Clinical Implications of Topographic Anatomy on the Ganglion Impar

Fifty sacra and coccyges were removed and dissected under the surgical microscope (Carl Zeiss, Oberkochen, Germany). Four to five pairs of sacral ganglia were found after removing the fascia and fat tissue. The caudal ends of the two trunks converged into a single ganglion at diverse sites on the coccyx. Distances of the ganglion impar along the medial line from the mid sacrococcygeal joint (X) to the tip of the coccyx (Y) were measured using a digital caliper (Mitutoyo, Kawasaki, Japan), and the relative index was calculated as X/X + Y. The distance from the mid sacrococcygeal joint to the tip of the coccyx was measured for the size of the coccyx. The Pearson correlation coefficient (r ) between the size of the coccyx and the distance of the ganglion impar from the tip of the coccyx was calculated.

The shape of the ganglion was classified as oval (26%), irregular (20%), triangular (14%), elongated (10%), rectangular (8%), and U shaped (8%). In 14% of the samples, two caudal ends of the sympathetic trunks were connected without forming a recognizable ganglionic shape. The average long and short diameters of the ganglion were 2.5 and 1.1 mm for the oval type, 4.2 and 2.5 mm for the irregular type, 1.9 and 1.3 mm for the triangular type, 1.8 and 0.7 mm for the rectangular type. The average length of the elongated type was 4.4 mm.

The average distances of the midpoint of the sacrococcygeal joint and the tip of the coccyx to the ganglion impar were 8.6 mm (0–19.3 mm) and 25.0 mm (10.7–37.4 mm), respectively. The relative index of the location of the ganglion impar was calculated from the determined distances, as described in the Materials and Methods. Its value varied from 0 to 0.6, with a median and average value of 0.3 (fig. 1). The frequency according to the distance of the ganglion impar from the tip of the coccyx was also calculated (fig. 2). The size of the coccyx ranged from 18.2 to 48.1 mm, with a mean of 33.3 mm. The relation between the size of the coccyx and the distance of the ganglion impar from the coccygeal tip was statistically significant (P < 0.001) (fig. 3).

The branch from the ventral ramus of the sacral nerve was observed to run close to the ganglion impar in one (4%) or both sides (2%). The shortest distance between the nerve branch and the ganglion impar ranged from 2.8 to 10.3 mm, with a mean of 6.3 mm. One or two coccygeal ganglia were observed in 12% of the samples.

Since the blockade of the ganglion impar was first introduced for the management of intractable perineal pain in 1990 by Plancarte et al. , a number of modified methods have been reported, including the transsacrococcygeal ligament placement of a needle,1the application of a curved needle,2and cryoablation through the sacrococcygeal disc.4However, the nature of the perineal pain that can be relieved by the ganglion blockade is neuropathic; hence, the perineal pain due to the somatic invasion of malignancies is not the appropriate indication for the blockade of the ganglion impar. Ganglion blockade was also reported to be effective in the treatment of hyperhidrosis in the perineum and buttock.10,11Although successful blockade of the ganglion impar depends on accurately locating the ganglion,4its location has been described inconsistently. Previous reports on the blockade of the ganglion depicted its location anterior to the sacrococcygeal joint,1–4but anatomy textbooks locate it anterior to the coccyx5–8or at the tip of the coccyx.9The current study makes plain the wide range of sizes of the coccyx and distances of the ganglion impar from the coccygeal tip, and the significant correlation between them. The diverse locations of the ganglion impar were represented by a relative index, and the value of this index varied from 0 (locating it at the sacrococcygeal joint) to 0.6 (below the midpoint of the line joining the midpoint of sacrococcygeal joint and the tip of the coccyx). The median and average index value was 0.3, which was the midpoint between the two sites with relative indexes of 0 and 0.6. This result implies that the needle for the blockade of the ganglion impar should be directed toward the site with an index value of 0.3 rather than at the sacrococcygeal junction, the conventional injection site in previous reports.1–4 

The branches from the ventral ramus of the sacral nerve were observed to run close to the ganglion impar in 3 of the 50 samples. Considering the risks of the development of neuritis and neuralgia after chemical neurolysis,12this finding suggests that the amount of blocking agents should be minimized to avoid possible injury of the sacral nerve branch. However, the determination of the minimal and optimal amounts of blocking agents requires further clinical investigation.

The authors thank Dong-Su Jang, B.A. (Research Assistant, Department of Anatomy, Yonsei University College of Medicine, Seoul, Korea), for his help with the figures.