While back problems account for the largest single chunk of American chronic pain, other conditions produce long-term pain problems -- periods of unremitting, often agonizing pain, resistant to most treatments and often completely disabling.

These include:

* So-called "phantom pain" felt by amputees.

* The pain some people have after a bout with the disorder shingles, which is caused by latent chicken pox herpes virus. Sometimes, long after the acute shingles episode is over, there is excruciating pain, called post-herpetic neuralgia.

* The often agonizing pain felt, usually in extremities, by diabetes patients when the illness has begun to cause nerve damage. This "diabetic neuropathy" is often manifested by what is called "burning foot" syndrome.

Similar pain can occur when permanent nerve blocks are used to deal with chronic pain. They help alleviate the pain at first, but, says National Institutes of Health pain specialist Ronald Dubner, "the live nerve that is left will start to send off its own signals and after a while, the brain sees these as pain messages, and it may get to be a worse, more constant pain than the one you started out with."

NIH is working with groups of patients with these pain disorders as well as those with back pain, headaches and the intractable pain of some cancers. Approaches include experiments on new drugs designed to work in different ways on different brain cells -- so that when one drug seems to lose its effectiveness, another can be substituted. A study for which volunteers are now being recruited will deal with chronic tension headaches using three approaches -- drug, nondrug and placebo. They are also recruiting persons needing third molar extractions. In addition, the pain specialists are probing the interactions between care-givers and patients and are studying deep muscle therapy and hypnosis. NIH Studies

Patients for pain research must be referred by their physician. Physicians should write PAIN, c/o Dr. Ronald Dubner, National Institute on Dental Research, NIH, Bethesda, Md. 20205.