Elevation of intracranial pressure affects the relationship between hemoglobin concentration and neuronal activation in human somatosensory cortex

During neuronal activation, a local decrease of deoxygenated hemoglobin concentration (deoxy-Hb) occurs which is the basis of functional brain imaging with blood oxygenation level dependent functional magnetic resonance imaging (BOLD-fMRI). Elevated intracranial pressure (eICP) has been shown to impair functional deoxy-Hb changes. This study investigated this effect and its relation to the underlying neuronal activity in the human primary somatosensory cortex (SI). Functional near-infrared spectroscopy (fNIRS) during somatosensory evoked potentials (SEP) monitoring was performed on 75 subjects during conditions of median nerve stimulation (MNS) and resting state, combined with normal breathing (NB) and eICP by escalating breathing maneuvers (breath holding [BH], Valsalva maneuver with 15 mmHg [V15] and 35 mmHg expiratory pressure [V35]). During NB, fNIRS revealed a typical oxygenated hemoglobin concentration (oxy-Hb) increase with deoxy-Hb decrease during MNS enabling SI brain mapping. Breathing maneuvers associated eICP produced a known global change of oxy-Hb and deoxy-Hb with and without MNS. When subtracting measurements during resting state from measurements during MNS, neither functional oxy-Hb nor deoxy-Hb changes could be recovered while SEPs remained unchanged. In conclusion, Valsalva-induced eICP prevents oxy-Hb and deoxy-Hb changes during neuronal activation in SI. This finding raises questions on the validity of oxy-Hb- and deoxy-Hb-based brain imaging (e.g., BOLD-fMRI) during eICP.