CLINICAL ARTICLE I dIopathIc normal pressure hydrocephalus (iNPH) is characterized by the clinical triad of gait disturbance, cognitive decline, and urinary incontinence in patients with ventricular enlargement and a normal mean intracrani- al pressure, occurring without secondary causes of NPH. 1,5 A precise diagnosis is very important because iNPH is treatable by permanent drainage of CSF. The international guidelines for iNPH recommended CSF tap and CSF drain- ABBREVIATIONS 123 I-IMP = 123 I-iodoamphetamine; ARG = autoradiography; CAP = patients with the CAPPAH sign; CAPPAH = convexity apparent hyperperfusion; CBF = cerebral blood flow; DESH = disproportionately enlarged subarachnoid space hydrocephalus; FAB = Frontal Assessment Battery; iNPH = idiopathic normal pressure hydrocephalus; iNPHGS = iNPH Grading Scale; MMSE = Mini–Mental State Examination; NCAP = patients without the CAPPAH sign; rCBF = regional CBF; ROI = region of interest; SEE = stereotactic extraction estimation; SSP = stereotactic surface projection; TMT-A = Trail Making Test A; TUG = Timed Up and Go. SUBMITTED May 3, 2017. ACCEPTED September 25, 2017. INCLUDE WHEN CITING Published online March 16, 2018; DOI: 10.3171/2017.9.JNS171100. Usefulness of the convexity apparent hyperperfusion sign in 123 I-iodoamphetamine brain perfusion SPECT for the diagnosis of idiopathic normal pressure hydrocephalus Takuma Ohmichi, MD, 1 Masaki Kondo, MD, PhD, 1 Masahiro Itsukage, MD, 1 Hidetaka Koizumi, MD, PhD, 1 Shigenori Matsushima, MD, PhD, 2 Nagato Kuriyama, MD, PhD, 3 Kazunari Ishii, MD, PhD, 4 Etsuro Mori, MD, PhD, 5 Kei Yamada, MD, PhD, 2 Toshiki Mizuno, MD, PhD, 1 and Takahiko Tokuda, MD, PhD 1,6 Departments of 1 Neurology, 2 Radiology, 3 Epidemiology for Community Health and Medicine, and 6 Molecular Pathobiology of Brain Diseases, Graduate School of Medical Science, Kyoto Prefectural University of Medicine, Kyoto; 4 Department of Radiology, Kindai University Faculty of Medicine, Osaka; and 5 Department of Behavioral Neurology and Cognitive Neuroscience, Tohoku University Graduate School of Medicine, Sendai, Japan OBJECTIVE The gold standard for the diagnosis of idiopathic normal pressure hydrocephalus (iNPH) is the CSF removal test. For elderly patients, however, a less invasive diagnostic method is required. On MRI, high-convexity tight- ness was reported to be an important fnding for the diagnosis of iNPH. On SPECT, patients with iNPH often show hyperperfusion of the high-convexity area. The authors tested 2 hypotheses regarding the SPECT fnding: 1) it is relative hyperperfusion refecting the increased gray matter density of the convexity, and 2) it is useful for the diagnosis of iNPH. The authors termed the SPECT fnding the convexity apparent hyperper fusion (CAPPAH) sign. METHODS Two clinical studies were conducted. In study 1, SPECT was performed for 20 patients suspected of hav- ing iNPH, and regional cerebral blood fow (rCBF) of the high-convexity area was examined using quantitative analysis. Clinical differences between patients with the CAPPAH sign (CAP) and those without it (NCAP) were also compared. In study 2, the CAPPAH sign was retrospectively assessed in 30 patients with iNPH and 19 healthy controls using SPECT images and 3D stereotactic surface projection. RESULTS In study 1, rCBF of the high-convexity area of the CAP group was calculated as 35.2–43.7 ml/min/100 g, which is not higher than normal values of rCBF determined by SPECT. The NCAP group showed lower cognitive func- tion and weaker responses to the removal of CSF than the CAP group. In study 2, the CAPPAH sign was positive only in patients with iNPH (24/30) and not in controls (sensitivity 80%, specifcity 100%). The coincidence rate between tight high convexity on MRI and the CAPPAH sign was very high (28/30). CONCLUSIONS Patients with iNPH showed hyperperfusion of the high-convexity area on SPECT; however, the pres- ence of the CAPPAH sign did not indicate real hyperperfusion of rCBF in the high-convexity area. The authors specu- lated that patients with iNPH without the CAPPAH sign, despite showing tight high convexity on MRI, might have comor- bidities such as Alzheimer’s disease. https://thejns.org/doi/abs/10.3171/2017.9.JNS171100 KEY WORDS cerebral blood flow; convexity apparent hyperperfusion sign; idiopathic normal pressure hydrocephalus; 123 I-IMP SPECT J Neurosurg March 16, 2018 1 ©AANS 2018, except where prohibited by US copyright law