Clinically silent brain lesions may accompany acute ON or myelitis attacks (most frequently seen in the corpus callosum, followed by the internal capsule, and the cerebral peduncles in AQP4-IgG-positive patients) [103, 168], but large tumefactive (even bilateral) lesions (as occasionally seen also in MOG-EM/MOGAD and MS), cavitary lesions, and residual T1 lesions (rare in MOG-EM/MOGAD but frequent in MS) may occur as well. Here, AQP4 is linked to myeloid sarcoma.