Classification of solitary pontine lesion–related trigeminal neuralgia

The letter by Di Stefano et al. (1) in this issue of Cephalalgia raises an important and timely issue regarding the classification of trigeminal neuralgia (TN) and, more broadly, the underlying principles in the International Classification of Headache Disorders (ICHD) (2). Their correspondence proposes that Solitary Pontine Lesion–Related Trigeminal Neuralgia (SPLTN) represents a distinct syndrome, worthy of separate classification. This concept gained momentum following the report by Tohyama et al. (3), which highlighted a reproducible association between solitary pontine lesion (SPL) and a typical TN phenotype, unrelated to MS.

Ideally, a condition proposed as a new classification entry should exhibit a well-defined clinical phenotype, a consistent pathophysiological mechanism, and clear therapeutic implications. In particular, effective treatment of the presumed causal mechanism should result in resolution of pain. SPLTN fulfills several of the criteria. Clinically, its presentation can be remarkably consistent and closely parallel the defining features of trigeminal neuralgia as delineated in ICHD-3 (3). However, the literature shows that other facial pain phenotypes can occur in association with a SPL, depending on lesion location, extent, and pathophysiology within the trigeminal pathways of the pons.

A SPL was initially thought to be most closely related to multiple sclerosis (MS), with the possibility that these are solitary sclerosis (4). However, assigning SPLTN to secondary trigeminal neuralgia due to MS seems inaccurate. SPLs differ from MS both in their restricted spatial distribution within the brainstem, their specific location, and associated diffusion tensor imaging abnormalities that are not typical of MS-related disease (3). SPLTN is consistently and notoriously resistant to any of the established surgical interventions for TN, further distinguishing it from both classical TN and MS-associated TN. This observation may have important practical implications when evaluating patients with idiopathic TN who have failed surgical treatment. At present, however, therapeutic alternatives remain limited, as few lesion-targeted therapies have been applied to SPLs (5). As a result, we are unable to unequivocally establish a causal relationship between SPLs and trigeminal neuralgia.

The pathophysiology of SPLs appears heterogeneous, encompassing several etiological mechanisms such as viral neuritis (6), ischemia (7), and focal infarction (8). In addition, incidental MRI findings suggest that there are asymptomatic SPLs. While this heterogeneity may initially seem to weaken the case for SPLTN as a discrete entity, it likely reflects the same degree of pathophysiological complexity already accepted for TN itself.

Although only ∼40 cases of SPLTN have been reported to date, rarity alone has not precluded recognition within the ICHD framework. Moreover, cohort-based estimates suggest that SPLTN may occur in approximately 1 in 20 patients with TN, indicating that it is potentially underdiagnosed rather than truly rare (3). Formal classification would facilitate systematic identification, reporting, and study of additional cases.

Taken together, there is a compelling argument for the inclusion of Solitary Pontine Lesion–Related Trigeminal Neuralgia as a distinct subgroup within Secondary Trigeminal Neuralgia in ICHD-4. The merits and implications of this proposal will no doubt be carefully considered by the ICHD subcommittee responsible for Painful lesions of the cranial nerves and other facial pain.