The authors from Norway state that cerebral palsy is the most common cause of physical disability in children and affects about 2 in every 1,000 births. It usually results in impaired movement that is associated with symptoms such as unsteadiness, involuntary movements and floppiness or rigidity of limbs and body.
The disease can affect an individual's functional abilities to varying degrees. In severe cases, a person with cerebral palsy is unable to walk, may have intellectual disabilities and may also experience blindness, deafness or epilepsy.
Cerebral palsy is believed to originate from damage to the immature brain, and although pregnancy risk factors such as preterm delivery, multiple fetuses and atypical intrauterine growth have been identified, the causes of the condition are largely unknown.
Other studies have found there could be a link between family and the development of cerebral palsy, but the Norwegian researchers say that positive findings have been hard to replicate. This has led to the objective of their new study: to investigate the risks of recurrence of cerebral palsy in family members in order to see if there are any patterns of hereditability.
'Over 2 million participants'
For the study, the researchers used data from the Medical Birth Registry of Norway, which is linked to the Norwegian social insurance scheme, to examine 2,036,741 Norwegians born between 1967-2002. This figure breaks down to 1,991,625 single births and 45,116 sets of twins.
The study investigated the cerebral palsy incidence across a broad range of family relationships. In over 2 million births, the researchers found 3,649 cases of cerebral palsy.
They also examined 1,851,144 pairs of first-degree relatives, 1,699,856 pairs of second-degree relatives and 5,165,968 pairs of third-degree relatives.
The researchers found 3,649 cases of cerebral palsy among the births - 1.8 cases for every 1,000 children. This rate was found to be higher in twins (5.1 per 1,000) than in single births (1.7 per 1,000). In twins of affected children the risk was even higher; if one twin had developed cerebral palsy then the risk of the other twin developing the condition was 15 times higher.
In families with an affected single child, there was also an increased risk for subsequent siblings. Subsequent first-degree relatives had a risk of developing the condition that was six to nine times higher, and subsequent second-degree relatives had a risk that was up to three times higher.
Parents affected by cerebral palsy were found to be 6.5 times more likely to have a child with the condition, compared with parents who were unaffected by cerebral palsy. On the other hand, there was little evidence for an increased risk associated with third-degree relatives (first cousins) who were affected by the condition.
The authors of the study also found that these increased risks were independent of sex and were still present after preterm births (an identified risk factor for cerebral palsy) were excluded from the data.
'Genetic component' but not sole cause
There were some limitations with the research that were acknowledged by the authors. There is a lack of information about the different subtypes of cerebral palsy, of which some may have different causes to others. Equally, the study is unable to distinguish cases of cerebral palsy with post-neonatal causes.
It is also noted that far fewer people with cerebral palsy went on to become parents, compared with people who were unaffected (11% as opposed to 51% in people 19 or older). The authors suggest that this is most likely to be "influenced by social challenges among people with impairments such as cerebral palsy, which limit opportunities to develop relationships."
In an editorial accompanying the article published by The BMJ, Prof. Peter Rosenbaum, pediatrician and Canada Research Chair in Childhood Disability, says that over time, "as adults with cerebral palsy have more opportunities to become biological parents, researchers will be able to explore inheritance with greater certainty."
The authors write that their findings suggest that "cerebral palsy includes a genetic component, with a stronger recurrence among relatives with closer genetic relationship." However, they also state that it is not a simple case of genetics being the sole influencing factor:
"The similar risks of cerebral palsy of co-twins affected like-sex and unlike-sex twin pairs suggest that genetic influences are only part of a wide range of causes."
The authors suggest that future studies should consider the possibility of genetic susceptibility to environmental causes as well as genetic causes.
Within the past year, Medical News Today helped to fund treatment for Daniel Pretty, a young boy with cerebral palsy. A story reporting on his life-changing surgery was published last month.