

• Leigh syndrome: infantile to childhood regression with ataxia, dystonia, brainstem signs, and symmetric basal ganglia / brainstem T2 lesions. Many genes; mtDNA and nuclear both.
• MELAS and other mitochondrial encephalopathies: ataxia may be a component alongside stroke-like episodes, lactic acidosis, hearing loss.

Congenital / structural cerebellar disorders

• Joubert syndrome: molar tooth sign on MRI, hypotonia, abnormal eye movements, irregular breathing in infancy, then ataxia. Ciliopathy.
• Pontocerebellar hypoplasias, Dandy-Walker, cerebellar hypoplasia syndromes: structural on MRI from birth.

Lysosomal / storage with ataxia

• Niemann-Pick disease type C: vertical supranuclear gaze palsy, splenomegaly, gelastic cataplexy, progressive ataxia. Miglustat is partially disease-modifying.
• Late-onset Tay-Sachs / GM2 gangliosidoses: ataxia, lower motor neuron findings, psychiatric features in adolescence.

1. MRI brain with attention to cerebellum, posterior fossa, and brainstem. Rules out tumor and structural malformations; the molar tooth sign, cerebellar atrophy pattern, and basal ganglia signal all narrow the differential.
2. Alpha-fetoprotein. Elevated in AT and AOA2; cheap and high-yield.
3. Vitamin E level, lipid panel (low cholesterol / triglycerides in abetalipoproteinemia), peripheral smear for acanthocytes.
4. Lactate, ammonia, plasma amino acids, urine organic acids, acylcarnitines for metabolic / mitochondrial.
5. Echocardiogram and ECG if Friedreich is on the table (hypertrophic cardiomyopathy).
6. Immunoglobulins if AT is suspected (low IgA / IgG2).
7. Targeted genetic testing or ataxia gene panel. FXN repeat sizing is a separate assay from sequencing panels; order it explicitly when Friedreich is on the differential, because expansions are missed by sequencing alone.