Comment on: Median nerve conduction parameters can vary between different arm positions due to altered vascular, mechanical, and functional properties

To the Editor

We were interested to read the article by Fidanci et al¹ on a prospective study on the effect of different arm positions (ortho position (P1), anteflexion (P2), abduction (P3)) on median nerve conduction tests performed between January and July 2023.¹ The median nerve compound muscle action potential (CMAP), mean/minimum latency and negative duration were lowest at P1 and the mean CMAP amplitude was higher at P3 than at P1 or P2.¹ It was concluded that the mean NCS differs between arm positions P1, P2 and P3.¹ The study is noteworthy, but several points should be discussed.

The first point is that the NCS of the median nerve may be highly dependent on the blood supply to the upper limb.² Since the blood supply can vary between different arm positions² and since the nerve function of all nerves depends on the blood supply to this nerve, it is conceivable that the differences in the median NCSs between the three arm positions are at least partly due to the variable blood supply to the median nerve in the different arm positions.

The second point is that the length of the median nerve can vary in different positions due to stretching or contraction. The mechanical stress on the nerve can greatly affect the conduction properties of the nerve. In particular, the excitability of neurons decreases when a nerve is stretched, as has been shown in the context of soliton theory.³ This thermodynamic theory for the propagation of nerve impulses is based on the existence of a fusion transition in the nerve membrane.¹ During this transition, the area of the nerve membrane and the nerve thickness change as a function of thermodynamic variables such as temperature and hydrostatic pressure.³

The third point is that different arm positions require a different pattern of muscle innervation and therefore a different intensity of contracted muscles to maintain a particular arm position. Different muscle contractions can exert different tensions on the nerves, and different tensions can lead to different propagation properties of the innervating neurons. The different pressure on the median nerve at different arm positions could contribute to the differences observed in the 20 healthy volunteers.

The fourth point relates to the definition of normality. Since the exclusion criteria only included patients with neurological diseases (mononeuropathy, polyneuropathy, muscle weakness, Martin-Gruber anastomosis, abnormal neurological examinations), it is conceivable that people with cardiovascular diseases were also included in the study cohort. Patients with cardiovascular disease may have reduced blood flow to the investigated nerve and therefore strongly influence the conduction properties and thus the results of the study. We should know how many of the subjects included had hyperlipidemia or arterial hypertension or were smokers. Were patients with cardiovascular disease also excluded from the study population?

The fifth point is that the current medication taken regularly by the subjects included was not specified. Since various medications can reduce nerve conduction parameters, it would have been imperative to exclude patients taking medications that reduce nerve conduction. Drugs that particularly reduce nerve conduction velocity include muscle relaxants, anticonvulsants, antidepressants, opioids and aminoglycosides. Did any of the people studied take these particular drugs?

To summarize, this interesting study has limitations that affect the results and their interpretation. Addressing these limitations could strengthen the conclusions and support the message of the study. All open questions need to be clarified before readers uncritically accept the conclusions of the study. The conduction parameters of the median nerve may vary between the different arm positions as the vascular, mechanical and functional properties change in the different arm positions.

Sounira Mehri, Biochemistry Laboratory, LR12ES05 Nutrition-Functional Foods and Vascular Health, Faculty of Medicine, Monastir, Tunisia

Josef Finsterer, Department of Neurology, Neurology & Neurophysiology Center, Vienna, Austria

Median nerve conduction in different arm positions

Sounira Mehri,

Genomics Platform, Institute Pasteur de Tunis (IPT), Tunis-Belvédère, Tunis, Tunisia

Reply from the Author

We would like to thank the authors for their interest in and valuable contributions to our article.¹ In this study, we investigated the effect of different arm positions on median nerve conduction studies and discussed the potential underlying mechanisms. The points raised regarding changes in blood flow and muscle-tendon length between arm positions were already addressed in the Discussion section, where we noted that these factors could potentially explain our findings. Another point raised was the possible inclusion of individuals with Martin-Gruber anastomosis; however, as stated in the Methods section, such individuals were excluded based on neurophysiological testing. Similarly, the critique regarding the inclusion of participants with chronic conditions such as cardiovascular disease or hyperlipidemia was also addressed in the Methods section, where we clarified that individuals with chronic diseases were excluded from the study. Nonetheless, as the authors rightly pointed out, we acknowledged in the Limitations section that future studies involving individuals with conditions known to cause neuropathy would be valuable. Although smoking status was not part of the exclusion criteria, we agree that studies specifically investigating nerve conduction in smokers may yield insightful results.

In conclusion, we consider the points raised by the authors to be valid and aligned with our perspective. As noted, arm position may indeed produce different effects on nerve conduction studies in individuals with chronic diseases associated with neuropathy, compared to healthy individuals

Halit Fidanci, University of Helath Sciences Adana City Training and Research Hospital,

Division of Clinical Neurophysiology, Department of Neurology

Yuregir, Adana, Turkey