A-waves were thought to be of clinically insignificant value; however, recently, they started to gain clinical importance and they are considered to be signs of peripheral nerve dysfunction [6, 7]. In the current study, we aimed at characterizing A-waves in ALS and demyelinating neuropathies (DN). Two groups of patients were included, ALS group and DN group (14 patients each), and their nerve conduction studies were reanalyzed to look for A-waves. A-waves were assessed in 61 nerves in the ALS group versus 64 nerves in the DN group.
The frequency of occurrence of A-waves was higher in the DN group than the ALS group which agrees with other studies . However, that did not reach statistical significance. The frequency of A-waves in the ALS patients in our study is higher than that reported in previous studies [1, 4]. Also, the frequency of A-waves in patients with DN is slightly higher than other studies , this could be attributed to the small sample size.
In the current study, A-waves were recorded more frequently from the DN group, and they tend to occur in multiple nerves of the same patients, which is in accordance with other studies . A-waves in the DN group were recorded more frequently from nerves of the upper extremities (median and ulnar). However, in the ALS group, there was no significant difference between nerves in the upper and lower extremities. One possible explanation for the presence of A-waves mostly in the median and ulnar nerves in the DN group is the predilection of some forms of DN (MMN) to affect upper limbs; median, ulnar, and radial nerves are most commonly affected in MMN .
In the current study, all A-waves detected in the ALS group were simple A-waves. However, complex A-waves (CAW) represented 28% of A-waves recorded from the DN group, and they were recorded more frequently from ulnar and median nerves. This was in accordance with other studies which found CAW a useful sign to distinguish between demyelinating neuropathies and anterior horn cell disorders [7, 8]. Thus, and according to the abovementioned studies and the current study, CAW seems to be a useful specific electrodiagnostic sign to distinguish DN from ALS.
In the current study, we did not find a significant difference between both groups regarding the timing of occurrence of A-waves in relation to F-waves. In both groups, A-waves with absent F-waves were recorded more frequently, followed by early A-waves (recorded before F-waves) and then intermixed A-waves. Thus, this was not useful in differentiating between both conditions. The frequency of occurrence of early A-waves in the ALS group in the current study (33.3%) was almost similar to that found in previous studies . However, we found that the occurrence of A-waves in the absence of F-waves was higher.
To the best of our knowledge, there were no previous studies comparing the characteristics of A-waves among ALS and DN patients, which gives credit to our work and will open the door to more studies in this area. Also, we included only patients that were newly diagnosed with ALS/DN and were not on specific treatment to make sure that our findings are not affected by medications, which again gives credit to our results. However, the study has some limitations that may have affected the results. The small sample size is one of the major limitations. Thus, we recommend a larger sample size for future studies to validate our results. Also, our sample was heterogeneous as patients within the same group were classified as definite, probable, or possible ALS/DN, which also could have contributed to the statistically insignificant results. Thus, a homogeneous sample is recommended in future studies.