DEAR DR. DONOHUE: I am 73 years old and started having kidney stones 47 years ago. I have had five attacks. A week ago, I started having pains in my side, and the CT scan showed stones in both kidneys. Do you have any idea why kidney stones develop? – C.M.

ANSWER: Kidney stones form when the concentration of certain minerals in the urine becomes so great that they don’t stay dissolved. It’s very much like putting too much sugar in a cup of coffee. No matter how vigorously you stir, you can’t get all the sugar dissolved; some settles in the bottom of the cup. In the kidney, the minerals that settle out stick together to form a stone.

You’re one of those people who has a propensity to form stones. It often is a genetic thing. Preventing stone formation centers on drinking a great deal of fluid to keep your urine dilute. You need to drink 2-3 quarts of fluid a day. Fluid doesn’t mean just water. All fluids count. You can tell if you’re drinking enough if your urine is clear or pale yellow.

Ask your doctor what kind of kidney stones you had. Calcium oxalate is the most common kind, and you’d think that avoiding calcium would prevent that sort of stone formation. It doesn’t. You do, however, have to limit the amount of meat you eat, and you have to go easy with salt and sugar. They promote calcium oxalate stones.

A repeat stone former should be investigated for conditions that foster stone production. An example is parathyroid hormone excess. The parathyroid glands are four small glands that lie behind the thyroid gland in the lower neck. They regulate blood calcium levels. If they’re making too much of their hormone, stones form.

DEAR DR. DONOHUE: I’m writing in hope that you will discuss how Coumadin can cause problems if people bump their head. My husband had to have surgery after he hit his head on a beam in our crawlspace. Unbeknownst to us, he began bleeding around his brain. As blood filled this area, his brain was pushed to one side, and this caused strokelike symptoms and three seizures. He was saved in surgery by a wonderful neurosurgeon.

Please warn Coumadin users that it’s not just cuts they have to worry about. – A.S.

ANSWER: Am I right in saying your husband had a subdural hematoma? A hematoma is a collection of blood, usually clotted blood. “Subdural” denotes the space under the outermost brain covering, the dura. Veins in that area are fragile, and it’s a place where bleeding is common after head trauma, even minimal trauma, especially in older people. The hematoma presses on the brain and causes symptoms similar to those of a stroke. Headaches and personality changes are other signs of a subdural hematoma.

Coumadin is a blood thinner that many people must take. It puts people at risk for bleeding, and subdural bleeding is among those risks.

I’m happy to hear that your husband made it through this unhappy experience. Thanks for alerting people to the danger.

DEAR DR. DONOHUE: How do the makers of flu vaccine know what virus to put in the vaccine each year? It changes, doesn’t it? – R.M.

ANSWER: The flu virus undergoes yearly changes. Vaccine manufacturers in the spring or early summer sample people in parts of the world where flu virus is prevalent and base their formula for this year’s vaccine on that information. They’re often right on the mark. If they miss a little, the vaccine is still effective.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475. Readers may also order health newsletters from www.rbmamall.com