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Most trials are comparisons of one drug versus no drugs, one drug versus another drug, or one drug given in different doses. Participants in a clinical trial usually have to meet a specified requirement—they must have a certain CD4 count, for instance, or a certain opportunistic infection.     These requirements are adhered to strictly. Though such strictness is frustrating to both the investigator and the potential participant, it is necessary for the trial to be a valid test of a drug and for the FDA to approve the drug.     Once in the trial, participants are assigned to groups called treatment arms. What the treatment arms are depend on what the trial is testing: one treatment arm might be high doses of a drug and another arm low doses; one arm might be one drug and the other arm another drug; one arm might be one drug and another arm no drug at all. In trials involving two or more arms, the participant is assigned to a treatment arm randomly, like flipping a coin. Random assignment is a process over which neither the participant nor the investigator has any control. This is necessary if the trial is to be scientifically credible.     The purpose of a trial is to compare one treatment arm against another. Some trials, called placebo control trials, compare a drug to a placebo, a pill that has no effect on the body. Placebo trials are done because people taking any pill, including a placebo, feel better not necessarily because of the physiological effect of the drug, but because of the psychological effect of taking a drug that might help. For example, 60 percent of the people treated with any drug, placebo or not, for arthritis claim that it improves their symptoms. The necessity for a placebo trial is less when the effects being evaluated are objective: weight, blood counts, tests for the virus, or frequency of opportunistic infections. Some effects, however, are more subjective and cannot easily be measured by scientific yardsticks: the sense of well-being, the level of fatigue, the number of headaches. For these more subjective effects, a placebo control trial is more important. In most trials, the results of a trial are evaluated in terms of both subjective and objective effects.     It should be emphasized that the rule of clinical trials is that, if a drug is known to work, no treatment can be a placebo; in that case, the drug known to work becomes the standard for comparison. Furthermore, once the trial shows a clear benefit, the trial must stop. Thus, in the AZT trials, when the analysis of data in September 1986 showed nineteen deaths in the placebo arm and only one death in the arm receiving AZT, the trial was promptly stopped and everyone was given AZT. Assurance on this point is a matter of medical ethics.     The best way to make the comparison in any trial is to double-blind. Double-blinding means that neither investigator nor participant knows which drug or which dose the participant is receiving. Since both investigator and participant are likely to have biases, double-blinding ensures that results will be evaluated objectively.     Although double-blinding is preferred, in some instances it is simply not realistic. For example, when the trial is to find out which way to administer the drug, by pill or by vein, proper double-blinding would have one treatment arm receive the drug by vein and a placebo by pill, and the other treatment arm receive the drug by pill and placebo by vein. But receiving a drug intravenously is inconvenient and can be risky, and it might be inappropriate for participants to receive placebo by vein simply to maintain the blind.     Not all trials are comparisons: some trials, called pilot trials (like pilot TV shows), simply gather enough background information to see if a larger trial would be justified. Other trials compare a new drug to an old drug tested previously—called a historical control. Many times, in an effort to gather additional information, the drug is just given with no second arm for comparison.*185\191\2*

HIV: HEAD AND NERVE PROBLEMS-HEADACHES: LYMPHOMA AND CRYPTOCOCCAL MENINGITISAnother common cause of focal neurologic problems in people with HIV infection is lymphoma. Lymphoma is a tumor of lymph cells that occurs in 1 to 3 percent of people with HIV infection. Almost any part of the body can be affected by lymphoma, but the brain is one of the parts most commonly affected.     Lymphoma is often suspected if treatment for suspected toxoplasmic encephalitis is ineffective. Lymphoma can be diagnosed by taking a small sample of brain tissue, a test called a brain biopsy. Treatment is with radiation and the chemotherapy drugs used to treat cancers.     Cryptococcal meningitis-Meningitis means inflammation (it is) of the meninges, the fibrous membrane that surrounds the brain and spinal cord. The symptoms of meningitis are usually fever and stiff neck; other symptoms can include seizures and double vision.      Cryptococcal meningitis is caused by a fungus called Cryptococcus that is found throughout the world and is transmitted when the fungus is inhaled. The infection itself cannot be transmitted from one person to another. Cryptococcus usually causes either a trivial disease or no disease at all until the immune system is weakened: it is the most common cause of meningitis in people with HIV infection. Cryptococcal meningitis is both serious and treatable, so it is important to see a physician as soon as symptoms appear.     The test for cryptococcal meningitis is a spinal tap. A spinal tap is done so that a sample of the cerebrospinal fluid can be examined for evidence of inflammation and for Cryptococcus. Treatment usually consists of the antibiotics amphotericin B (given by vein) or fluconazole (given by vein or by mouth) or a combination of both. Treatment is usually successful, but the infection tends to recur when treatment is discontinued. For this reason, treatment is usually continued for extended periods.*133\191\2*