Finger length probed in motor neurone disease
“The length of a person’s fingers could reveal their risk of motor neurone disease,” reported the BBC. It said that a study had tested whether the most common form of the disease was related to the length of the ring and index finger in adults.
In this study, researchers measured the finger lengths in 110 men and women. Just under half had amyotrophic lateral sclerosis (ALS) the most common form of motor neurone disease. Both men and women with ALS were found to have relatively longer ring fingers than index fingers.
This research works towards the important goal of identifying exposures in the womb that increase the later risk of diseases in adulthood. However, the study had a number of limitations that affect the strength of its conclusions, one of which is its small size. Larger studies of a more robust design are needed to confirm this theory.
The study does not mean that everyone with a relatively long ring finger is at a higher risk of motor neurone disease. Experts believe there are several genetic and environmental factors that contribute to the disease’s development.
Where did the story come from?
The study was carried out by researchers from the Institute of Psychiatry, London. It was supported by the Medical Research Council and the Motor Neurone Disease Association of Great Britain. The study was published in the (peer-reviewed) Journal of Neurology, Neurosurgery and Psychiatry.
The study was reported accurately by the BBC, which included in its report comments from an independent expert.
What kind of research was this?
This case control study aimed to test the theory that high testosterone levels in the womb are a risk factor for the later development of amyotrophic lateral sclerosis (ALS) the most common form of motor neurone disease. It is unknown what causes ‘sporadic’ ALS (occurring in people with no known family history of the disease) or what the risk factors for it are.
In general, a case control study is not the best study design to answer this sort of question, as the cases will differ from the controls on many known and unknown characteristics anyway. Ideally, a group of patients at high risk of the disease should be assessed for finger length and then followed up over time.
The researchers say that prenatal factors are known to influence the development of ALS and blood testosterone levels are thought to play an important role in the normal function of motor neurons, the nerve cells that control muscle function. They say that a relatively longer ring finger (compared to the index finger) and measured by a ratio, is considered a surrogate marker for high levels of testosterone during a baby’s development.
In this study, they looked at the difference in length between the ring and index fingers in people who have and did not have ALS. Although the male sex is associated with both higher prenatal levels of testosterone, and with an increased risk of ALS, the researchers thought that this association would be independent of gender and therefore the link would also be present in women.
What did the research involve?
The researchers recruited patients diagnosed with ALS and unrelated individuals from a specialist referral centre for the disorder. Using a digital camera, they photographed people’s hands with the fingers fully flattened out. They used four independent scorers who were 'blinded' to the disease status of participants (they did not know which were ALS patients and which were not) to measure the length of the fingers, using a computer programme. The measurements were usually taken from the photo of the right hand.
The researchers excluded any photos where the fingers were difficult to measure, for instance where the fingers could not be fully flattened owing to muscle contraction. Using results from the four scorers, they calculated the average ratios between the ring and index fingers of participants. A statistical analysis was then carried out to examine whether there was any association between this ratio and ALS. The researchers adjusted their figures to take account of the sex ratio between groups.
What were the basic results?
Out of 141 people whose hands were photographed (73 with ALS and 68 controls), the researchers excluded 21 whose fingers could not be measured accurately because of muscle contracture. This could represent up to 29% of the ALS group.
They excluded a further 10 who could not be measured by one of the four scorers. The remaining 110 photographs were included in the analysis, 47 of which were from patients with ALS.
The researchers found the ratio of the index to the ring finger length was lower in people with ALS, compared to the controls. This means that people with ALS were more likely to have a longer ring finger, relative to their index finger. This finding was independent of gender.
How did the researchers interpret the results?
The researchers conclude that patients with ALS have a lower ratio of ring to index finger length, which is consistent with the theory of higher prenatal circulating levels of testosterone. They say this indicates that prenatal levels of testosterone could be a factor in the risk of developing ALS as an adult.
Although male sex is associated with both higher levels of prenatal testosterone and an increased risk of ALS, the researchers say it is prenatal testosterone levels rather than gender itself that is the risk factor for ALS. This is borne out by their finding that the link was independent of gender.
Conclusion
This study has a number of limitations:
- As the researchers note, people with muscle contractures were excluded from the analysis. Since the disease can cause the muscles of the hands or fingers to contract, those excluded were more likely to be patients with ALS. This could have affected the results, but the numbers are not reported.
- The study may have been too small to detect any effect of gender on the relative length of the ring finger. This is because the already small sample would have been even smaller when analysed separately by sex. This undermines the conclusion that the association between the risk of ALS and prenatal levels of testosterone (as indicated by a long ring finger) is independent of gender.
- The researchers do not describe how the cases and controls were selected or any details on other diagnoses, age, sex or other factors. This means that it is not possible to tell how different the groups were or what neurological conditions the control group had for example.
- Importantly, the researchers did not actually measure testosterone, in either its bound or unbound forms, in the adult participants. If there is a strong link between prenatal and adult hormone concentrations, a correlation between the finger measurements and adult hormone concentrations could be expected. This would be important evidence to collect to support or reject the theory.
Many studies have been conducted on the link between finger length and a range of adult conditions. The study of prenatal factors that may influence the development of motor neurone disease in later life is important as it could lead to the development of valuable preventative measures.
The underlying mechanism needs to be better understood and larger studies of a more robust design are needed to test the theory that levels of sex hormones in the womb are a contributory factor in non-inherited motor neurone disease.
