High-Resolution MRI Findings following Trigeminal Rhizotomy

Discussion

Studies to date on patients post-rhizotomy have primarily focused on clinical outcomes, notably comparison of different forms of rhizotomy with each other and with microvascular decompression. While these articles offer occasional images of complications associated with rhizotomy, no studies to date, to the authors' knowledge, have evaluated patients after rhizotomy in an effort to describe postprocedural findings on MR imaging. This study was performed to determine what findings can be encountered on postprocedural MR imaging.

Subjective changes, such as decreased CISS SI and clumping of nerve roots, are commonly encountered (65%) after rhizotomy; however, these changes are rarely seen in treatment-naïve patients (6%). These results suggest that the radiologic findings are associated with the rhizotomy procedure rather than the underlying pathophysiology of trigeminal neuralgia. Objective measurements of signal intensity in Meckel's cave support our subjective findings by showing a statistically significant decrease in CISS SI in Meckel's cave after rhizotomy. The mechanism underlying these changes is presumed to be the result of chemical- or heat-induced neurolysis of the nerve rootlets in Meckel's cave. Notably, the patients with changes in Meckel's cave had undergone, on average, more rhizotomy treatments (n = 2.9) compared with those who did not have such changes (n = 1.3). Although the cause is unknown, similar changes demonstrated in procedure-naïve patients could reflect a prior inflammatory event predating the development of trigeminal neuralgia. Conversely, there could be recall bias in our study, with the possibility that some subjects failed to report their rhizotomy at another institution to the clinician.

Small foci of encephalomalacia of the temporal lobe adjacent to Meckel's cave were encountered in 31% of patients post-rhizotomy and not in the control group. One such focus demonstrated minimal blood products. These findings could be due to the adjacent temporal lobe being within the thermal zone of the radiofrequency probe and/or direct surgical manipulation.^10,11 No adverse clinical signs or symptoms related to the temporal lobe encephalomalacia were reported in these patients.

Weakness of muscles of mastication frequently occurs after trigeminal rhizotomy, with a recent review noting weakness in 16% of patients, which is often transient lasting 6–12 months.¹² In our review, atrophy of the muscles of mastication was present in patients with and without rhizotomy, with no statistically significant difference between groups. No enhancement of trigeminal nerve roots was encountered after treatment, though it is unclear whether more immediate postprocedural imaging would perhaps reveal enhancement in the acute injury phase after treatment. Additional complications such as vascular injury (cavernous carotid fistula, internal carotid artery pseudoaneurysms), abscess, meningitis, and cranial nerve III and VI injury, noted in previous publications, were not encountered clinically or on imaging in our series.^{10⇓⇓⇓⇓⇓⇓⇓–18}

There were several limitations in this study, including the inherent biases associated with the retrospective study design. Subjects were grouped according to a review of the medical record, depending on whether the patient was documented as having previously undergone rhizotomy. The patients were seen by experienced clinicians in a dedicated trigeminal neuralgia clinic, and documentation of prior procedures is standard, but we cannot exclude the possibility that subjects had undergone rhizotomy at another institution and failed to report this to the clinician. All patients in our study who had rhizotomy underwent glycerol rhizotomy or glycerol in combination with radiofrequency thermocoagulation rhizotomy. No patients had balloon compression rhizotomy, a method not used at our institution due to the preference of the surgeons. Another limitation of this retrospective study was lack of pre-rhizotomy imaging in those patients who underwent rhizotomy, with the exception of 1 patient. In the case of the 1 patient who underwent high-resolution imaging both before and after treatment, no difference was noted within Meckel's cave or the adjacent structures between studies. Additionally, no correlation between the degree of clumping or decreased CISS SI in Meckel's cave and the degree of symptoms could be made due to the retrospective nature of the study and in the absence of clinically reported grading of symptoms.

The timing of imaging after rhizotomy greatly varied, which could potentially influence the degree of changes seen in Meckel's cave, though the average time of imaging in those with and without nerve clumping was similar. Additionally, more acute changes within Meckel's cave after rhizotomy (<1 month) could not be accurately assessed. Last, the pain of trigeminal neuralgia is known to recur frequently following rhizotomy. Many of the subjects imaged after rhizotomy experienced recurrence of symptoms. Because patients who responded positively to trigeminal rhizotomy are unlikely to have undergone follow-up imaging, whether or to what extent these findings might correlate with degree or duration of therapeutic response remains a subject for further investigation.