The etiology and incidence of known metabolic and hereditary disorders associated with unilateral or bilateral structural cerebellar abnormalities, defined by CT and/or MRI, were determined in 78 children examined at the University Hospital Aachen, Germany, and Katholieke Universiteit Leuven, Belgium. Lesions were bilateral in 62 and unilateral in 16, both cerebellar hemispheres were involved in 38, the vermis in 15, and pontocerebellar in 9. Hemisphere atrophy was static in 10 and progressive in 28. MRI was superior to CT in definition of lesions. Genetic/metabolic causes were found in more than half the cases of ponto-cerebellar hypoplasia or progressive cerebellar atrophy, but in none with unilateral cerebellar lesions. These included amino and organic acidurias, lactic acidosis, lysosomal and peroxisomal disorders, Menkes kinky hair disease, molybdenum cofactor deficiency, and autosomal dominant ataxias. Other causes of cerebellar and pontocerebellar hypoplasia included intrauterine ionizing radiation, phenytoin exposure, cytomegalovirus, chromosomal syndromes, hypogonadism, Ito’s hypomelanosis, and carbohydrate-deficient-glycoproteins syndromes. Investigations should include EEG, EMG and NCS, abdominal ultrasound, urine screening for amino and organic acids, and blood tests for acanthocytes, liver function, protein electrophoresis, ammonia, lactate and pyruvate, copper and ceruloplasmin, immunoglobulins, VLC fatty acids, and glycoproteins. An overview of the literature is also presented. 
COMMENT. Pontocerebellar hypoplasia or progressive cerebellar atrophy defined by MRI is an indication for biochemical and neurophsiological tests for hereditary or degenerative neurological disorders.
Focal cerebellar lesions and associated learning impairments were detected in 8 patients cf 6 control subjects tested in a serial reaction-time task at the Catholic University, and University of Rome ‘La Sapienza’, Rome, Italy . Cerebellar patients had longer reaction times than controls when stimuli were presented in sequence.