Disentangling renal function in the context of irreversible ethambutol optic neuropathy

Abstract

We read with great interest the meta-analysis recently published in International Ophthalmology concerning risk factors for ethambutol optic neuropathy (EON) [1]. According to the meta-analysis, risk factors include impaired renal function, longer duration of ethambutol treatment, higher dose ([25 mg/ kg/day) of ethambutol and older age. Noticeably, Talbert and Sadun reported that solely 4% of published case reports (i.e., one case report by Nair et al. [2]) have provided the necessary information to document absence of renal failure [1]. Nevertheless, close examination of the underlying data in the case report by Nair et al. seems not to provide conclusive evidence about the renal function of the patient, as the latter exhibited higher serum levels of ethambutol at 7 h (3 lg/ml) than at 3 h (2.25 lg/kg) after a single oral dose; this may not obligatorily reflect normal renal function, as the half-life of ethambutol is 3.1 ± 0.4 h [3]. In this Letter, we present a case of EON in an elderly woman and suggest that the Cockroft–Gault formula [4] may be a valuable tool in the context of EON, so as to reliably evaluate the underlying renal function. As older age and renal impairment are two closely related counterparts, the clarification of renal function seems indispensable to clarify any confounding effects of age. Taking into account the patient’s age, it seems that the Cockroft–Gault formula may be particularly valuable so as disentangle whether the (elderly) patient truly has renal failure or is just at risk. The EON case presented below is free of renal failure, as demonstrated by the Cockroft–Gault formula. Specifically, we briefly present the case of an 84-year-old female patient. Due to TBC osteomyelitis, the patient received rifampicin (600 mg/day), isoniazid (300 mg/day), ethambutol (15 mg/kg/day) and pyrazinamide (2.5 g/day). Prior to the initiation of treatment, the patient had Best Corrected Visual Acuity 9-10/10 at both eyes, normal colour vision, normal fundus and normal intraocular pressure. Concerning her history, the patient had undergone uneventful bilateral cataract surgery. I. P. Chatziralli (&) L. Papazisis Department of Ophthalmology, Veroia General Hospital, Veroia, Greece e-mail: eirchat@yahoo.gr