Sodium channel inactivation is vital for the proper cessation of action potentials. The decreased inactivation caused by these mutations, then, is expected to cause prolonged action potentials and repetitive firing. Such altered firing will cause increased pain sensation and increased sympathetic nervous system activity, producing the phenotype observed in patients with PEPD. Pathophysiology
There are a total of 8 mutations that account for the disorder in 8 of 14 studied families. These mutations are clustered in four regions throughout the channel: the linker between domains 2 and 3 (D2-3), the intracellular segment linking segments 4 and 5 in domain 3 (D3S4-5), the linker between domains 3 and 4 (D3-4) and the intracellular segment linking segments 4 and 5 in domain 4 (D4S4-5). The mutations in the D3S4-5 region (I1461T, F1462V and T1461I) are located in or next to an IFM motif that is conserved across all voltage-gated sodium channels. Mutagenesis studies of this region have shown that it acts as part of the inactivation gate, pivoting to block the central pore. Not surprisingly then, the two of these mutations that have received further study show incomplete inactivation. When the IFM motif pivots to block the central pore it interacts with residues in the D3S4-5 region. There are three mutations in this region (V1298F, F1298D and V1299F) that are believed to alter the interaction with the inactivation gate. While this region has been studied by mutagenesis these specific mutations have not all received attention, though they are expected to produce changes similar to the aforementioned IFM region mutations.
While other anti-epileptic drugs, gabapentin and topiramate, have limited effect in some patients, they have not been shown to be generally effective. Opiate derived analgesics are also largely ineffective, with only sporadic cases of beneficial effect. Epidemiology
PEPD is an extremely rare disorder with only 15 known affected families. There are some cases, however, of individuals originally diagnosed with epilepsy who are later determined to have PEPD. This suggests that rates of PEPD may be higher than currently believed. History
PEPD was originally described by Hayden and Grossman in 1959. At that time it was not given a specific name. A later report, by Dugan in 1972, labeled this disorder as familial rectal pain syndrome. This name was used for 33 years, until a consortium of patients and clinicians was formed in the hopes of discovering the genetic cause of PEPD. During this process a number of patients expressed dissatisfaction with the name and after considerable discussion between patients and clinicians the name paroxysmal extreme pain disorder was agreed upon in 2005. References


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