Tuberous sclerosis (TSC) is a condition well known to paediatricians for causing severe epilepsy, learning difficulties and behaviour problems in about half of those affected.1 It also causes cardiac rhabdomyomas, skin lesions, renal cysts and angiomyolipomas, subependymal giant cell astrocytomas and retinal astrocytomas.2^–5 TSC results from damage to one of two genes: TSC1 on chromosome 9 and TSC2 on chromosome 16.6 7 The protein products of the TSC1 and TSC2 genes are called hamartin and tuberin, respectively. Both genes function as tumour suppressor genes, and, when they are damaged, as in TSC, there is a tendency to form benign tumours (hamartomas) in most organs of the body.8 9 Occasionally, people have contiguous deletions of the TSC2 gene and the adjacent adult-onset polycystic kidney disease gene (PKD1), and they develop early-onset polycystic kidney disease as well as the other features of TSC.

In the central nervous system, the hamartomas take the form of the cortical and subcortical hamartomas (tubers) that develop before birth and which are responsible for the epilepsy and some of the learning difficulties that affect many affected patients. There is a relationship between the number of tubers and the age of onset and degree of severity of both epilepsy and learning difficulty. However, it is not possible in any one person to give a prognosis for learning difficulty based on the number of their cerebral lesions.10 Those with one lesion can be badly affected, whereas those with many lesions can escape both epilepsy and learning difficulty. If a child with TSC has developed normally up to 5 years of age, then it is likely that their development will continue along a normal trajectory. Permanent developmental regression does not occur with late-onset (ie, after 5 years of age) epilepsy in TSC.1 …