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Paroxysmal extreme pain disorder
Paroxysmal extreme pain disorder is a rare disorder of abnormal pain sensation.
Several families have been described. Prevalence is unknown but there is evidence that this condition may be underdiagnosed.
This condition presents in the first year of life with episodes of severe rectal pain following bowel movements. This is often accompanied by reflex anoxic seizures (RAS). Finally there is a colour change down one half of the body; either horizontally from the waist down or involving one half of the body as in the harlequin phenomenon. Adults experience similar attacks although they are rare. Triggers include unexpected falls, fights, sexual activity and vivid dreams but not defecation. Older children and adult patients are affected by painful eye and jaw crises. Patients may have any or all of the different episodes.
In several families it is caused by mutations in the sodium channel gene SCN9A. This results in a sodium channel that is constantly firing and hyperexcitable.
Diagnosis is based on a careful history, a normal examination and normal EEG, ECG and nerve conduction studies. Screening for SCN9A mutations should now be feasible.
Differential diagnoses include primary erythermalgia and hyperekplexia (see these terms).
Antenatal diagnosis may be possible in the future.
Paroxysmal extreme pain disorder follows an autosomal dominant pattern of inheritance.
Management and treatment
In children it is important to manage the inevitable constipation that ensues from stool withholding secondary to fear of defecation. The episodes themselves respond to the medication used in the management of chronic neuropathic pain disorders, namely anticonvulsants. Carbamazepine, a sodium channel blocker, is the most effective of these treatements.