Heralding Extramedullary Blast Crisis: Horner's Syndrome with Brachial Plexopathy in a Patient with Chronic Myelogenous Leukemia.

Chronic myelogenous leukemia (CML) blast crisis is an ominous clinical event that is challenging to treat. This can develop at extramedullary sites rarely and is defined as the infiltration of blasts outside the bone marrow irrespective of proliferation of blasts within the bone marrow. We aim to report an unusual clinical presentation characterized by Horner's syndrome, ipsilateral arm weakness, and cervical lymphadenopathy as the first signs of extramedullary blast crisis in a CML patient. To the best of our knowledge, the extramedullary locations involving the brachial plexus along with cervicothoracic paraspinal chloroma have not been previously reported in the literature.