Trigeminal neuralgia in patients with cerebellopontine angle tumors: should we always blame the tumor? A case report and review of literature

Pathophysiology

Trigeminal neuralgia secondary to brain tumors has been reported to have an incidence of between 1 and 9.9% [8], [9], [10], [11], [12]. Other causes for secondary TN include multiple sclerosis and vascular malformations. Tumors causing TN have been reported by multiple studies and the mechanism of onset of TN is proposed to be due to the following [7], [8], [9, 13]:

1. Direct compression/distortion of the nerve by the tumor,

2. Nerve encasement by the tumor,

3. Intrinsic tumor of the trigeminal nerve

4. Distortion of local anatomy by the tumor, causing:

   1. the nerve to be pushed towards a vessel that can cause compression or,

   2. the vessel to be pushed towards the nerve by the tumor, (the vessel usually being peritumoral).

These mechanisms have been used to understand the possible etiology of TN contralateral to a tumor [8, 9]. Janetta et al. have argued that even in patients with tumors, the etiology remains to be NVC rather than tumoral compression, although this remains debated [8]. The most commonly encountered tumors in this subset of patients in order of frequency are epidermoids, meningiomas and schwannoma, although others such as squamous cell carcinoma, lipoma, osteoid osteomas have been rarely encountered [7, 9].

However, we would like to emphasize that TN in the subset of patients with cerebellopontine angle/clival tumors does not necessarily arise due to the tumor causing direct compression or indirect neurovascular conflict. Neurovascular conflict can exist as a separate, coexisting pathology in a small minority and differentiating between the two etiologies is extremely difficult both radiologically and intraoperatively. In the illustrated case, both pathologies coexisted and were seen on the MRI, making the treatment decision process challenging.

Diagnosis and non-surgical management

Diagnosis of TN is mainly clinical and supplemented with MRI [3]. While MRI is not necessary for the diagnosis of TN, it is required in order to determine who should undergo MVD, and also to identify secondary TN [2, 3, 10]. Higher strength MRI imaging may be more useful in identifying NVC due to increased resolution, and combining this with time of flight MR angiography may increase yield [2, 14]. Careful attention has to be paid to direct neural compression/distortion by the tumor. Presence of any NVC must also be noticed. However, it is difficult to determine the etiology of TN from MRI alone as in many cases, both tumor and NVC may be present, and rarely, asymptomatic individuals have NVC detected incidentally on MRI [2].

Medical management has been tried in secondary TN with varying success, with many patients requiring dose escalation or polypharmacy and commonly experiencing treatment failure. Although radiosurgery has been used, it does not have success rates similar to microsurgery [15, 16]. Targets vary with studies, and include nerve only, tumor only and combined tumor and nerve. Targeting the tumor and nerve seems to have better results, although complication rates may be higher [16].

Surgical management

Tumor excision has been the cornerstone in the management of TN which presents with a tumor in proximity to the trigeminal nerve [17]. Although tumor excision itself may be sufficient, there have been authors who have recommended optional vascular decompression as well although evidence for this may be of low quality [2, 8]. Microvascular decompression is the standard of care for classical TN and offers significant relief over long durations, although its role in secondary TN is not as well established [2, 3, 18, 19]. In addition to the more comprehensive studies concerned with the subject of tumors and TN, a literature review revealed two similar cases of concurrent vascular compression in addition to a tumor [6, 20].

Marinelli et al. described the case of a 51 year old female presenting with left sided TN alone. MRI revealed a 1.6 cm enhancing mass in the left cerebellopontine angle (CPA), extending into the internal auditory canal. Although the tumor was mildly displacing the tumor, NVC due to a branch of the superior cerebellar artery (SCA) was also confirmed. Intraoperatively, the schwannoma was noted to be arising from the facial nerve and hence tumor excision was abandoned. MVD was performed and the patient was noted to be pain free at last followup (15 months). She later underwent radiosurgery for the tumor. As the tumor was not excised, the authors could not state definitively if the tumor was causing vascular compression indirectly [6]. Another case report by Cha et al. detailed a 64 year old male with long standing right sided trigeminal neuralgia that was diagnosed to be secondary TN after a small tumor was found in the right infratemporal area on MRI in the trigeminal nerve course. As the patient did not find improvement after both medical management and radiosurgery to the tumor, an exploratory suboccipital craniotomy was performed. During surgery, a branch of the SCA was identified and MVD was performed. Postoperatively, pain was noted to have completely disappeared up to 1 year of followup [20].

Tumor excision has been noted to be sufficient to relieve TN in patients with large posterior fossa tumors [21]. In addition, our case illustrates the importance of always keeping an MVD as part of the surgical plan when treating a patient with TN suspected to be secondary to a smaller tumor. A unique aspect of our case, compared to the others is the presence of a vessel completely unrelated to the tumor, as it was passing through the rootlets of the trigeminal nerve. Rarely, as these cases show, the TN may be classical in nature, with the coincidental presentation of a tumor in closeness, proximity. In patients with secondary Trigeminal neuralgia suspected due to a tumor, conventional wisdom dictates that surgical resection of the tumor should produce pain relief. However, it is important to always be cognizant of the presence of NVC in these patients, especially those with small tumors. Microvascular decompression must always be borne in mind when attempting a tumor excision and every effort must be made intraoperatively to conclusively rule out neurovascular conflict. In cases such as the one illustrated here, microvascular decompression is a necessary, separate and non-optional part of the surgical management.