The nervous system has two parts: the central nervous system and the peripheral nervous system. The central nervous system is made up of the brain, where thinking takes place, and the spinal cord, which is a bundle of nerves that carries directions from and to the brain. The peripheral nervous system is composed of the nerves that bring sensory messages to the brain and deliver commands to the muscles. Both the central nervous system and the peripheral nervous system can be affected by HIV infection. The central nervous system—primarily the brain—is somewhat more likely to be affected than the peripheral nervous system, either by HIV itself or by an opportunistic infection or tumor. The most common symptoms of central nervous system involvement are (1) mental slowing, with memory loss and impaired concentration; (2) seizures; (3) weakness or paralysis; (4) poor coordination; and (5) headache that is often severe or different from the usual headache. All of these symptoms suggest infection in the brain or meninges (the membrane surrounding the brain) and require medical treatment. In many instances, the person with these symptoms will then be referred to a neurologist, a specialist in diseases of the nervous system. The most frequent and serious diseases of the central nervous system are toxoplasmic encephalitis, cryptococcal meningitis, lymphomas of the brain, and AIDS dementia complex. Other diseases also affect the central nervous system: Kaposi’s sarcoma, cytomegalovirus, progressive multifocal leukoencephalopathy, Mycobacterium avium, tuberculosis, and the herpes viruses. All these diseases cause similar symptoms. Diagnosis, therefore, requires special tests. The tests usually done begin with a neurologic examination that includes a physical examination of the nervous system to determine coordination, strength, sensations, reflexes, and mental functioning. An important laboratory test is a lumbar puncture, also called a spinal tap. The lumbar puncture is done to obtain the cerebrospinal fluid that surrounds the spinal cord and brain; the fluid is then examined for any inflammatory cells or microbes that will provide clues to the diagnosis. Other major laboratory tests are computerized tomography (CAT scan) and magnetic resonance image (MRI) of the brain. Both tests are methods of viewing the brain in three dimensions to look for specific changes. These changes indicate the location of the problem and its probable cause. Diagnosis of central nervous system problems, then, is based on the symptoms, the results of a neurologic examination, the results of examination of the cerebrospinal fluid, and any changes in the images of the brain. Many diseases of the central nervous system can be treated successfully, especially early in the course of the disease. Many of the symptoms suggesting central nervous system infections, however, occur even when there is no problem in the central nervous system at all. Weakness, seizures, and mental changes, for instance, can be caused by medications, changes in the balance of electrolytes in the blood, and fever due to some other infection. Particularly difficult to sort out are headaches: 90 percent of all people, with or without HIV infection, have periodic headaches. The final part of this section on head and nerve problems will discuss the problems HIV infection causes with the peripheral nervous system.*130\191\2*
