Neurologists around the world say that a small subset of patients with Covid-19 are developing serious impairments of the brain.

Although fever, cough and difficulty breathing are the typical hallmarks of infection with the new coronavirus, some patients exhibit altered mental status, or encephalopathy, a catchall term for brain disease or dysfunction that can have many underlying causes, as well as other serious conditions. These neurological syndromes join other unusual symptoms, such as diminished sense of smell and taste as well as heart ailments.

In early March, a 74-year-old man came to the emergency room in Boca Raton, Fla., with a cough and a fever, but an X-ray ruled out pneumonia and he was sent home. The next day, when his fever spiked, family members brought him back. He was short of breath, and could not tell doctors his name or explain what was wrong — he had lost the ability to speak.

The patient, who had chronic lung disease and Parkinson’s, was flailing his arms and legs in jerky movements, and appeared to be having a seizure. Doctors suspected he had Covid-19, and were eventually proven right when he was finally tested.