New York,  Patients hospitalised with severe Covid-19 infection are highly at risk of developing cognitive impairment -- including depression and delirium -- both during the treatment and after it, finds a study.

Delirium is a serious disturbance in mental state wherein a patient is confused, agitated and unable to think clearly.

The study, published in BMJ Open, looked at nearly 150 patients hospitalised for Covid at the beginning of the pandemic. Of these, 73 per cent had delirium.

Patients with delirium tended to be sicker, with more comorbidities like hypertension and diabetes, and appeared to have more severe Covid-related illness as well, said study author Phillip Vlisides, from the Department of Anesthesiology at Michigan Medicine, University of Michigan.

"Covid is also associated with a number of other adverse outcomes that tend to prolong hospitalisation and make recovery difficult," he added.

Using patient medical records and telephone surveys following hospital discharge for a group of patients hospitalised in the intensive care unit between March and May 2020, the team attempted to identify common threads amongst patients who developed delirium.

The disease itself can lead to reduced oxygen to the brain as well as the development of blood clots and stroke, resulting in cognitive impairment. In addition, inflammatory markers were greatly increased in patients with delirium. Confusion and agitation could be a result of inflammation of the brain.

In addition, care teams often were unable to perform standard delirium reduction techniques, such as exercises designed to get a patient moving or allowing visitors or objects from home to orient patients while in the hospital due to the infectious Covid disease.

Further, there was a correlation between the use of sedatives and delirium -- patients with delirium were sedated more often and frequently at higher doses, the team found.

The study also found that cognitive impairment can persist even after discharge. Almost a third of patients did not have their delirium marked as resolved in their chart upon leaving the hospital and 40 per cent of these patients required skilled nursing care. For many the condition lasted for months.

"Overall, this study highlights another reason why getting vaccinated and preventing severe illness is so important. There can be long term neurological complications that perhaps we don't talk about as much as we should," Vlisides said.