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Persistent idiopathic facial pain (PIFP): An unexplored entity

An update on the diagnosis and management of PIFP, a difficult-to-treat and under-recognised condition

Terminology

PIFP is the new term suggested by International Headache Society (HIS) for non-specific chronic facial pain with a code of IHS-ICHD — 13.12. Previously, various terms have been used, including atypical facial pain (AFP), medically unexplained chronic orofacial pain and ‘facial pain of unknown origin’.

Issues

PIFP is an unrecognised and unhelpful diagnosis but a chronic pain condition with a big diagnostic challenge. Patients frequently are misdiagnosed or attribute their pain to a prior event, such as a dental procedure or facial trauma. The vast majority, 90 per cent, have constitutional symptoms like depression, headache and non-specific back pain.

Epidemiology and prognosis

The estimated incidence of PIFP has been reported to be around 1/100,000, which could again be an underestimate or overestimate, as diagnostic criteria could vary among physicians across different specialities.

A German study reports a lifetime prevalence of PIFP to be 0.03 per cent. In orofacial pain clinics, PIFP may account for around 10-to-21 per cent of all pain conditions. PIFP mainly affects adults and is rare in children, and is more common in females than males. Overall prognosis remains very poor. It represents a big challenge to the primary care and pain physicians who are left with limited options to treat.

Diagnosis

Diagnostic criteria — PIFP: IHS — International classification of Headache Disorders ICHD — 13.12 (see Table 1).

Associated psychological disorders: Like other chronic pain conditions, depression and anxiety remain the most common association, followed by other conditions (see Table 2).

Differential diagnosis: The condition that should be separated from PIFP is painful post-traumatic trigeminal neuropathy, which has different diagnostic criteria. Other conditions which could closely mimic PIFP are trigeminal autonomic cephalgias (TACs) and regional myofascial pain (see Figures 1 and 2).

Pathophysiology: Clearly, the pathophysiology of PIFP remains elusive. Injury of the trigeminal nerve could be a possibility. PIFP is a disproportionate response to mild injury, like complex regional pain syndrome (CRPS). Infectious causes should also be considered. On one hand, PIFP can be considered a neuropathic pain syndrome based on some studies which reveal increased neuronal excitability at the brain stem level, disturbed inhibitory function, and alterations in the dopamine systems. On the other hand, PIFP may not always be a neuropathic pain syndrome, as suggested by negative quantitative sensory testing (QST) in many patients. Neck pain associated with facial pain could be explained by the concept of cervico-trigeminal nucleus (CTN) complex activation, the role of which is being widely considered now in neck pain associated with facial pain or headache.

Investigations

QST may demonstrate small-fibre dysfunction (A delta and C). Neurography of the inferior alveolar nerve (IAN) may be helpful. SSEP, facial CT, SPECT-CT and MRI/MRA are important diagnostic tools in some cases. Functional imaging studies like fMRI/PET scan should be reserved for research purposes only.

Treatment

Treatment of PIFP is multidisciplinary. The lack of a clear pathophysiological basis precludes the establishment of a treatment protocol. Education is needed to clarify the diagnosis, and certainly the patient should be discouraged from any further invasive interventions aimed at pain relief in the absence of clear associated pathology.

Psychological intervention: Early psychiatric screening should be performed even though there is insufficient evidence to support the claim that psychological factors are the primary cause. Understanding the associated psychological problems improves overall outcome. Simple psychological treatments like cognitive behavioural therapy, mindfulness and hypnosis have been shown to be useful in  persistent pain and demonstrate small but consistent improvements in pain, disability and quality of life compared to standard care.

Drug treatment: Carbamazepine (200-1200mg/d) and oxcarbazepine (600-1800mg/d) can be tried as first-line therapy but the treatment can be hampered by side-effects, including serious hyponatraemia. Other medications can  also be tried, like lamotrigine (100-400mg/day), gabapentin (300-3600mg/day), pregabalin (150-600mg/day), phenytoin (100-600mg/day) and baclofen (10-90mg/day). In myofascial facial pain local injections of botox (25-100 units) every three months have been found to be useful. A new Nav1.7 selective state-dependent, sodium channel blocker (vixotrigine) is under development.

Non-drug non-surgical pain interventions: Interventional pain procedures like trigeminal nerve block, PRF/neurolysis of the sphenopalatine ganglion, PRF/neurolysis of the trigeminal ganglion, occipital nerve block and stellate ganglion block carry more risks than benefits. Serious complications can occur with trigeminal ganglion block or ablation, one of which is infection (meningitis). However, overall incidence of meningitis is generally very low.

Non-drug surgical interventions: They have very little role to play in such a non-specific pain condition. Non-invasive interventions include occipital stimulation, repetitive transcranial magnetic simulation of the motor cortex contralateral to pain, gamma knife radiation surgery and low-level laser therapy. Invasive surgical interventions include microvascular decompression (MVD), repeat posterior fossa exploration and trigeminal tractotomy-nucleotomy under CT guidance.

Level of evidence various treatment: In brief, the available evidence suggests that well-designed studies regarding the clinical effectiveness of interventions for PIFP are currently lacking.

Drug treatment: Unfortunately, few randomised controlled trials for the treatment of PIFP exist.

Non-surgical interventions: Limited evidence of the clinical effectiveness of non-surgical interventions (drug and non-drug) was reported by five non-randomised studies and two randomised controlled trials, where non-surgical approaches were found to attenuate pain symptoms in some patients, but not all patients responded well to these treatment modalities.

Surgical interventions: Seven non-randomised studies on surgical procedures for atypical facial pain were identified that reported generally poor outcomes related to pain relief and complications or adverse events among this patient population. The identified evidence-based guideline recommends the use of conservative pharmacologic intervention as first-line therapy, followed by minimally-invasive surgical intervention for patients refractory to drug treatment. These recommendations were based on a small number of low-quality studies. Patient outcomes relating to pain relief and adverse events or complications following surgical intervention are generally poor among this patient population; in addition, the rate of pain recurrence is of significant concern.

Recommendations

PIFP does not have a clear aetiology that can be easily targeted for treatment, and it has been suggested that neurosurgery and other invasive treatments are not effective for this condition. In clinical practice, evidence regarding the safety and effectiveness of treatment options for PIFP, as well as guidelines for their use, is required.

A multidisciplinary approach should be based on experiences with comparable chronic headache disorders, including involvement of headache and facial pain specialists. Treatment should include medications, relaxation training, psychological interventions and physiotherapy. At any cost, invasive treatment should be avoided.

Conclusions

PIFP is often a relatively featureless headache that may mimic other facial pain syndromes. Inclusion criteria rather than a ‘diagnosis of exclusion’ should be considered. Often, PIFP seems to be initiated by minor trauma, suggesting a shared pathophysiology and a clinical spectrum with painful traumatic neuropathies. Careful interdisciplinary collaboration is needed to establish the diagnosis and management of PIFP. In the current climate, with the lack of resources to treat facial pain and the focus on a mix of skills, oral and maxillofacial (OMF) surgeons and pain specialists are likely to have an increasing role in the treatment of these patients. Evidence-based guidelines are needed to be identified to manage PIFP.

References on request

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