Neurologists in Wuhan, China, where the outbreak started, were among the first to report the symptoms in a preliminary paper published online in February.
Since that report, specialists observed similar symptoms in Germany, France, Austria, Italy and Holland as well as the United States, including among patients under 60, Dr. Stevens said.
Some doctors have reported cases of patients who were brought in for treatment because of their altered mental state, and who ultimately tested positive for Covid-19, although they had none of the classic symptoms like fever or cough.
Four elderly patients who came into Danbury Hospital in Connecticut with encephalopathy ultimately tested positive for Covid-19, although they had no other symptoms, said Dr. Paul Nee, an infectious disease specialist at the hospital. Two of the four went on to develop low grade fevers and needed oxygen briefly, but two did not, he said.
While it is not unusual for elderly people to experience confusion when they develop other infections, “the striking thing is we have not seen any real respiratory illness in these patients,” Dr. Nee said. They have continued to test positive and cannot be discharged, even though they are not really ill, he said.
But earlier reports had indicated that severely ill individuals with more typical symptoms were more likely to exhibit the rare neurological conditions, which ranged from dizziness and headaches to impaired consciousness, stroke and musculoskeletal injury. The Chinese study in February said that about 15 percent of those patients with severe illness experienced a change in mental status, compared with 2.4 percent of those who did not have severe illness, according to that study.
Another study, published in the British Medical Journal in late March, found that of 113 patients from Wuhan who died of Covid-19, 22 percent had experienced disorders of consciousness, ranging from somnolence to deep coma, compared with only 1 percent of another group of patients who recovered from the illness.