Key points

Introduction

Atypical facial pain (AFP), otherwise known as persistent idiopathic facial pain (PIFP), is a chronic and diffuse distribution of facial pain along the territory of the trigeminal nerve. This condition is unique in that it occurs in the absence of any neurologic deficit or any other obvious etiology. AFP is one of the most challenging conditions to diagnose due to the lack of clear diagnostic criteria. As a result, AFP is a diagnosis by exclusion of other known etiologies of facial pain and have no distinguishable lab markers or abnormalities.

In 1924, the first known diagnosis of AFP was recorded by Frazier and Russell who determined that 10% to 15% of patients who presented with chronic facial pain had symptoms that differed from the characteristic clinical pattern of trigeminal neuralgia, leading them to coin the term, atypical neuralgia . The diagnosis and terminology for this condition are surrounded by controversy and disagreement, with a wide variation in names adopted by different organizations and societies. The World Health Organization has adopted the term AFP whilst the International Headache Society and the International Association of the Study of Pain use the terminology, persistent idiopathic facial pain . Owing to the fact that patients with this disorder experience pain that neither follows the distribution of the peripheral nerve nor responds to antiepileptic agents, labeling this condition as atypical has served to distinguish it from the typical trigeminal neuralgia. Although this provides a means of categorizing patients with similar pain history and profiles, the basis on which this disorder is distinguished can be considered to have diagnostic limitations because the condition is defined by exclusion rather than inclusion.

Clinical presentation

Clinical presentation of AFP is largely variable and depends on the patient. Generally, patients suffering from this disorder experience pain that presents as poorly localized, deep, dull, aching, burning, pulling, and involving diffuse areas of trigeminal nerve distribution in the face. Additionally, the pain is long in duration, presents daily, and tends to last most of the day. Pain can be continuous or intermittent with periods of no pain. Stress and fatigue may elicit symptoms. At onset, pain can be confined to a limited area, which usually is unilateral and then may spread to a diffuse, larger area. In some cases, pain can present as sharp, shooting, and bilateral. Patients with AFP often report that analgesics are ineffective and this pain has been present for several years. This condition seems to have a predilection for the maxilla, women, and the middle aged–elderly, with most ages 30 years to 50 years.

Epidemiology

The estimated incidence and prevalence of AFP diverge significantly. In a study of Dutch primary care patients, the incidence was 39.5 per 100,000 person-years. In an epidemiologic study conducted in Germany, Mueller and colleagues estimated the prevalence of AFP as 0.03%, whereas other studies suggested it can be more than 1%. This large discrepancy in estimates of incidence and prevalence is owing to the absence of clear diagnostic guidelines. An inability to distinctly evaluate and diagnose this condition has likely contributed to both underestimates and overestimates in the various studies. As such, there are insufficient data providing evidence of the incidence, prevalence and predilection of this condition.

Etiology

The etiology attributable to AFP is not well understood. Researchers have suggested that the underlying causes of AFP are associated with injury to the trigeminal nerve, peripheral central demyelination, or minor trauma, such as a dental extraction. Some literature suggests that it is possible that an abnormal sensitization of the trigeminal nociceptive system may play a crucial role in the onset of AFP, whereas other researchers have suggested that AFP is a centrally mediated pain and may even be psychological in origin because they have found associations with underlying psychological disorders, such as depression and anxiety. Research has not provided clarity as to whether these psychological disorders are responsible for AFP or whether AFP plays a role in the onset of these conditions because other conditions of chronic pain are commonly associated with these psychological disturbances. Altogether, research thus far has yielded suggestion of associations and possible etiologies of AFP; however, the associations are weak and the suggested etiologies lack a scientifically evident justification.

Diagnosis

Given the unclear etiology of AFP and high variability in presentation of symptoms, there are no clear studies or tests that can confirm an accurate diagnosis of AFP. Currently, AFP is a diagnosis made based on a good clinical assessment by an experienced oral surgeon who can eliminate all other causes of facial pain; it is diagnosed by means of exclusion. Some guidelines have been put in place to offer a diagnostic criterion. The International Headache Society has offered diagnostic criteria of AFP. These criteria were evaluated by Zebenholzer and colleagues, who, using these criteria, suggested that most patients could be classified with accuracy and comparisons of management made easier. Their classification and definition are as follows.