Finger length clue to motor neuron disease

Motor neuron disease is a serious neurodegenerative disease that results in progressive paralysis and eventually death from respiratory failure. On average, a person survives two years after being diagnosed. The cause of the disease is still mainly unknown, although prenatal factors are thought to be important.
 
Genetic factors are known to contribute to motor neuron disease susceptibility and athleticism is thought to be an environmental risk factor.
 
In both men and women, motor neurons are dependent on testosterone for survival. Men are more likely than women to develop motor neuron disease and are also exposed to higher levels of testosterone before birth.
 
UK researchers suspected that it was the high prenatal testosterone level rather than male sex itself that was a risk factor for the development of motor neuron disease in later life, perhaps because this makes the adult motor neurons less sensitive to testosterone.
 
The researchers from King’s College London and Oxford University Nuffield Department of Clinical Neurosciences, looked at the ratio of the lengths of a person’s index finger to ring finger – known as the 2D:4D ratio. This is calculated by dividing the length of the index finger of the right hand by the length of the ring finger.
 
A low ratio means the ring finger is relatively long compared with the index finger and is thought to be an indicator of high prenatal testosterone levels in men and women (and is probably the reason why on average men have longer ring fingers than index fingers, while women often do not).
 
They carried out a study with 141 participants, 110 of whom provided useable results, including patients attending a specialist tertiary referral centre for motor neuron disease and people without the disease.
 
Using a digital camera to photograph hands, finger length was measured and ratios were calculated between fingers.
 
The 2D:4D ratio was consistently lower for people who had the ALS form of motor neuron disease compared to those without.
 
The researchers said their results suggested that testosterone levels in a person during their development might affect the subsequent risk of the ALS form of motor neuron disease.
 
The authors conclude: "Our reported association between low 2D:4D ratio and ALS nonetheless supports the possibility that very early factors in embryonic development might influence adult-onset motor neuronal degeneration."

Source: BMJ