The ‘dark’ side of the stomach

In a previous edition of Experimental Physiology, an interesting study is presented exploring, for the first time at high resolution, the electrophysiological events along the posterior surface of the stomach (Du et al. 2016). Although all scientific publications in principle provide novel findings, the exploration of a relatively large and unknown area in the mammalian body is a relatively rare occurrence, possibly meriting closer attention. At first glance, the results presented by Du et al. (2016) were to be expected. In short, the major electrical event in the stomach, the slow wave, propagates as large waves from the orad area located close to the larger curvature, in large sweeping arcs along the corpus and the antrum, towards the pylorus, essentially demonstrating the same pattern as has been shown to occur along the ventral surface of the stomach. In that sense, this exploration resembles that performed many years ago by the Russian space probe Luna 3 that orbited, for the first time, the ‘dark’ unknown side of the moon (Yankovsky, 1960). Also in that exploration, essentially a similar type of topography was seen as present on the visible side of our satellite, although there were some local variations. A closer look at the results of Du et al. (2016) also points out some remarkable differences between the anterior and the posterior side of the stomach, which may indicate that a more complex behaviour is possible in the stomach than hitherto believed. First, as expected, abnormal propagation patterns may also occur in the posterior serosa, such as re-entry (‘rotors’), ectopic foci, retrograde propagation, etc. Second, such abnormal propagation patterns along one side of the stomach may and do occur while relatively normal propagation occurs along the other side of the stomach. From a clinical point of view this could be crucial, as recordings from one side may not suffice in categorizing a stomach’s behaviour. Finally, the authors quite rightly point out the fact that propagation across the greater curvature may induce complicated interactions from one side of the stomach to the other, which would enhance or prolong gastric dysrhythmias. But an even closer inspection of the results shows more interesting details. For example, in the propagation maps displayed in Fig. 1 of Du et al. (2016), in which simultaneous propagation patterns along both the anterior and the posterior side of the stomach are presented, as recorded in a normal dog, significant differences in propagation velocity can be seen, with slower propagation along the posterior surface than along the anterior surface (Fig. 1B of Du et al. 2016). This cannot be caused by slight misalignment of the two electrode arrays, because the interelectrode dimensions in both arrays were the same. Could this difference be caused by an ectopic location of the slow-wave pacemaker? Or is it explained by the fact that the posterior serosa, during the experiment, was pressing against the dorsal abdominal wall, in contrast to the anterior surface, which was facing the opened abdominal cavity? Similar significant variations in slow-wave propagation between the anterior and the posterior surfaces can be seen in all five animals (Fig. 2 of Du et al. 2016), but the reasons for these are not explored. Finally, another big unknown needs to be mentioned here which, owing to technical limitations, could not be addressed in this study; that is, the possibility of slow-wave propagation from one side of the stomach to the other, not along the major curvature but along the lesser curvature of the stomach. Such a hypothesis was formulated many years ago by Kelley & La Force (1972) and suggested again more recently (Lammers, 2015), forming a potential area of propagation that might also induce a new type of gastric arrhythmia, circumferential re-entry, as recently demonstrated for the first time in the porcine small intestine (Angeli et al. 2013). In summary, this first peek at the ‘dark’ side of the stomach is not a trivial expansion of a technology, high-resolution electrical mapping, a technique which has had a major impact on the study and treatment of cardiac pathology as recently well summarized by one of the pioneers in cardiac electrophysiology (Wit, 2015). This first look at the other side of the stomach, at this early stage, can offer but a tantalizing look at the spatial complexity of gastric electrophysiology and motility.