DEAR DR. DONOHUE: Please give me some information on sleep apnea. What are the causes, the effects, the cures? - C.C.

ANSWER: I have to know the type of sleep apnea you have. There are two forms, you know. I'll briefly discuss both.One is obstructive sleep apnea, the most common variety, in which air passages become so narrow they cannot carry air to the lungs. Loud snoring followed by brief absence of breathing (the apnea) is a cardinal sign of this along with consequent daytime fatigue. (Apnea episodes may occur by the score in a single night.)

Some people with this obstructive apnea are overweight and the overabundance of loose tissue in the back of the throat causes the obstruction by caving in the airway when lying down. If this is the setting, then losing weight helps relieve symptoms.

Some people use mechanical aids, such as CPAP (continuous positive airway pressure). It forces oxygen through the obstruction and into the lungs during sleep. Lastly, some may require surgery to clear away excess tissue; for example, the redundant tissue that can form at the back of the throat.

The other kind of sleep apnea is called central. Here, something's wrong with the brain's breathing control center. Breathing impulses misfire and the person gasps for breath. There are drugs to help such people. If you write again, be specific about the kind of sleep apnea you have, although it's a safe bet it is the obstructive kind. For more on sleep, see report No. 30. Order by writing Dr. Donohue/No.30, Box 830, Gibbstown, NJ 08027-9909, enclosing a long, stamped, self-addressed envelope and $2.

DEAR DR. DONOHUE: I have a granddaughter who has inverted nipples. My son is making a big fuss over this. She is 9. Is this a problem? Will she outgrow it? Will she be able to nurse if she has a child? I think my son is making a big fuss over nothing, but maybe he's right and I'm wrong. - C.G.

ANSWER: Most inverted nipples aren't really inverted, but buried in surrounding tissue. A bit of pressure applied to the area will evert them in such cases. Childhood pudginess might exaggerate this phenomenon.

That's fact one. Fact two is that unless normal nipples suddenly become inverted, they are not a sign of any illness. Women who have sudden inversion of nipples ought to be examined.

Fact three is that inverted nipples don't have to preclude nursing. The La Leche League offers help for such women. Special nipple shields evert the nipples for the purpose.

For peace of mind all around, have the child seen by a pediatrician and a final judgment made. It is easy to transmit parental preoccupation with such things to the child itself.

DEAR DR. DONOHUE: We were debating about rapid eye movement (REM) sleep and whether it is the deepest sleep stage. Isn't it a period of fitful sleep? - C.M.

ANSWER: We sleep in five stages, four of non-REM sleep and the REM stage. REM sleep comes within two hours of falling asleep. During it, the brain is aroused, almost as much as when awake. Eyes dart back and forth, we dream, and the heart speeds up. The period of deepest sleep is the last non-REM stage. We go through the five stages four or five times a night, the REM stage becoming longer as morning approaches.

DEAR DR. DONOHUE: I am a 21-year-old female who had post-meningitis hydrocephalus at 6 months of age. I have had a drainage shunt in place since. I am writing in response to an item you had on "normal pressure hydrocephalus." What is that? If you have normal cranial pressure, you shouldn't have hydrocephalus at all! - P.G.

ANSWER: I'll discuss your specific situation later. However, any hydrocephalus is pressure in the brain canals, which circulate irrigating fluid. So you have the right idea, and normal pressure hydrocephalus does sound like an oxymoron. But here's what happens:

Normally, the fluid I spoke of is absorbed uniformly as it circulates in brain caverns, called ventricles. Fluid then seeps outward to the brain surface and spinal cord, where special channels permit release, thus maintaining normal pressure within.

When something blocks the fluid flow, backup occurs and pressure builds, compressing brain tissue. Pressure can be relieved by a shunt, a plastic tube inserted into a ventricle. Drainage fluid is often shunted into a vein.

Sometimes, brain compression from a long ago blockage of flow can linger, producing chronic symptoms, even though the fluid pressure itself may have normalized. Somehow, lowering the normal pressure relieves the compression. The signs of brain compression subside, and I am speaking of an unsteady gait, memory loss and loss of other functions, like kidney control.

Having said all this, I hasten to add that childhood meningitis hydrocephalus has nothing to do with normal pressure hydrocephalus. Your continuing hydrocephalus and need for the shunt reflect a continuation of your childhood illness.

In fact, normal pressure hydrocephalus tends to occur in older persons, often from an unclear cause. The letter below, which came to me in the same mailing yours did, indicates successful use of the shunt procedure in normal pressure hydrocephalus.