Hypothalamic hamartoma: Neuropathology and epileptogenesis

Hypothalamic hamartomas (HHs) are congenital malformations of the ventral hypothalamus resulting in treatment-resistant epilepsy and are intrinsically epileptogenic for the gelastic seizures that are the hallmark symptom of this disorder. This paper reviews the neuropathologic features of HHs associated with epilepsy, with an emphasis on characterizing neuron phenotypes and an ultimate goal of understanding the cellular model of ictogenesis occurring locally within this tissue. We also present previously unpublished findings on Golgi staining of HH. The microarchitecture of HH is relatively simple, with nodular clusters of neurons that vary in size and abundance with poorly defined boundaries. Approximately 80-90% of HH neurons have an interneuron-like phenotype with small, round soma and short, unbranched processes that lack spines. These neurons express glutamic acid decarboxylase and likely utilize γ-aminobutyric acid (GABA) as their primary neurotransmitter. They have intrinsic membrane properties that lead to spontaneous pacemaker-like firing activity. The remaining HH neurons are large cells with pleomorphic, often pyramidal, soma and dendrites that are more likely to be branched and have spines. These neurons appear to be excitatory, projection-type neurons, and have the functionally immature behavior of depolarizing and firing in response to GABA ligands. We hypothesize that the irregular neuronal clusters are the functional unit for ictogenesis. Further research to define and characterize these local networks is required to fully understand the cellular mechanisms responsible for gelastic seizures.