Effect of Digit Ratio on Contact Heat Evoked Potentials in China
Contact Heat Evoked Potentials (CHEPs) have emerged as a significant electrophysiological diagnostic tool for small fiber neuropathy, a condition characterized by the involvement of myelinated Aδ and unmyelinated C fibers due to various etiologies. CHEPs are influenced by several factors, including gender, age, and body height. However, the effect of the second to fourth digit ratio (2D:4D) on CHEP parameters has not been previously investigated. The digit ratio, defined as the length of the index finger divided by the length of the ring finger, is typically smaller in men than in women. This study aimed to explore whether the digit ratio influences CHEPs and to enhance the understanding and methodology of CHEPs.
The study recruited healthy participants who underwent comprehensive neurological examinations, laboratory tests, and nerve conduction studies (NCSs). The NCSs were performed on bilateral median, ulnar, tibial, fibular, and sural nerves. The eligibility criteria for participants were detailed in the supplementary material. The digit ratio was measured by determining the length of the fingers from the bottom crease to the tip on the palmar surface of the hands.
CHEPs were conducted at four specific sites: the forearm (FA), the leg (LE), the seventh cervical vertebrae (C7), and the twelfth thoracic vertebrae (T12). The FA site was located at the upper border of the distal third volar aspect of the right and left forearms. The LE site was above the lateral malleolus at the upper border of the distal third of the right leg. The C7 and T12 sites were at the spinous processes of the respective vertebrae. For each condition, a series of ten stimuli were administered, with an interstimulus interval set between 10 and 18 seconds. The N2 latency, P2 latency, and N2-P2 amplitude were determined by two independent clinicians.
Statistical analysis was performed using SPSS 19.0. The correlation between CHEP parameters and the digit ratio, age, gender, and height was analyzed using linear regression. A two-tailed P value of less than 0.05 was considered statistically significant. The study enrolled a total of 137 healthy participants, including 72 men, with an average age of 34 ± 14 years.
The results indicated that the N2 latency of the right FA was correlated with age, height, and the right digit ratio. The linear regression equation for this relationship was Y = -42.354 + 0.720 age + 0.796 height + 237.795 right digit ratio. The P2 latency of the right FA was correlated with the right digit ratio, with a linear regression equation of Y = 13.583 + 447.915 right digit ratio. The N2-P2 amplitude of the right FA was correlated with age, with a linear regression equation of Y = 191.771 – 0.904 * age.
For the left FA, the N2 latency was correlated with age, with a linear regression equation of Y = -428.714 + 1.192 age. The P2 latency of the left FA was correlated with the left digit ratio, with a linear regression equation of Y = -739.764 + 1236.292 left digit ratio. The N2 latency of C7 was correlated with age, with a linear regression equation of Y = -203.837 + 8.980 age. The P2 latency of C7 was also correlated with age, with a linear regression equation of Y = -246.253 + 9.689 age.
The N2 latency of T12 was correlated with age, with a linear regression equation of Y = -221.157 + 9.927 age. The P2 latency of T12 was correlated with age, with a linear regression equation of Y = -276.103 + 12.502 age. The N2 latency of the LE was correlated with age, with a linear regression equation of Y = -309.588 + 13.181 age. The P2 latency of the LE was correlated with age, with a linear regression equation of Y = -354.242 + 15.375 age. The N2-P2 amplitudes of the left FA, C7, T12, and the LE were not correlated with the digit ratio, age, gender, or height.
The 2D:4D digit ratio is considered a marker of the ratio of testosterone and estrogen levels during embryogenesis, with a smaller ratio typically observed in men than in women. It has been associated with various conditions, including migraine and tension-type headache. In this study, the N2 and P2 latencies of the right and left FA were correlated with the right and left digit ratios, respectively, but were not correlated with gender. This discrepancy may be due to the fact that the digit ratio is primarily influenced by fetal sex hormones and is established during early embryogenesis. In contrast, the relationship between the digit ratio and adult sex hormone levels is less clear, as many studies have found this association to be statistically non-significant.
This study suggests that the digit ratio is an influencing factor on CHEPs, independent of gender. Further research with larger sample sizes is necessary to confirm these findings and to explore the underlying mechanisms. The results of this study contribute to the growing body of knowledge on CHEPs and their potential applications in diagnosing and understanding small fiber neuropathy.
doi.org/10.1097/CM9.0000000000001868
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