Persistent idiopathic facial pain (PIFP), originally known as atypical facial pain, refers to pain along the territory of the trigeminal nerve that does not fit the classic presentation of other cranial neuralgias. [1, 2] The pain is usually of long duration, lasting most of the day (if not continuous), is unilateral, and is without autonomic signs or symptoms. It is described as a severe ache, crushing sensation, or burning sensation. Upon examination and workup, no abnormality is noted.
Within the group of chronic facial pain syndromes, PIFP represents a particular diagnostic challenge. Patients frequently are misdiagnosed or attribute their pain to a prior event, such as a dental procedure or facial trauma. Psychiatric symptoms of depression and anxiety are prevalent in this population, further compounding the diagnostic conundrum. Treatment of PIFP is typically less effective than that of other facial pain syndromes, and a multidisciplinary approach is required to address the many facets of this pain syndrome.
Pathophysiology and Etiology
The Headache Classification Subcommittee of the International Headache Society defines PIFP as follows  :
Pain is in the face
Pain is present daily and persists for all or most of the day
Pain is confined at onset to a limited area on one side of the face and is deep and poorly localized
In addition, the pain is not associated with sensory loss or other physical signs, with no abnormalities in laboratory or imaging studies
PIFP usually does not have a specific cause; however, injury of the trigeminal nerve proximally or distally may lead to this disorder. Demyelination, either central or peripheral, may initiate PIFP symptoms. Infectious causes should also be considered.
Epidemiology and Prognosis
Because of a lack of agreement on classification criteria, accurate figures on the frequency of PIFP are difficult to obtain. The estimated incidence has been estimated to be 1 case per 100,000 population (0.001%), though this number may be an underestimate.  A German study that included 3336 participants estimated the lifetime prevalence of PIFP to be 0.03%. 
For patients who do not respond to the available pharmacologic and nonpharmacologic therapies, the prognosis is poor.
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