Kidney disease and inner ear impairment: a simpler and closer pathogenic analogy?

Abstract

The association of chronic kidney disease (CKD) and hearing disorders is well known, and was first reported more than 80 years ago by Alport, who described a case of familial kidney disease associated with hearing loss. Subsequently, a number of rare conditions or syndromes were described as occurring with a close link between hearing impairment and CKD (see Table 1). Moreover, some observational, cross-sectional, population-based studies concerning patients with CKD, or on dialysis therapy, confirmed this association suggesting the possibility that the linkage between the ear and the kidney is more than casual. In particular, Vilayur et al. report several physiological, ultrastructural and antigenic similarities between the kidney and the cochlea that strongly support the link between the hearing impairment and CKD [1]. The inner ear and the kidney share a series of basic processes for water and ion regulation as well as some specific cellular water channels known as aquaporins, which are known to have a crucial role in the functional activity of both organs. Moreover, the presence of a kidney-specific ion transport system has recently been demonstrated in the rat and human endolymphatic sac that, to date, represents the only known localization outside the kidney [2]. Once again this underlines a strict correspondence between the kidney and the inner ear, with both organs involved in highly energy-requiring processes linked to the strict necessity of maintaining the balance of ions and a stable pH. In particular, a derangement in the mechanisms regulating fluid and electrolyte homeostasis has been hypothesized both in the cochlear striavascularis, and in the kidney, to explain the possible association of inner ear and kidney diseases. A further contribution in order to clarify the importance of the analogies between kidney and inner ear comes from the progress in genetic research, revealing a possible common ground of dysfunction [3]. Finally, some cardiovascular risk factors such as age, diabetes, hypertension, electrolyte disorders and hemodialysis can affect both the ear and the kidney, and separately contribute to the development and the progression of organ-specific diseases [1]. Nevertheless, these reported connections and analogies between the ear and the kidney deserve to be considered according to a more comprehensive hypothesis. In particular, we speculate that a baseline condition of hemodynamic imbalance leading to a different extent of transient ischemia and hypoxia may elicit similar mechanisms of disease in both organs. Over the last decade, we have provided several observations supporting the hypothesis that some inner ear disorders of undetermined origin may depend on a large decrease in blood pressure values associated with an abnormal peripheral vasoconstriction [4]. This mechanism can be activated either in subjects without major cardiovascular risk factors (other than a generic sympathetic hyperreactivity), or in patients treated for hypertension or chronic congestive heart failure. In ‘‘healthy’’ subjects, the proposed model can provide a theoretical explanation for the cases of inner ear diseases usually labeled as ‘‘idiopathic’’, as well as for the different inner ear disturbances including the largely unexplained Meniere’s disease [5]. In patients with cardiovascular diseases, the symptoms of A. Pirodda Department of Specialist Surgical and Anesthesiological Sciences, S. Orsola-Malpighi University Hospital, Via Massarenti 9, 40138 Bologna, Italy