A new study has shown that low IQ in children with a difficult-to-treat form of epilepsy is associated with having had seizures when very young (under 12 months old). IQ does not depend on how many seizures the child has, or on what the underlying cause of their epilepsy is.

Temporal lobe epilepsy (TLE) is relatively common. The temporal lobe is the part of the brain which coordinates the storing of new memories and the retrieval of old ones. In adults, the most common type of difficult-to-treat TLE is caused by scarring of brain tissue (sclerosis). Adults with TLE often have problems with their memory but a normal IQ. Children's brains differ from adults' (especially in the first year of life) because they are still developing. A range of conditions can cause TLE in children, and these conditions are associated with a range of problems with cognitive ability and IQ.

Dr Francesca Cormack and colleagues from the Institute of Child Health in London therefore investigated IQ, cognitive ability and behaviour in a group of 79 children who were preparing for surgery to treat their TLE, since anti-epileptic drug treatment had not worked. The children were between 3 and 18 years old. Fifty-seven percent of the children had an IQ of less than 79 points (well below average) and 14% had such severe cognitive or behavioural difficulties that their IQ could not be assessed.

The patients' first seizures had occurred on average, at age 3-4 years, though age at first seizure ranged between birth and 13 years of age. Lower IQ was definitely associated with earlier age of onset of epilepsy: patients whose seizures started when they were less than 12 months old had significantly lower IQ than those whose seizures started later (55 points compared with 81).

Unlike in adults, IQ did not depend on the length of the period for which the children had experienced seizures. It also did not depend on the number of seizures they'd had or the underlying cause of their epilepsy. These causes included tumours, abnormal development of brain tissue, or sclerosis.

These results, published in the journal Epilepsia in January 2007, indicate that infants' brains are particularly vulnerable to damage by seizures. It is the epilepsy itself that causes the damage, reducing the density of cell growth in the "grey matter" of the brain (for more on the neocortex, click here). The development of neurones in the first year of life appears to lay down the basis for developing cognitive skills later on.

Early treatment for patients like these (with unusually severe epilepsy) is probably advisable, before the seizures cause more damage. Though TLE is reasonably often resistant to treatment with anti-epileptic drugs, surgery has a good success rate. Sixty-three percent of the children in this study became seizure-free after their operation.