This is a 55-year-old man, allergic to tetracyclines and beta-lactams with a history of arterial hypertension being followed up by the Chronic Pain Unit due to a clinical picture of low back pain and left lumbosciatica of spondyloarthrosis origin with an associated myofascial component.

Medical treatment was started with tramadol, pregabalin and duloxetine at the usual doses and diagnostic-therapeutic infiltration with local anaesthetic and corticoids of deep muscles (left psoas and quadratus lumborum). After a clinical improvement of more than 50%, a new infiltration was scheduled, according to hospital protocol, of the psoas and left quadratus lumborum muscles, with botulinum toxin serotype A (Dysport®) 200 IU per muscle in the same session and without incident. Four months later, the patient presented a self-limited episode of eyelid ptosis without consulting for it. Since then, he reported episodes of ptosis, which improved after infiltration with local anaesthetic and corticosteroids of trigger points in the shoulder girdle. Six months later, he presented with an episode of palpebral ptosis associated with diplopia and mandibular weakness, for which he consulted the Neurology Department of the hospital and was diagnosed with myasthenia gravis (MG), starting treatment with corticosteroids and pyridostigmine with improvement of the clinical picture.