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The causes of epilepsy: inheritance THE CAUSES OF EPILEPSY: INHERITANCE
Until about 40 years ago most doctors believed that inheritance was a major factor in causing epilepsy. This belief is still strong amongst the population at large. Doctors are often told 'It can't be epilepsy because there is nothing like that in the family,. These views were held in the past with such force in some states of America and in some Scandinavian countries that it was illegal for people with epilepsy to marry. It is certainly true that genetic factors do play a part in epilepsy, but not an overwhelming part.
There are some genetic diseases in which inheritance is through a dominant gene. Genes come in pairs, one from each parent. One member of the pair may always be dominant in influencing the structure or biochemistry of the offspring. If a child or adult has the gene, then, in broad terms he or she has the disease, although there are variations in the severity of the disease. One of the parents; carrying the gene will therefore not only show the effects of the gene himself or herself, but will transmit the effective gene to, on average, half his or her children. Tuberous sclerosis and neurofibromatosis, both disorders affecting the structure of nerve cells and surrounding tissue, are transmitted in this way.
There are other genetic diseases in which the gene is recessive. In recessive inheritance, the effects of the gene are only expressed if a child has a double dose of the relevant gene—one abnormal gene from each parent. The parents, although themselves carriers, do not show the abnormality as the other member of their pair of genes is normal. There are certain rare disorders of metabolism of the brain, collectively known as the lipidoses, which are inherited by recessive genes. Fatty substances known as lipids are important constituents of the membranes surrounding the nerve cells. A disorder of the structure and function of the cell membrane may well lead to paroxysmal discharge of nerve cells—an epileptic seizure.
It must be stressed that these diseases are rare. They have been mentioned first only because the mechanism of their inheritance is most clearly understood.
There is, however, also good evidence that primary generalized idiopathic epilepsy is also inherited. In order to explain this, it is easier to trace back from a child with epilepsy to his parents, rather than first considering the chances of a prospective parent with epilepsy having an epileptic child. The characteristic EEG is seen in about 40 per cent of brothers and sisters of children with primary generalized epilepsy, even if these brothers and sisters have not had any apparent seizures. That is to say, the abnormality which causes the abnormal EEG record is inherited, but this abnormality is not necessarily expressed in clinically apparent seizures. A smaller proportion of the parents of children with primary generalized epilepsy will also show the characteristic EEG changes. We know from following the children with these EEG changes that the characteristic discharges become much less frequent with age, so the absence of discharge in adult life does not mean that the parent did not have unrecorded and unapparent discharges in childhood. From mathematical studies of the proportion of the abnormal EEG records of many families with primary generalized epilepsy, it is possible to calculate that the pattern of inheritance is probably that of a dominant gene.
Energetic research studies are ongoing in several centres to identify the gene. It will probably turn out that there is more than one gene, each giving a similar clinical picture. This has already been shown to be true for the much more clearly defined disorder of tuberouss sclerosis, a disorder in which there are nests of abnormally developed nerve cells and their supporting cells (known as glial cells), some of them calcified and some sufficiently large to be seen on a brain scan. It is now known that two dominant genes on separate chromosomes can result in what appears to be an identical picture. (A chromosome is the microscopically visible structure within the nucleus of a cell which contains the genetic material—the DNA.)
As we have already explained, for a child to show an abnormality in cases of dominant inheritance, only one member of the pair of relevant genes (one from the father and one from the mother) needs to be abnormal. However, the effects of other gene pairs may to some extent succeed in suppressing this gene from expressing itself in obvious seizures. This means that about only one third of the children to whom it is transmitted will have seizures. Furthermore, even if the gene is expressed in seizures, the result may only be a few absence seizures in childhood.
The variability in clinical expression of the genetic abnormality accounts for the occurrence of primary generalized epilepsy in a child of parents neither of whom has ever had a known seizure. In such an instance one assumes that one parent does indeed have the gene, and, had an EEG been recorded in his or her childhood, the typical EEG discharge would have been seen.
Another aspect is the inheritance of a convulsive threshold. Any one of us can be made to have a seizure ii the stimulus is strong enough, and some of us do at lower levels of stimulus—at lower thresholds—than others. The inheritance of this level of threshold is probably
polygenic—that is to say, several genes, some recessive and some dominant, interact to produce the final result. Another example of polygenic inheritance is height. Tall parents tend to have tall children, but height is not determined by a single gene.
This inherited convulsive threshold is a background, as it were, to the whole of the area. It influences even those cases in which epilepsy clearly seems to be secondary to some obvious cause, such as a severe head injury causing local cortical scarring. Head injuries obviously are not inherited as such. Nevertheless there is a slight tendency for those who develop epilepsy after head injury to have a family history of epilepsy more often than those who do not develop epilepsy after what may be regarded as a comparable injury. What is being inherited here, through a number of different genes, is a lower-than-average convulsive threshold. The children of such head-injured parents are not likely to have seizures unless some additional cerebral damage affects them. It would be an unlikely family in which two members suffered severe head injuries, so that the risk of 'inheriting' epilepsy from a parent with epilepsy secondary to some structural brain damage is small. It follows that one good reason for a paediatrician or neurologist to do his or her best to find a 'cause' for epilepsy is so that they can best advise about the risk of brothers or sisters or daughters or sons being affected.
There is, however, one group of people in whom inherited and acquired characteristics interplay in a complex way. The tendency to febrile convulsions is inherited, through one or more genes.
A febrile convulsion, if very prolonged (lasting longer than 20-30 minutes), may rarely damage one or other temporal lobes of the brain through lack of oxygen occurring during the seizure. The scar in the temporal lobe may then act as a focus from which paroxysmal discharge—seizures—spread in later childhood and adult life.
The first duty of a doctor asked by a young couple, one of whom has epilepsy, about the chances of any child of theirs having epilepsy, is to characterize the seizure type as accurately as possible, using the description of the seizures and the EEC if it is clear that the prospective parent is having partial seizures, of generalized seizures which have a clear focal onset, either clinically or demonstrated on the EEG then, these seizures are almost certainly secondary to some area of cortical scarring or developmental abnormalities. The risk of any child of this marriage having epilepsy is only moderately higher than the risk of the population at large. It is, however, somewhat higher than the risk of the average child because of the inheritance of the convulsive threshold. If it is clear that the prospective parent has primary generalized epilepsy, then we have to say that about half his or her children will carry the gene, but only about one child in six will have definite seizures. The chances of epilepsy being a significant problem in the life of a child of a parent with primary generalized epilepsy is no more than of the order of five per cent, the others perhaps having only a minor EEG abnormality if that is specifically looked for.
A small fraction of the genetic material (DNA) is carried outside the nucleus in small particles in the cell known as mitochondria. These are only derived from the maternal ovum, not being present in sperm. There are therefore a few rare disorders in which inheritance is only through the maternal line. Some of these are associated with epilepsy.
*17\188\2*
Epilepsy
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