Medical professionals are urging individuals to be vigilant about the often-overlooked symptoms of pulmonary embolism, a life-threatening condition responsible for the death of renowned actress Catherine O’Hara.

O’Hara, known for her roles in “Home Alone” and “Schitt’s Creek,” passed away last month at the age of 71. A coroner’s report revealed that she was hospitalized with breathing difficulties and subsequently succumbed to a pulmonary embolism, a condition caused by a blood clot in the lungs.

This serious health issue claims the lives of nearly 3,000 people in the UK each year. Despite its prevalence, many individuals are not familiar with its early warning signs, according to Professor Rasha Al-Lamee, a cardiologist at Imperial College London.

Professor Al-Lamee emphasizes that certain risk factors can increase the likelihood of developing a pulmonary embolism. Fortunately, there are straightforward measures people can take to mitigate these risks.

What should you look out for to detect the early signs of this condition, and how can you prevent it from developing?

A pulmonary embolism occurs when a blood clot obstructs the arteries in the lungs, disrupting blood flow, placing strain on the heart, and depriving the body of essential oxygen.

The embolism often starts as deep-vein thrombosis, a clot forming in the veins of the leg or the pelvis. Often, according to Professor Al-Lamee, this can cause pain, swelling and tenderness in the leg – often in the calf.

However, she says the symptoms of an embolism will differ depending on its severity. A serious embolism – one where the clot is big, completely blocking blood flow in the lungs – is often unmistakable.

Schitt’s Creek and Home Alone star Catherine O’Hara was taken to hospital struggling to breathe

‘If a patient is suffering from a large pulmonary embolism they will experience sharp chest pain, a racing heart and can feel really unwell, alongside breathlessness’, says Professor Al-Lamee.

In some cases, patients may experience bluish lips or fingernails or a sense of anxiety or doom.

In 2023, actress Emily Chesterton, 30, died of a pulmonary embolism after an NHS worker mistook her symptoms for anxiety. Emily, from Manchester, also experienced calf pain, a swollen and hot leg, shortness of breath and found it increasingly difficult to walk.

A coroner concluded her life could have been saved if she had been to A&E and treated for a pulmonary embolism. Treatment typically involves anti-clotting medicines to reduce the size of the blockage. In extreme cases, lung surgery to remove the clot may be required.

Studies show that a severe embolism of this size can kill within an hour – or even minutes. Speedy treatment also does not necessarily guarantee survival. Research shows that around half of patients who experience a large pulmonary embolism die, even with treatment.

But, Professor Al-Lamee adds, small embolisms – where blood flow is not completely blocked – can be equally dangerous. This is because, while not instantly deadly, studies show they are often missed because the symptoms are mistaken for something less severe.

Without treatment, these clots do not disappear and can eventually grow larger, fully blocking off blood flow.

The last photo of O’Hara, seen left with her husband, Bo Welch, at the Angel Awards at the Proper Hotel Santa Monica on October 16 last year. Right, at the Emmys in September

Research shows that around 400 patients in the UK die every year due to their pulmonary embolism being misdiagnosed.

‘Smaller embolisms can recur over a period of time and patients often first notice a reduction in their ability to exercise, breathlessness, fatigue and sometimes blood-stained phlegm,’ says Professor Al-Lamee. ‘For this reason, these smaller clots are hard to pick up.’

Professor Al-Lamee also says that certain people are more prone to pulmonary embolisms – and so need to be more alert to the early warning signs.

One of the biggest risk factors, she says, is having cancer, which raises the chances of a deadly lung clot by six times.

This is research shows that cancer cells make the blood ‘stickier’, leaving it more prone to clots which can then travel to the lungs.

Experts say this might explain Ms O’Hara’s death. Her death certificate revealed that the actress also had rectal cancer at the time of the embolism, something she never made public while alive.

‘In Catherine O’Hara’s case this was likely the underlying cause of her embolism,’ says Professor Al-Lamee. ‘Other common risk factors for embolisms are pregnancy, smoking, obesity, and recent surgery.’

For this reason, she says, losing weight and quitting smoking are crucial for avoiding a pulmonary embolism.

Crucially, Professor Al-Lamee adds, long plane journeys – or any other extended period of immobility – greatly increases the chances of a deep vein thrombosis in the leg, which can then make its way to the lungs.

‘This is why when on a long haul flight it is important to wear compression socks and make sure you go for regular walks as this reduces the risk,’ she says.