Human herpes virus 6
In the mid-1980s, human herpesvirus 6 (HHV-6) was suggested as a possible causal link between febrile convulsions and mesial temporal lobe epilepsy. However, although the virus is found in temporal lobe tissue at surgery for TLE, it has not been recognised as a major factor in febrile seizures or TLE. Reelin
Dispersion of the granule cell layer in the hippocampal dentate gyrus is occasionally seen in temporal lobe epilepsy and has been linked to the downregulation of reelin, a protein that normally keeps the layer compact by containing neuronal migration. It is unknown whether changes in reelin expression play a role in epilepsy. Pathophysiology
Neuronal loss
In TLE, there is loss of neurons in region CA1 and CA3 of the hippocampus. There is also damage to mossy cells and inhibitory interneurons in the hilar region of the hippocampus (region IV) and to the granule cells of the dentate gyrus. In animal models, neuronal loss occurs during seizures but in humans, neuronal loss predates the first seizure and does not necessarily continue with seizure activity. The loss of the GABA-mediated inhibitory interneurons may increase the hyperexcitability of neurons of the hippocampus leading to recurrent seizures. According to the "dormant basket cell" hypothesis, mossy cells normally excite basket cells which in turn, inhibit granule cells. Loss of mossy cells lowers the threshold of action potentials of the granule cells. GABA reversal
In certain patients with temporal lobe epilepsy it has been found that the subiculum could generate epileptic activity.
The M1627K mutation in the D4S4-5 region may also affect a residue involved in interacting with the IFM inactivation motif. This would explain the observed alteration of inactivation and the broadening of a window current. One of the affected families with the R996C mutation, pedigree 12, has a single individual who also has the V1298D mutation. The individual in this family with the compound mutation is the most severely affected, suggesting that the R996C mutation may cause a less severe phenotype. The less severe phenotype of the pedigree 4 family is in concordance with this theory. It is unclear how the R996C mutation affects channel function. Diagnosis
Hematological, biochemical and metabolic investigations on blood and urine between attacks are normal, as are karyotyping and EKG recordings. EKG recordings during attacks show sinus tachycardia. CT, MRI, EMG and nerve conduction studies produce normal results. EEG recordings are normal between attacks but show early-onset tachycardia during attacks. On the Neuropathic Pain Questionnaire patients indicated that pain during attacks is extremely unpleasant and typically felt deep, though also superficial on occasion. Aside from presentation of typical symptoms (see Signs and symptoms above) mutation of the gene SCN9A aids in appropriate diagnosis as this gene is mutated in 8 of 14 studied families. Treatment
Carbamazepine is at least partly effective at reducing the number or severity of attacks in the majority of PEPD patients. High doses of this drug may be required, perhaps explaining the lack of effect in some individuals.