Patients with COVID-19 who have been admitted to the intensive care unit are very likely to experience unusually persistent delirium, according to emerging research.

Delirium is a medical term used to describe confused thinking and reduced awareness of surroundings - a not uncommon state of mind for the sickest hospitalized patients.

As it turns out, severe cases of COVID-19 are enough to trigger something similar. In fact, initial investigations have suggested delirium occurs in up to 80 percent of ICU patients with COVID-19, possibly as a result of loss of oxygen to the brain or widespread inflammation.

Now a new analysis of critically ill COVID-19 patients at a single hospital in Michigan has found even more evidence that delirium is a very common symptom of the disease - one that could possibly slow patient recovery if it's not addressed.

Using medical records and discharge surveys from 148 patients checked into the ICU between March and May 2020, researchers have found more than 70 percent of the cohort experienced a prolonged disturbance in their mental abilities.

In most cases, the delirium lasted for days. But nearly a third of participants left hospital without demonstrating they'd fully recovered from their delirium.

Of those who were discharged with signs of cognitive impairment, nearly half required skilled nursing care to get by at home. Their persistent confusion reduced their ability to look after themselves, according to follow-up phone surveys conducted between month one and month two of being discharged.

"These results align with previous data demonstrating a high incidence of delirium in critically ill patients with COVID-19," the authors conclude.

"Moreover, the median duration of delirium (10 days) is relatively long compared with other critically ill populations."

It's not yet clear whether these severe impairments are a result of the SARS-CoV-2 virus itself, which seems to cause an unusual number of neurological symptoms that can persist for six months or more, or if it's a sign of critical illness more broadly.

Generally, cognitive impairment is seen in about 20 percent of patients in acute care facilities, so it's expected to a certain extent. But the current pandemic seems to have at least tripled that number.

While the mechanism behind COVID-19 delirium remains a mystery, researchers in Michigan say it is clear that ICU patients infected with the coronavirus are experiencing "considerable neuropsychological burden" both during their hospital stay and after being discharged.

"Overall, this study highlights another reason why getting vaccinated and preventing severe illness is so important," says anesthesiologist Phillip Vlisides from Michigan Medicine.

"There can be long term neurological complications that perhaps we don't talk about as much as we should."

Early on in the pandemic, for instance, checking patients for symptoms of delirium was not commonplace.

Even when delirium was observed, exercise regimes and other novel strategies for improving cognitive performance, like face-to-face time with family or breathing trials, were rarely introduced, possibly because protective equipment was not easily available at the time.

The likely result is that many patients with severe cases of COVID-19 have been discharged from hospital with serious cognitive impairments, which were not addressed properly.

And that's a big problem. Delirium is generally associated with prolonged hospitalization and illness recovery.

In the new Michigan study, for instance, those patients experiencing delirium had longer stays at the hospital and ICU. They also spent more time relying on mechanical ventilation.

"Whatever creative ways we can implement delirium prevention protocols is likely to be very helpful," says Vlisides.

"That includes consistent communication with family members, bringing in pictures and objects from home, and video visits if family cannot safely visit."

As it turns out, those patients disproportionately vulnerable to severe forms of COVID-19, like those from racial and ethnic minority communities, are also the most likely to experience delirium while hospitalized.

In fact, researchers in Michigan found half the patients in the delirium group were African American - a damning reflection of ongoing disparities in US healthcare.

Further research at more acute care facilities and among larger and more diverse cohorts will be needed before we can say with any certainty who is most at risk of experiencing delirium when hospitalized with COVID-19.

While the study in Michigan found female patients are more likely to fall in the delirium group, other initial studies suggest male patients in the ICU are more susceptible to cognitive impairment.

If it turns out that delirium really is such a common experience for those with severe COVID-19, we need to start to recognize and treat the symptoms as early as possible. Otherwise, it could prove much harder for the sickest COVID-19 patients to get back on their feet.

The study was published in BMJ Open.