Doctors must identify the origins and type of stroke a person has had to prescribe the best form of treatment to minimise the risk of a second stroke. However, about a third of all cerebrovascular accidents in Singapore are categorised as cryptogenic strokes, meaning they have no known cause.
We speak to Dr Reginald Liew Kay Choon, senior consultant cardiologist from the Harley Street Heart and Vascular Centre about why it’s important to know what caused a stroke, and how specialists are getting better at detecting asymptomatic cerebrovascular events.
Q: What is a stroke? And are there different kinds?
A stroke is an event affecting the arteries that supply the brain with oxygen and nutrient rich blood. Strokes can be divided into two basic types, although there are subtypes within the two divisions.
Haemorrhagic strokes account for about 13% of strokes and occur when a blood vessel ruptures and leaks blood into the surrounding brain tissue. The leading causes of haemorrhagic stroke are high blood pressure and aneurysms, balloon-like bulges in an artery that can stretch and burst. Most haemorrhagic strokes are intracerebral haemorrhages, i.e. a small artery in the brain which bursts but there is a less common form called subarachnoid haemorrhage when blood leaks into the space between two of the membranes surrounding the brain.
Ischaemic strokes occur when an artery in the brain becomes blocked by a blood clot or plaque, cutting off the blood supply to a part of the brain. The root cause of most ischaemic strokes is atherosclerosis, a narrowing of the arteries due to gradual cholesterol deposition forming a plaque on the artery walls. If the arteries become too narrow, blood cells may collect and form blood clots or more rarely, bits of plaque might break off.
Ischaemic stroke can be divided into a thrombotic or embolic stroke. A thrombotic stroke occurs when diseased or damaged cerebral arteries become blocked by the formation of a blood clot within the brain itself. An embolic stroke is when a clot forms in another part of the body and then breaks off, forming an embolism, which then travels through the blood stream until it gets lodged in one of the narrow vessels in the brain.
Q: What is a cryptogenic stroke?
A cryptogenic stroke is a stroke with no known cause. In most cases, strokes including cryptogenic ones, are caused by clots but sometimes, despite a thorough examination and testing, the source of the clot can’t be found.
Q: Why is it important to identify the type of stroke a person has had and what caused it?
It’s important to identify the type of stroke a person has had to know what would be the best form of long-term care and treatment to minimise the risk of a second stroke. In order to prescribe the correct medication to limit the risk of recurrence, we need to know what caused the first stroke because the different types of stroke are treated with different medications.
For instance, a patient who’s had a haemorrhagic stroke is likely to be given medications to reduce their blood pressure but not anticoagulants (blood thinners) which might increase the risk of a second stroke. Whereas long-term treatment for most ischaemic stroke patients includes antiplatelet medication such as aspirin which, depending on the sub-type of ischaemic stroke it was, is often combined with blood thinning medication such as direct oral anticoagulant.
Q: How often are strokes categorised as cryptogenic?
It is difficult to give an exact percentage. Although cryptogenic stroke seems common, the term lacks specificity and leads to great variability, especially globally, in definition and recording methods so that it is variously reported as accounting for 15 to 40% of all ischaemic strokes in different studies.
In Singapore, they probably account for around a third of strokes each year.
Q: Are doctors getting better at discovering the origins of cryptogenic strokes? If so, how?
Yes, doctors are getting better at detecting asymptomatic episodes of atrial fibrillation (AF) in patients after strokes with improvements in cardiac monitoring technology. AF is a common cause of stroke but can be intermittent and difficult to detect with conventional cardiac monitoring equipment, such as use of a 24-hour Holter monitor. Use of insertable cardiac monitors (ICM), small electronic devices which can be easily implanted underneath the skin on the chest wall, allow the heart rhythm to be monitored for up to three years and greatly improve the detection rate of AF. New research in the field has shown that the use of ICMs in patients diagnosed with cryptogenic stroke can detect AF in up to 40% of patients, which was previously undiagnosed.
Definition of atrial fibrillation (AF): Normally your heart contracts and relaxes to a regular beat. In AF, the upper chambers of the heart (atria) beat irregularly (quiver) instead of beating effectively to move blood into the ventricles, the two main chambers of the heart. AF can lead to blood clots, strokes, heart failure and other related complications. Strokes due to AF are more severe, causing disability in over 50% of patients and generally worse outcomes than strokes due to other causes.
Q: How long after a cryptogenic stroke can these new tests be done?
Cardiac tests, such as an ECG, Holter monitor and ultrasound scan of the heart (echocardiogram) are usually done within the first few days when the cryptogenic stroke is first detected. If no abnormality is detected, then an insertable cardiac monitor can be implanted within the first week after the cryptogenic stroke.
Q: What proportion of cryptogenic strokes recategorised as ischaemic or Transient Ischamic Attacks (TIA) are likely to be associated with atrial fibrillation?
As many as 40% of cryptogenic strokes recategorised as ischaemic strokes are associated with atrial fibrillation, according to recent studies.
Q: What prevention options are there for patients with strokes that have been identified as being caused by AF?
The importance of diagnosing AF and reclassifying the cryptogenic stroke as ischaemic is that patients can then be treated with appropriate blood thinning medication, usually a direct oral anticoagulant, which can significantly reduce the chances of them getting another stroke.
Q: How does the treatment for haemorrhagic strokes differ from that for ischaemic strokes or TIAs?
The treatment for haemorrhagic strokes is to reduce the factors that caused the stroke and bleed in the first place, i.e. high blood pressure or cerebral artery aneurysms. In contrast, the treatment for ischaemic strokes or TIA is to start blood thinning medication if the cause was due to a thromboembolism (blood clot travelling to the brain), as is the case in patients with AF, or atherosclerosis which result in the furring up arteries in the neck.
Dr Reginald Liew Kay Choon is a senior consultant cardiologist at the Harley Street Heart and Vascular Centre in Gleneagles Hospital, Mount Elizabeth Novena Hospital, and Mount Elizabeth Medical Centre.