To the Editor:—
We have obtained interesting information about spinal puncture using magnetic resonance imaging. It is important to understand the anatomy of the cauda equina when performing spinal anesthesia. Previous studies using computed tomography or magnetic resonance imaging have shown that in the supine position, the cauda equina lies symmetrically in the dorsal subarachnoid space. 1–3However, a patient is usually placed in the lateral decubitus position during spinal puncture. Thus, it is necessary to obtain detailed anatomic information about the cauda equina in the lateral decubitus position. Magnetic resonance imaging reveals interesting and important information about the anatomy of the cauda equina. In seven healthy volunteers, axial views of magnetic resonance imaging of the cauda equina during both the supine and left lateral decubitus positions were obtained and compared. In all subjects, a movement of the cauda equina was observed by changing position. The cauda equina lay symmetrically at the dorsal side of the subarachnoid space when the patient is in the supine position (fig. 1A). However, it moved to the left side of the subarachnoid space when the patient was placed in the left lateral decubitus position (fig. 1B). Our observations are similar to those that Fink et al. reported in an abstract at the 1993 Annual Meeting of the American Society of Anesthesiologists (published in Anesthesiology 1993; 79:A828). These results suggest that the cauda equina has considerable mobility in the cerebrospinal fluid. During the lateral decubitus position, it may dynamically move to the gravity-dependent side. This phenomenon may alter our thought on spinal anesthesia. First, we should care about cauda equina syndrome. If a spinal needle is inserted downward in the lateral decubitus position, it may increase the possibility of injury to the cauda equina. Second, it is necessary to consider the specific gravity of local anesthetics for spinal anesthesia. Differences in anesthetic effects between hyperbaric and hypobaric local anesthetics may contribute to the deviation of the cauda equina by changing position. Although additional investigations concerning the anatomy of the cauda equina are necessary, this observation transforms our knowledge about spinal anesthesia.