Painful nervus intermedius neuropathy attributed to Bell's palsy: The need for independent diagnostic criteria

The International Classification of Headache Disorders, 3rd edition (ICHD-3) distinctly recognizes pain resulting from the involvement of the trigeminal, glossopharyngeal, and intermedius nerves in chapter 13: “Painful Lesions of the Cranial Nerves and Other Facial Pain” (1). These conditions are further classified into neuralgias and painful neuropathies based on the nature of the pain and associated clinical features. Neuralgias are characterized by recurrent, intense, paroxysmal, electric shock-like pain episodes within the distribution of the affected nerve, often triggered by innocuous stimuli. Conversely, painful neuropathies present with near-continuous, burning-like pain and might be accompanied by sensory deficits or other signs of nerve injury.

The nervus intermedius, a branch of the facial nerve (cranial nerve VII), carries parasympathetic fibers to the nasopalatine and lacrimal glands and somatosensory fibers from the inner ear and tongue (2). Accordingly, pain related to nervus intermedius is typically localized in the auditory canal and the mastoid region, with possible irradiation to other areas. Following the classification of other cranial nerve-related pain syndromes, conditions involving the nervus intermedius are categorized as either nervus intermedius neuralgia (classical, secondary or idiopathic) or painful nervus intermedius neuropathy (attributed to herpes zoster, post-herpetic, attributed to other disorders or idiopathic) (1).

Bell's palsy is a relatively common acute or subacute condition characterized by facial nerve inflammation, typically presenting around the age of 40 years (3). Although its exact pathophysiology remains unclear, reactivation of herpes simplex virus 1 is widely postulated as a potential trigger of the inflammatory process. The clinical manifestations of Bell's palsy reflect the diverse functional components of the facial nerves. These include unilateral facial paralysis (motor fibers), eye discomfort due to reduced lacrimation (parasympathetic fibers of the greater superficial petrosal branch), hyperacusis (fibers innervating the stapedius muscle) and pain or altered sensation (somatosensory fibers of the intermedius branch). A burning-like, near-continuous pain localized to the distribution of the nervus intermedius region is observed in approximately half of patients with Bell's palsy, typically during the early phase of the disease (4,5). By contrast, paroxysmal, electric shock-like episodes characteristic of neuralgia are rarely reported. Notably, cold exposure to the ear is a recognized trigger of nerve intermedius neuralgia and is frequently described in the days preceding Bell's palsy onset. Prospective studies suggest that this pain is usually self-limited, with fewer than 10% of patients reporting persistent pain beyond two months from symptom onset (4). As such, most cases of Bell's palsy meet clinical, anatomical, and pathophysiological criteria consistent with a transient painful nervus intermedius neuropathy. Despite this, the condition remained under-recognized and is notably absent as an independent diagnosis from the ICHD-3, the International Classification of Orofacial Pain, and much of the literature on pain attributed to cranial nerves (1,6,7).

Given the annual incidence of Bell's palsy (approximately 20 per 100,000 individuals) (3) and trigeminal neuralgia (approximately 10 per 100,000 individuals) (6), and considering that approximately 50% of Bell's palsy cases are associated with pain (4), the incidence of painful nervus intermedius neuropathy related to Bell's palsy might be comparable to that of trigeminal neuralgia, and yet it remains largely overlooked. Adding an official ICHD-3 diagnosis to painful nervus intermedius neuropathy attributed to Bell's palsy might substantially help increase attention towards this clinical condition.

Importantly, Bell's palsy should be differentiated from Ramsay Hunt syndrome. This neurological disorder occurs when the varicella-zoster virus reactivates in the facial nerve, which can lead to a combination of ear pain, vesicular ear rash, and peripheral facial palsy (8). Some cases of Bell's palsy associated with ear pain may, in fact, represent misdiagnosed cases of Ramsay Hunt syndrome. Nevertheless, it is plausible that ear pain also occurs in uncomplicated Bell's palsy, given the impairment of the nervus intermedius. An accurate clinical diagnosis of Bell's palsy requires careful assessment beyond motor symptoms.

We propose the following criteria for a diagnosis of Painful nervus intermedius neuropathy attributed to Bell's palsy, which might be included as a subsection of diagnosis 13.3.2 – Painful nervus intermedius neuropathy:

Description: Unilateral continuous or near-continuous pain, with or without brief paroxysms superimposed, in the distribution of nervus intermedius and associated with Bell's palsy.

Diagnostic criteria:

Unilateral continuous or near-continuous pain^a in the distribution of nervus intermedius^b and fulfilling criterion C

Peripheral facial palsy (Bell's palsy)

Pain developed in temporal relation to Bell's palsy

Not better accounted for by another ICHD-3 diagnosis.^c

Notes:

Brief paroxysms may be superimposed, but are not the predominant pain type.

In the auditory canal, auricle and/or region of the mastoid process.

A diagnosis of varicella-zoster virus infection should be excluded clinically (vesicles on the tympanic membrane, auditory canal, auricle and/or skin overlying the mastoid process) and via blood or cerebrospinal fluid testing. A diagnosis of “probable painful nervus intermedius neuropathy attributed to Bell's palsy” can be formulated if varicella-zoster virus infection is excluded clinically, but not via blood or cerebrospinal fluid testing.

In conclusion, painful intermedius nerve neuropathy is an under-recognized feature of Bell's palsy, with an estimated incidence comparable to that of trigeminal neuralgia. As diagnostic criteria for headache and facial pain syndromes continue to evolve, this clinical entity might be acknowledged in updated classification systems (9,10).