Papanastassiou, Alexander M. M.D.; Schwartz, Richard B. M.D., Ph.D.; Friedlander, Robert M. M.D., M.A.
ABBREVIATIONS: CISS, constructive interference in steady-state; MRI, magnetic resonance imaging; TN, trigeminal neuralgia
Trigeminal neuralgia (TN) is defined as a sudden, usually unilateral, severe but brief, recurrent stabbing pain in the distribution of one or more branches of the trigeminal nerve (25). The incidence of TN is 4.7 per 100,000 persons (20). Although most cases of TN are associated with vascular compression of the trigeminal nerve or are idiopathic, some cases are associated with central lesions such as tumors or, rarely, aneurysms or arteriovenous malformations. Although many general references do not include Chiari I malformation as a cause of TN, there are case reports in which TN occurs in the setting of a Chiari I malformation, and surgical treatment of the Chiari I malformation has been found to relieve TN. Here, we present what, to our knowledge, is the 19th such case reported in the English-language literature. In addition, we propose a potential mechanism for the etiology of TN in this patient.
A 63-year-old right-handed man initially presented in 1993 with left-sided lancinating facial pain in the V2 distribution of the trigeminal nerve; pain was triggered by certain movements, tactile stimulation, or a hot shower. He complained of occasional aching, numbness, or soreness in this area as well. The neurological examination showed hyperesthesia in all three distributions of the left trigeminal nerve, with the worst component being in the maxillary division. Magnetic resonance imaging (MRI), performed on the initial evaluation, was reported to have shown a vessel loop contacting the left trigeminal nerve. The patient was diagnosed with TN and was treated for approximately 10 years with up to 1600 mg/d of carbamazepine, until worsening of his facial pain prompted a referral to neurosurgery for consideration of microvascular decompression in January 2005. The preoperative neurological examination showed a mild decrease in light-touch, pin-prick, and temperature sensation on the left side of the patient's face, as well as slight gait unsteadiness. There were no other pertinent signs or symptoms related to a syrinx. Preoperative evaluation included an MRI scan, which showed a Chiari Type I malformation with a cervical spine syrinx, as well as the superior cerebellar artery coursing just superior to the trigeminal nerve (Fig. 1). Review of his initial MRI from 1993 did show evidence of the Chiari I malformation and syrinx, but this study was not available to us.
The patient underwent uncomplicated suboccipital craniectomy, C1 laminectomy, and duraplasty for Chiari decompression on January 28, 2005. Intraoperatively, descent of the tonsils to C1 with compression of the medulla was observed. The tonsils were extended to an equal extent on both sides. Postoperatively, the patient's examination continued at baseline, and he reported that his pain was mostly better. His pain continued to improve, and he was able to wean his carbamazepine to 600 mg/d by 2 weeks postoperatively and then to 200 mg/d by 6 months postoperatively. One year after surgery, the patient was off of carbamazepine and all pain medicines, and he continues to be pain-free, without the use of pain or antiepileptic medications, as of December 2007. Postoperative MRI scans at 1 month, 6.5 months, and 10 months after surgery demonstrated progressive reduction in the size of the syrinx (Fig. 2, A and B). A slight increase in cerebrospinal fluid space between the superior cerebellar artery and trigeminal nerve is present (Fig. 2C).
TN that is not caused by a structural lesion is initially treated medically. The first-line agent is carbamazepine, which has a patient response rate of approximately 70% (7,21,28,44). Many other anticonvulsant medicines are also used for TN. The indication for surgery in this situation is symptoms refractory to medical therapy, for which microvascular decompression is performed (2).
The presence of a symptomatic accessible mass lesion is also an indication for surgery. Such symptomatic lesions are termed, by the International Association for the Study of Pain, “secondary trigeminal neuralgia from central nervous system lesions” (25). Cushing (10) first reported that mass lesions can cause TN. Current series show that 0.8 to 5.7% of patients with TN or facial pain harbor tumors affecting the trigeminal nerve (4,9,37).
In a series of 215 patients who underwent posterior fossa craniotomy for section of the trigeminal nerve for treatment of TN, Dandy (11) described two cases in which TN was associated with congenital malformations at the base of the cranium. His description of a malformation with reduced intracranial space and cerebellar tonsillar herniation fits the modern definition of a Chiari I malformation. The illustration from this initial report is shown in Figure 3. Since this time, 16 additional patients have been reported in the English language for which TN or facial pain is associated with a Chiari I malformation, and they are summarized in Table 1 (8,14,15,29,31,34,37,40,41). Of the 18 cases, 15 patients experienced pain typical of TN, whereas 3 were reported to have facial pain. Treatment by suboccipital craniectomy with or without laminectomies, duraplasty, or tonsillar aspiration was performed in 13 of the 18 patients. Postoperatively, nine of these 13 patients (approximately two-thirds) were pain-free. Two additional patients were pain-free after additional microvascular decompression, whereas the two remaining patients experienced improved or bearable, but residual, pain.
