AQP2 and Menkes disease: An immunohistochemical and ultrastructural investigation of the human endolymphatic sac in MD revealed both the secretion of glycoproteins and the possible existence of hypersecretions of endolymph in the sac (8–10), and an increased expression of aquaporin-2 in the endolymphatic sac epithelium of patients with MD was proposed to be involved in the pathophysiology of EH (11), which may support the assumption that an increased secretion outweighs a decreased absorption resulting in increased pressure in the inner ear (12).