Are disproportionate subarachnoid spaces and stroke history predictors of external lumbar drainage outcome in iNPH?

We comment the article by El Ahmadieh et al. (Lumbar drain trial outcomes of normal pressure hydrocephalus: a single-center experience of 254 patients. J Neurosurg [epub ahead of print January 4, 2019. DOI: 10.3171/2018.8.JNS181059]), which pursues the aim to find predictors of one of the most practiced and accurate tests to select idiopathic normal pressure hydrocephalus (iNPH) patients for shunt surgery. We argue that the development of a bedside clinical prediction tool allowing the clinician to identify a subset of iNPH patients with a very high a priori probability to have a negative or positive ELD trial would be an important step forward. Those patients with a very high likelihood of having a negative ELD outcome could be confidently screened out and spared from undergoing both ELD and subsequent surgery. Likewise, those patients identified as having a very high a priori probability of a positive ELD outcome could be safely assigned to surgery without undergoing ELD beforehand. Eventually, this would avoid undue discomfort and complication risks associated with ELD and surgery for a subset of iNPH patients, with obvious savings for healthcare providers.