Of the five patients who did not undergo suboccipital craniectomy, two with hydrocephalus experienced complete relief of symptoms after ventriculoperitoneal shunting or third ventriculostomy. Another was pain-free with carbamazepine treatment, and data are not available for the other two patients. The patient described here had lancinating pain characteristic of TN, as well as hyperesthesia and occasional aching, numbness, or soreness that is not typical of TN. As of December 2007, the patient continues to have no pain without taking pain or antiepileptic medications.
The mechanism by which Chiari malformation could cause TN is not clear. Four mechanisms have been proposed: 1) symptoms of TN could be caused by vascular compression at the nerve root entry zone, which could be affected by hydrocephalus or anatomic factors related to the Chiari malformation, such as a small posterior fossa; 2) the Chiari I malformation could lead to stretching of the trigeminal nerve, resulting in demyelination; 3) the TN may suffer from microischemic changes; and 4) direct brainstem compression may result in symptoms of TN.
The following discussion explores these suggested mechanisms, drawing on available data on the pathophysiology of TN and Chiari I malformations. The first two hypotheses are appealing because of their consistency with the body of data supporting vascular compression and subsequent demyelination of the trigeminal nerve root entry zone as the cause of typical TN (11,17,18,37). Similarly, nerve root specimens from multiple sclerosis patients with TN also show demyelination in the nerve root entry zone. Taken together, these observations support the hypothesis that nerve root entry zone demyelination and the resulting ectopic generation of spontaneous nerve impulses with ephaptic conduction to neighboring axons are the common mechanism by which various pathological processes cause TN (5,6,12,19,23,24,32,33,36,38,39). Although constructive interference in steady-state (CISS) images in this patient show the superior cerebellar artery just superior to the trigeminal nerve, there are no reports with histopathological evidence that the Chiari I malformation leads to stretching or demyelination of the trigeminal nerve.
The third proposed mechanism that microischemic changes cause TN in the setting of a Chiari I malformation seems plausible, but no direct support for this hypothesis has been reported. Several case reports indicate that, central lesions such as a small infarction or a cavernous malformation of the nerve root entry zone or pons rarely may cause TN, showing that ischemia can cause TN in other clinical settings (1,16,22,27,30,35,42).
Regarding the fourth hypothesis, that the Chiari I malformation causes TN as the result of mass effect on the brainstem, one might expect to observe histopathological changes in the trigeminal nucleus in such cases. However, we are not aware of postmortem studies in any patient with TN and a Chiari I malformation. Additionally, although the authors of many studies detail the gross and microscopic findings of syrinxes in Chiari malformations, relatively few reports exist on the microscopic parenchymal findings in the Chiari I malformation (3,43). Typical histological parenchymal findings associated with syringomyelia include the existence of vascular dilation, fibrosis, or simple hyalinization of the walls of blood vessels. In the spinal cord parenchyma, there may be a proliferation of Schwann cells, termed schwannosis, although this finding is not specific (43). The relationship between these features and TN associated with Chiari I malformations remains unestablished. Although central processes such as increased excitation and decreased inhibition in the trigeminal nuclear complex likely contribute to TN, whether brainstem compression could contribute to such processes through demyelination of intramedullary trigeminal fascicles or cellular effects remains unexplored (13).
Pathological findings have been reported in a 60-year-old woman with Chiari I malformation and multiple, mostly left-sided neurological findings but no trigeminal pain. Relevant neurological findings included decreased tactile and pain sensation in the left face and nearly absent left-sided corneal reflex (3). Autopsy showed an elongated medulla oblongata with flattening of its right side from tonsillar herniation, as well as reduced size of the left half of the medulla. Multiple nuclei and tracts in the medulla showed a decrease in size. In particular, the right corticospinal tract showed demyelination, and the left trigeminal complex was hypoplastic. This patient was not reported to have had facial pain, but the brainstem pathology described makes a central pathogenesis for TN symptoms in the setting of a Chiari I malformation seem plausible. In a similar vein, axial morphology of nonneoplastic spinal cord cavities has been shown to correlate with histopathology and clinical findings (26). Evaluation of this hypothesis will await postmortem analysis in a patient with TN associated with a Chiari I malformation.
We propose a novel mechanism by which the presence of a cervical syrinx with a Chiari I malformation may contribute to TN symptoms. Concordant with hypotheses 1 and 2, detailed previously, a cervical syrinx indirectly may result in vascular compression or stretching of the trigeminal nerve at the nerve root entry zone. On the other hand, a syrinx may affect the brainstem directly. Given that the sensory fibers arising from the trigeminal nucleus descend down to the level of C2 and then course rostrally before they exit the brainstem, it is possible that the presence of a syrinx may impact the excitability of these fibers. In the patient described in this report, it is of interest to note that his TN decreased in parallel with the reduction of the volume of the syrinx (Fig. 2, A and B). This is clearly an association and not a direct proof. Also, this mechanism is only relevant in cases in which a syrinx is present. In two previous reports of TN and Chiari I malformation, identification of a syrinx has been reported (31,34).
TN is usually caused by vascular compression, but a small percentage of cases result from mass lesions such as tumors or vascular malformations. Chiari I malformation is important to consider as a rare cause of TN that responds to surgical therapy. MRI evaluation of patients with TN is necessary to detect these treatable lesions.
