QUESTION: My daughter was diagnosed as having eosinophilic fasciitis. She is on prednisone and penicillamine. Could you please give us any information about this illness? We are going crazy watching her go downhill. She is going through physical therapy. Our doctor is giving us great help through this ordeal, but we would just like to hear what you have to say about this. - M.S.A.

ANSWER: Fascia (FASH-ee-uh) are sheets of tissue deep under the skin that wrap around muscles and body organs, packing and protecting them. Fasciitis is inflammation of that material. The eosinophilic kind involves an abnormal rise in certain kinds of white blood cells, called eosinophils.We cannot say why this rise occurs along with the inflammation. It is a strange combination of events. Often, the person can recall an episode of very strenuous exercise preceding the onset. Symptoms include a swelling of the skin and a tenderness.

I hope that this aggravation is merely a memory for your daughter soon. She is getting the accepted treatment - the prednisone and penicillamine (no relation to penicillin, incidentally). All this must be upsetting, but you can take some comfort in the thought that eosinophilic fasciitis usually goes away. That, I must add, can take one to three years. Tell your daughter to continue with the physical therapy, another important component of treatment.

QUESTION: My husband has emphysema. He is discouraged when he reads that the damage to the lungs it causes cannot be repaired. This idea seems to cancel out all the doctor's urging for him to exercise, etc. Can you clarify this? - Mrs. J.M.

ANSWER: I have also made the observation here that the damage caused to the tiny air passages in emphysema cannot be repaired. That is true. However, the point should also be made clearly that there is rationale behind exercise and giving up damaging practices like smoking.

The exercise gives the breathing muscles a chance to improve, to take advantage of the remaining lung function. Similarly, quitting cigarettes removes a major roadblock to getting the most out of remaining lung capacity. I am sending him a copy of the emphysema-bronchitis material. Other readers can order by writing Dr. Donohue/No.10, Box 830, Gibbstown, NJ 08027-9909, enclosing a long, stamped, self-addressed envelope and $2.

QUESTION: Why do you hear so much about potassium these days? I notice you refer to it constantly. - H.H.

ANSWER: Yes, potassium is becoming a regular subject in most medical columns. Why? It's not because we've just discovered it. You can blame the widespread use of diuretics, as in treatment for high blood pressure. Many diuretics cause a depletion of body potassium.

Potassium is a very important mineral. Without it, nerves cannot fire, muscles cannot contract and the heart cannot maintain normal regularity of action. No wonder we get so much mail on the topic. If one is not on diuretics, a deficiency is most unlikely to occur, since the mineral occurs widely in most foods.

QUESTION: Please discuss rotator cuff injury and what is done for it if it's a tear. Do you need surgery? - N.N.

ANSWER: The rotator cuff truly is a kind of cuff affair, a sleeve comprising several back muscle tendons that pass over the shoulder on their way to insertion in the upper arm bone.

Although we hear the term, rotator cuff tear, the injury often is not a tear at all, but a swelling and bleeding resulting from overuse. You can readily understand why many baseball pitchers find rotator cuff injuries the bane of their existence.

You must get rid of the initial swelling and bleeding, for if you don't then a state of scar formation and chronic inflammation sets in. The usual treatment is rest, physical therapy, and anti-inflammatory drugs, like aspirin, as needed. You always walk a narrow line in rehabilitation from such an injury. Complete rest can lead to a frozen shoulder, while too much exercise can compound the unhealed injury. You really need some professional therapy to guide you back to shoulder use.

Now, in the event the injury is really a tear of the cuff, the same basic treatment holds, with surgery the treatment of last choice, and that is seldom needed to repair the damage.

FOR MRS. L.: Yes, at most 30 percent of calories should be from fat. That means that with your 2,000-calorie daily intake, about 600 at most should be fat calories. One gram of fat gives 9 calories, therefore 600 calories of fat amount to 67 grams of it. There are 30 grams in an ounce, so your daily fat intake of 67 grams is a little over two ounces.

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.