QUESTION: Please explain orbital cellulitis, which my sister has developed in her right eye. Is it serious? Can she go blind? She is being treated by an eye specialist. Please fill me in. - Ms. H.D.

ANSWER: Before antibiotics, this was a most serious infection. Now it can, when diagnosed, be treated handily with few lasting effects."Orbital" refers to the bony socket that holds the eyeball - its orbit. Cellulitis is an infection of the cellular tissue that lines this orbit.

It may be difficult to determine the source of the infection. It can arise from nearby infected sinuses, or it can arise from bacterial invasion of the area through a scratch or bite. Vein infection in the orbit can lead to the problem. One of the potential serious complications from this orbital cellulitis is spinal meningitis. A spinal tap rules that out.

We can thank antibiotics for removing orbital cellulitis from the ranks of the serious ailments of man. In prior days it led to death or blindness in a relatively high percentage of cases.

QUESTION: When the doctor took me off Dyazide and put me on Vasotec for my high blood pressure, it made my tongue swell. I also developed headache and diarrhea and nausea. I am 76, a female, and weigh 132. Could you explain what happened? My blood pressure is now 160/88. What is Vasotec? - G.P.

ANSWER: Only a very few users of enalapril (Vasotec) have the kind of reaction you had. But it does happen.

I'm presuming you reported all this to the prescribing doctor. You shouldn't go on taking medicine that causes such dramatic symptoms. Sometimes it is the dosage that's to blame, and often an adjusted dosage is tolerated. Often, too, a patient may have such symptoms after taking the first dose of a medicine, and not thereafter.

Enalapril is a newer kind of blood pressure lowering drug. It interferes with the body's production of angiotensin, a substance that is responsible for pressure rises. Vasotec inhibits this substance, thereby falling into the ACE inhibitor category of drugs. ACE is short (very short) for angiotensin converting enzyme.

It's unfortunate that you had such a bad time with Vasotec, an excellent medicine for pressure control. Perhaps by the time you read this your body will have adjusted after that initial intolerance.

QUESTION: What is elemental calcium? - T.I.J.

ANSWER: "Elemental" roughly translates as "pure." With calcium supplements, for example, the mg number for elemental calcium tells the amount of calcium, not counting substances with which it is mixed, such as gluconate. If a package says a tablet contains, say, 500 mg of elemental calcium, you can figure that amount in as part of the daily 1000 or 1500 mgs of calcium you are trying to get.

QUESTION: I am taking a pill called a "potassium sparer." All I can tell you is that it lowers my blood pressure. Please explain. - Mrs. B.J.

ANSWER: I am sure you are referring to a kind of diuretic pill that does not cause excess urinary loss of valuable body potassium. This can happen with some of the diuretics.

An example of a potassium-sparing diuretic pill is triamterene. Others include spironolactone and amiloride. These drugs are sometimes used in conjunction with the other non-potassium-sparing drugs.

Dr. Donohue's booklet No. 23, "Osteoporosis, Prevention and Treatment, " explains this debilitating disease and what you can do about it. For a copy, write Dr. Donohue/No. 23, P.O. Box 830, Gibbstown, NJ 08027-9909. Enclose a long, self-addressed, double-stamped envelope and $2.

Dr. Donohue welcomes reader mail but regrets that, due to the tremendous volume received daily, he is unable to answer individual letters. Readers' questions are incorporated in his column whenever possible.