A Mayo Clinic study revealed that 23% of adults and 31% of children diagnosed with NMOSD were AQP4-IgG seronegative.43In these cases it is important to consider additional MRI features and to carefully exclude diseases that may mimic NMOSD, such as neurosarcoidosis, infectious diseases, and other inflammatory CNS disorders.1As mentioned, seronegative patients should be retested, and the combination of clinical and supportive MRI features should be used to fulfill the IPND 2015 diagnostic criteria.1 Here, AQP4 is linked to neurosarcoidosis.