This work was presented at the New England Neurosurgical Society Annual Meeting on June 9, 2006.
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Papanastassiou et al. presented a patient with a Chiari I malformation, cervical syrinx, a superior cerebellar artery near the trigeminal nerve, and trigeminal neuralgia (TN), which was improved after foramen magnum decompression. Eighteen other occurrence of Chiari I malformation and TN from the literature are reviewed.
The question is whether to do a foramen magnum decompression or another neurosurgical procedure (microvascular decompression, needle rhizotomy, or gamma knife radiosurgery) when TN is associated with a Chiari I malformation. Of the 19 patients, the follow-up interval is available for only six patients: all reported relief of TN, but follow-up for 2 patients treated only for hydrocephalus was 3 months and follow up for the other 4 patients treated with foramen magnum decompression (3 also had a syrinx) was from 1.5 to 2.9 years. The question as to the best way to treat these patients remains open, but it would seem from the data, on the basis of the relatively short follow-up, that foramen magnum decompression is a reasonable option, especially if there is also a syrinx.
New York, New York
An association of TN and Chiari I malformation has been reported previously but is quite rare. Even rarer is its association with syringomyelia. In this report, Papanastassiou et al. described the case of a 63-year-old man who presented with a long-standing history of left TN in association with a Chiari I malformation and syringomyelia. The possible mechanisms of TN were felt to include vascular compression of the trigeminal nerve within the volumetrically small posterior cranial fossa, direct compression of the brainstem, and secondary effects of syringomyelia such as vascular compression, stretching of the trigeminal nerve at the root entry zone, and intramedullary damage of the descending tract of the trigeminal nucleus. The latter hypothesis would have been stronger if there were magnetic resonance imaging scans with thin axial sections showing the anatomic relationship of the syrinx to the left dorsolateral quadrant of the spinal cord between C2 and the lower brainstem. Regardless of the exact cause of TN, the patient was significantly improved after a standard posterior fossa decompression for Chiari I malformation and was pain free and not taking medications at the 1-year follow-up. Postoperative magnetic resonance imaging scans demonstrated a reduction of syrinx size and a slight increase in the cerebrospinal fluid space between the left superior cerebellar artery and ipsilateral trigeminal nerve, but there was insufficient evidence to establish a direct correlation with pain relief. Overall, this is a useful case report that adds to the small but important body of literature dealing with the association of Chiari I malformation and TN.
Thomas H. Milhorat
Manhasset, New York
Papanastassiou et al. presented an interesting rare symptom of Chiari I malformation with TN. The patient did have a cervical syrinx. He underwent posterior fossa decompression with duraplasty, and his postoperative magnetic resonance imaging scan showed progressive reduction in the size of the syrinx.
The sensory loss in trigeminal distribution to pain and temperature has been reported in large series. However, TN with the hyperesthesia documented in this individual is a rare phenomenon.
Arnold H. Menezes
Iowa City, Iowa
This case report of TN in a patient presenting with syringomyelia caused by a Chiari I malformation is interesting because TN was relieved by simple occipitocervical decompression that subsequently produced a decrease in the size of the cervical syrinx. In their article, Papanastassiou et al. also provided a fruitful discussion on possible anatomic mechanisms of the neuralgia.
In their discussion they seem to favor the role of the syrinx. It would have been useful to have an axial section of the cervical spinal cord to see whether the syrinx was (even slightly) lateralized or strictly median and symmetrical. We have noticed on the constructive interference in the steady state oblique sagittal view that the trigeminal root was angulated at the crossing of the upper petrous ridge.
We have seen a patient with TN who had such a syrinx; we also have seen 9 cases of TN with Chiari I malformations without any significant associated syringomyelic cavity. Four of these 10 patients were cured by simple posterior cervico-occipital decompression; the other 6 underwent secondary additional microvascular decompression, which showed arterial compression (the superior cerebellar artery) in 3 and a pure venous compression in the 3 others, namely the inferior transverse pontine vein. In all 6 patients, the cerebellopontine cistern was “virtual” owing to the small size of the posterior fossa and the trigeminal nerve was found compressed, squeezed between the pons and the posterior aspect of the petrous pyramid. In the three patients with the transverse pontine vein as the compression, the vein was making a groove at the inferior aspect of the root just after its exit from the porus of Meckel's cave. At this point the root was segmentally atrophic and of a grayish color owing to focal demyelination. The vein was coagulated and divided. In all 6 of these patients, secondary microvascular decompression relieved the trigeminal pain.
More generally, we did notice in our series, although we did not study this aspect in a quantifying way, that in the younger population with TN, the posterior fossa was frequently of small capacity, with virtual cisterns including cisterna magna. At surgery, the cerebellopontine angle was found to be “crowded,” with a trigeminal root squeezed between the brainstem and petrous wall and vessel(s) packed along the root, with a special emphasis on the inferior transverse pontine vein engrooved in the root. Only a sufficiently wide “keyhole” craniotomy and systematic careful checking of the whole root, from the brainstem to the porus of Meckel's cave, will ensure that nothing compressing the root is missed.
Marc P. Sindou