**3.2 Glucose transporter type 1 deficiency**

Glucose transporter type 1 (Glut1) deficiency syndrome is caused by heterozygous, mostly de novo, mutations in the SLC2A1 gene encoding the glucose transporter GLUT1. This glucose transporter is the most important energy carrier of the brain through the blood–brain barrier [40]. Glut1 deficiency syndrome is classically characterised by the presence of childhood epilepsy, developmental delay, acquired microcephaly, cognitive impairment, spasticity, ataxia, and dystonia.

Paroxysmal manifestations including seizures and nonepileptic paroxysmal episodes are also part of the phenotype [40, 41]. Clinical severity may vary from mild to severe neurological dysfunction. In children with mild phenotypes, paroxysmal manifestations, such as exercise-induced dyskinesia or episodic ataxia, may be the only manifestations of this condition [41]. Low CSF level of glucose is a characteristic finding in Glut1 deficiency syndrome and mutation analysis of the SLC2A1 gene confirms the diagnosis [38].

The ketogenic diet provides an alternative source of energy by switching brain metabolism from glucose to ketone bodies, it is considered the gold standard treatment and can improve or reverse symptoms, especially if started as early as possible.

### **3.3 CAPOS syndrome**

CAPOS syndrome, named after its symptoms (cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss) is a rare condition caused by variants in the ATP1A3 gene that encodes the α3 subunit of Na+/K+ -ATPase, an integral membrane protein responsible for the regulation of sodium and potassium concentrations over the cell membrane [42]. Three classic phenotypes (rapid-onset parkinsonian dystonia, alternating infantile hemiplegia, and CAPOS syndrome) were initially described, but recently an increasing number of cases presenting with atypical and overlapping features have been reported [43].

The symptoms usually start in childhood between 1 and 5 years of age, and patients typically present an acute episode of fever-induced ataxia associated with encephalopathic features and weakness [42, 43]. Recovery from the episode is complete in most cases and a recurrent course is characteristic. Afterwards, the patients show a slow progression of the disease [42, 44]. Patients classically experience two to three episodes of acute ataxia during life before transitioning to a slowly progressive evolution [44]. There is no specific treatment for CAPOS syndrome, acetazolamide has been used to prevent relapses of the acute episodes [42].

### **3.4 Others**

Other causes of intermittent ataxia in children include metabolic disorders like pyruvate dehydrogenase deficiency [45, 46], pyruvate decarboxylase deficiency, urea cycle defects, aminoacidopathies, organic acidopathies, recurrent ADEM, multiple sclerosis, benign paroxysmal vertigo [36], vestibular migraine, between others [35].
