Absence of Bone over the Geniculate Ganglion

ALBERT L. RHOTON, JR., M.D., JACK L. PULEC, M.D., GEORGE M. HALL, M.D., AND ALLEN S. BOYD, JR., M.D. Sections of Neurosurgical Research and Otolaryngology, Mayo Clinic and Mayo Foundation, and Mayo Graduate School of Medicine, Rochester, Minnesota

ANDY recognized that the geniculate TABLE 1 ganglion "occasionally" protrudes Frequency and amount of exposure of geniculate D through a congenital defect in the roof ganglion and genu of the seventh nerve of the petrous temporal bone. In their book on in 50 autopsy examinations trigeminal neuralgia, Stookey and Ransohoff16 noted that in rare instances the petrous bone is Amount exposed (mm) Number defective over the geniculate ganglion. This fact has been considered of such minor sig- 0.54).9 2 1.0-1.9 5 nificance that it is largely disregarded in de- 2.0-2.9 6 scriptions of neurosurgical procedures for 3.0-4.0 2 treatment of trigeminal neuralgia in which the roof of the petrous temporal bone may be ex- Total 15 posed, and it is not among the anatomical variants listed in standard anatomy text- graphs were taken with the original paint books, 9,x3 although it has been mentioned in intact to illustrate the extent of exposure of the otological literature. House and Crabtree 1~ the nerve (Figs. 1-4). reported the incidence of this bony anomaly to be 5 %, presumably on the basis of their clin- Results ical material. Batson3 stated that the genicu- In 15 bones (15 %), all or part of the genicu- late ganglion is not covered by bone in early late ganglion and genu of the seventh nerve childhood. was found exposed (Table 1). The exposed This project was undertaken to evaluate the ganglion was on the right in eight and on the incidence of dehiscence of bone over the left in seven; the bony defect was bilateral in geniculate ganglion in the adult and to make five and unilateral in five. The entire genu and available an adequate photographic display of ganglion were found exposed in only two the phenomenon. bones. In 15 other specimens the geniculate gan- Method and Material glion was completely covered but there was At 50 autopsies, 100 adult human temporal no bone extending over the greater petrosal bones were removed and examined under a nerve (Table 2). In the remaining 70 bones dissecting microscope. The dura was removed the ganglion and some portion of the proximal from the bone, and the greater petrosal nerve part of the greater petrosal nerve were was exposed and traced to the facial hiatus. If covered by bone. More than 50 % of the speci- the ganglion and a portion of the greater mens had less than 2.5 mm of greater petrosal petrosal nerve were embedded in bone, their nerve covered. The greatest length of greater covering was removed, and the length of petrosal nerve covered by bone was 6.0 mm. greater petrosal nerve covered was deter- mined. If the bony covering of the geniculate Discussion ganglion was deficient, the size of the bony Our results show that there is significant defect was determined. In some of the speci- danger of injuring the facial nerve at the time mens the exposed portion of the ganglion of elevating the dura from the roof of the tem- was painted, and then the bone was removed poral bone during surgery for trigeminal neu- over the course of the seventh nerve. Photo- ralgia. Facial paralysis occurs in less than 10% of the cases, 14,16 an incidence that falls Received for publication April 10, 1967. within limits which can be explained by direct 48 Absence of Bone over the Geniculate Ganglion 49

FIG. l. Interior of base of skull as viewed from above. Operative defect for temporal approach to trigeminal ganglion is at right. Area from which specimens shown in Figs. 2, 3, and 4 were taken is indicated by circle (specimen for Fig. 2 was from left and for Figs. 3 and 4 from right). trauma. The fact that the onset of facial weak- There are several reasons why the absence ness is delayed, appearing 3 to 4 days after of bone over the geniculate ganglion is not operation, suggests that direct trauma may readily noted during operation for treatment not be the only cause. However, the delay in of trigeminal neuralgia. When exposed by the onset of facial paralysis does not exclude its absence of bone, the genu and ganglion do being caused by delayed swelling induced by not protrude from the bony defect but remain direct manipulation of the genu and ganglion. flush with or slightly depressed from the sur- In cases in which only a small portion of the rounding bony surface. Also, the bony defect ganglion is exposed, trauma could produce most often corresponds to only a portion of delayed swelling and facial paresis, while the diameter of the genu, the exposed portion trauma to those nerves fully exposed could being the same or only slightly larger than the produce the facial weakness immediately first portion of the greater petrosal nerve, for postoperatively. Peet and Schneiderx4 noted which it may be mistaken. When such a bony facial weakness immediately after operation defect is viewed from the side, as in the usual in 2.8 % of their trigeminal neuralgia patients. extradural subtemporal approach for tri- Others 1~ also have noted its presence im- geminal neuralgia, the irregularities of the mediately after operation although in most floor of the middle fossa could hide it or make instances it appeared later. it barely perceptible. Because bleeding may