Hypomagnesemia with secondary hypocalcemia (HSH) is an autosomal recessive genetic disorder affecting intestinal magnesium absorption. Decreased intestinal magnesium reabsorption and the resulting decrease in serum magnesium levels is believed to cause lowered parathyroid hormone (PTH) output by the parathyroid gland. This results in decreased PTH and decreased serum calcium levels (hypocalcemia). This manifests in convulsions and spasms in early infancy which, if left untreated, can lead to mental retardation or death. HSH is caused by mutations in the TRPM6 gene. Pathophysiology
HSH is caused by decreased intestinal magnesium reabsorption through TRPM6 channels. When expressed in cells, TRPM6 produces outwardly rectifying currents with the outward portion composed of Na+ ions and the inward portion of divalent cations (particularly magnesium and calcium). Inward flow of sodium ions is blocked by extracellular divalent cations. Increased intracellular magnesium concentrations also decrease current through TRPM6 channels. There are currently more than 30 known mutations in TRPM6 that are associated with HSH and these mutations are spreading throughout the gene (table 1). Of the eight HSH mutations that have been tested, none have shown to produce whole-cell current. The S141L mutation, one of the few missense mutations, has been of particular interest to researchers. They have found that it prevents coassembly with TRPM7 (and presumably other TRPM6 subunits) and lacks the ability to traffic to the membrane.
Because the areas of the cerebellum which determine increases and decreases in muscle tone are close together, people experiencing atonic seizures are most likely experiencing myoclonic ones too, at some point. This may play a role in therapy and diagnostic.One surgical approach, selective posterior callostomy, can greatly decrease instances of drop attacks and improve function and behavior in patients with intellectual disability. References


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