Unfortunately, strokes remain common.

The most common are ischemic in origin – the result of a block in an arterial branch, whether a local blood clot or an embolus that deprives the affected region of the brain of the oxygen and glucose it needs to survive.

Less common are hemorrhagic strokes.

Most are related to hypertension and rupture of relatively small degenerative penetrating arteries in the base of the brain, which supply the basal ganglia (putamen), thalamus, brainstem or cerebellum.

The outlook very much depends on the site of the bleeding and whether the hemorrhage continues to enlarge and compress the nearby brainstem.

Less familiar might be the hemorrhages, which occur between the tough outer membrane – called the dura – and the underlying flimsier arachnoid membrane.

Both are interposed between the inner surface of the skull and a third delicate membrane – the pia matter – which closely invests the contours of the brain, including the sulci between the brain’s gyri.

Between the dura and arachnoid membrane there is an extensive network venous channels.

If these channels are torn by trauma – or because they are more fragile, as they become in the later life – may bleed, creating a localized collection of blood between the inner side of the dural membrane and the arachnoid membrane (the subdural space) and thus the name, subdural hematoma.

Acute subdural hemorrhages are dangerous because, if large enough, they can compress the underlying brain and even push the brain downward, jamming the upper brainstem and causing coma or death.

That’s why acute subdural hemorrhages have a high mortality rate.

However, recognized early, they can be effectively treated by removing the blood, stopping the bleeding, and sometimes temporally removing the overlying skull flap to decompress the swollen brain.

The outcome depends on the severity of the injuries to the brain, often due to a car crash or injuries from sports such as football or hockey.

Subdural hemorrhages in the elderly behave differently. The brain begins to atrophy in the third decade and especially in the latter decades of life.

This creates more space between the dura matter and underlying the arachnoid membrane and surface of the brain, and stretches those bridging veins, which being more fragile in later life, are much more likely to be torn and bleed in response to minor blows to the head or perhaps, no injury at all.

Many subdural bleeds in the elderly follow trivial injuries, minor enough in some cases to be forgotten.

Some happen to patients who are on anticoagulant drugs because they have atrial fibrillation or some other condition to justify taking anticoagulant or antiplatelet drugs.

Subdural clots in the elderly may be asymptomatic or cause a bewildering range of symptoms ranging from headaches to epileptic seizures, impaired balance, difficulty focusing and fluctuating drowsiness.

Oddly, speech and vision are rarely affected.

Treatment involves draining the subdural through a burr hole in the skull and sometimes including a small drain.

But sometimes they recur making it necessary to take out the connective tissue membrane.

Allan Ropper, a former colleague of mine in Boston, wrote about such case at Massachusetts General Hospital when Dr. Raymond Adams was asked to see a colleague who had become “confused and dull over several weeks.”

Ropper said Adams found grasp responses (fingers and thumb flex when the palm is stroked) and mild weakness on the left side without a Babinski response (big toe goes up when the sole of the foot is stroked).

He percussed each side of the skull, and while departing, decreed, “Chronic subdurals – they need to come out.” Adams was right (without any CT or MRI imaging) and they did.

I relate this story because Adams was one of the best neurologists in the world in the 20th century, famous for being right most of the time and an excellent clinical teacher and neuropathologist.

He was a close colleague of Miller Fisher, a Canadian born in southern Ontario, who became equally famous for his work on stroke.

Between the them, they set the standard for excellence in North America in their time.

Their standard continues in the person of Ropper, a Harvard professor of neurology and neurology editor for the New England Journal of Medicine.

Dr. William Brown is a professor of neurology at McMaster University and co-founder of the InfoHealth series at the Niagara-on-the-Lake Public Library.