Is it Hereditary?

This is a question that is often put to hospital specialists by patients who have been diagnosed with a Chiari malformation. The answer is that it depends on what you mean by Chiari malformation and what you mean by hereditary. The term hereditary refers to a physical trait, or some aspect of a person’s being, that has been inherited from parents, grandparents or other forebearers. Such a feature will therefore have a genetic basis, being dictated by the code carried by DNA molecules, contained in the 46 chromosomes that we have in almost all cells in our body. The science known as molecular biology seeks to unravel the details of this genetic code and we can anticipate details emerging, in coming years, about which part or parts of individual chromosomes control the multitude of structures and physiological processes that underlie our very existence.

It might be helpful at this point to draw a distinction between the terms hereditary and congenital. The word congenital refers to how we are each made up as a result of our parentage, which is clearly similar in meaning to our heredity. When used to refer to medical conditions, however, the term is normally describing an abnormality that is present at birth. Such abnormalities are usually structural in nature, although they may not be externally apparent at the time. Importantly, they are not always genetic in origin but may have been caused by some environmental influence whilst in the womb. This is why a lot of attention is paid to possible harmful effects of any medicines or drugs taken during pregnancy. Hereditary conditions, on the other hand, whilst being part of an individual’s make-up and therefore present at birth, may not be evident at the time, only coming to light later in life. In summary, not all congenital abnormalities have a genetic origin and not all genetically based conditions are evident at birth.

As with congenital, the word hereditary can have different meanings according to the context in which it is used. An hereditary condition is generally seen as one in which there is an established genetic component. Geneticists can often provide estimates of the likelihood of such a condition being passed on to offspring. Many other conditions, however, which are not categorised as being hereditary disorders, are recognised as having hereditary components, or representing a familial trait. The risk of some cancers, for example, may be higher for an individual who has a history of the condition in the family. Even appendicitis has a tendency to run in families but would not normally be classified as an hereditary disorder.

So where do Chiari malformations fit in? Type 2 malformations form part of the spectrum of spina bifida malformations and both spina bifida and Chiari type 2 malformations are commonly accompanied by the condition known as hydrocephalus. Most cases will be evident at or shortly after birth and are therefore normally categorised as congenital malformations. Spina bifida itself is something that can have a familial component, and therefore so can type 2 malformations.

Chiari type 1 hindbrain hernias, on the other hand, most often come to light in adolescence or in early to mid-adult life. Chiari type 1 is not, therefore, usually categorised as being congenital. The issue is further complicated by our increasing appreciation that Chiari type 1 is not a single condition but, rather, an internal anatomical feature (often an incidental radiological finding) that can result from a number of different underlying causes. The commonest cause is a mismatch between the size of that part of the brain referred to as the cerebellum and the part of the skull in which it is contained, the so-called posterior fossa. In other words it is a consequence of the size and shape of a particular body structure, albeit an internal one.

The question, then, is how likely it is that parents with Chiari might pass on this anatomical feature to their offspring. We cannot answer this with any confidence and certainly cannot give a percentage likelihood of it happening. It is a bit like asking how likely it is that a child will develop the same shape of nose as one or other parent. Furthermore, whereas we can, sometimes, pick out a specific facial characteristic of a parent, in a child, more often than not such a resemblance is partial only. Whilst it may be reasonable to suggest that some cases of Chiari type 1 malformation could, in part, have a genetic basis, being the result of DNA programming, we do not regard Chiari malformation as an hereditary disorder. Genetic counselling is certainly not currently advocated by specialists dealing with Chiari malformations, although genetic studies are sometimes undertaken in a research setting.

We can’t rule out the influence of environmental factors, affecting development inside the womb, as a cause of Chiari type 1 malformations, but there is no strong evidence to indicate that this is the case. Furthermore, given the high prevalence of low cerebellar tonsils in the population as a whole, as revealed by MR imaging (about 1% overall), it is possible that some “clustering” could occur in a family simply by chance, without any genetic influence.

The answer to the original question, therefore, is “possibly, in some cases”. A parent with Chiari type 1 malformation may pass the same genetic trait to a child but the likelihood of this happening is not high and the consequences, should it happen, will not necessarily be troublesome. Should a child of a Chiari parent develop symptoms, which can be similar to or different from those experienced by the parent, then the child can always be referred for assessment and counselling by a suitably experienced specialist. A need for any subsequent intervention is far from inevitable. People with Chiari type 1 malformations need not avoid starting a family, just on the basis of their condition. There may be obstetric considerations around the time of delivery but these are separate issues to be discussed at the time, between the individual, her obstetrician and her neurosurgeon. With Chiari type 2, and its associated hydrocephalus and spina bifida, it is likely that such discussions will take place anyway.