"STATIC" BRAIN IMAGING
"DYNAMIC" BRAIN IMAGING
5 - HIAA EXCREATION
To get a right diagnosis disorders have to be defined reliably. The major depressive disorder is defined very precisely now. The following criteria must be present:
- A disturbance in mood characterized by depression, sadness, loss of interest or pleasure, or being "down in the dumps."
- Persistence for a two-week period of at least four of the following symptoms:
- Change in appetite or weight (poor appetite, weight loss, increased appetite, or weight gain).
- Impaired sleeping patterns (either insomnia or slipping excessively).
- Objective evidence of either extreme restlessness or extreme decrease in motor activity.
- Decreased interest or pleasure or decrease in sex drive.
- Decreased energy or a tendency to tire easily.
- Feelings of worthlessness, self-reproach, or excessive or inappropriate guilt.
- Difficulty in thinking or concentrating.
- Thoughts of death or suicide, or suicide attempts.
Laboratory tests may help illuminate the physical processes underlying the patient's symptoms. These tests are a relatively new, and none provides definitive proof for any particular diagnosis. Tests currently in use include a variety of techniques for brain imaging, the use of blood and urine measures of neuroendocrine and neurotransmitter function, and the use of the electroencephalogram to evaluate the electrical activities of the brain.
"Static" Brain Imaging
- "Static" Imaging techniques can be used to look for abnormalites only in brain structure.
The most widely used technique is computerized tomographic (CT) scanning. The brain is X-rayed, and computerized techniques are used to reconstruct relatively precise pictures of brain structures. CT does not provide a true diagnostic laboratory test but it does provide a large amount of significant information about pathological processes occurring in the brain. For example, venticular enlargement is relatively common in schizophrenia, and this fihding may confirm the diagnosis of schizophrenia.
Another imaging techniquenuclear magnetic response (NMR) scandone by nuclear magnetic resonance. Like CT it is painless. As in the case of CT scanning, computerized methods are used to reconstruct the pictures obtained. It has two main advantages over CT scanning. First of all, it produces images of extremely high resolution. Structures in the brain can be see in incredibly fine detail, with very clear differentiation between small gray and white-matter structures. The second advantage is that NMR achieves these pictures of living tissue at essentially no risk, Unlike CT scanning, which uses a small amount of radiation, NMR uses none.
- "Dynamic" Brain Imaging
- "Dynamic" brain imaging techniques permit to actually observe how the brain works when it performs certain tasks. In other words, it provides a way of measuring function rather than structure. The two commonest techiques for dynamic brain imaging are studies of regional cerebral blood flow (RCBF) and positron-emission tomographic (PET) scanning.
The technique of RCBF involves the use of radioactive tracaers, xenon-133, which are taken in brain tissue and can be used to visualize which parts of the brain are most active. The labeled xenon emits photons, which can be measured either by mapping flow on the surface of the brain or with a computer-assisted tomotgraph much like that used in CT scanning.
Patients suffering from particular kinds of mental illness have greater or lesser blood flow in particular areas of the brain than do normal people. For example, patients suffering from depression may have a decreased metabolic rate in their brains, which may be either a cause or a symptom of the disease. When the depression goes away with treatment, the metabolic rate (as reflected by cerebral blood flow) goes up. Thus, one application in this technique may be to monitor response to treatment.
PET scanning is a technique that combines the best of both worlds. It gives good resolution, much likes CT scanning, permitting the clinician to see brain structures in relatively fine detail. Futher, like RCBF, it is a dynamic technique that permits the neuroscientist to watch the brain at work and to observe which parts of it become more active in response to various kinds of stimuli.
A brain imaging procedure involves the use of positron-emitting substances that are injected into the body and taken up by the brain. The radiation that they emit is measured and used to construct a picture of the brain.
Pet scanning has a number of drawbacks. It is a very expensive (positron-emitting isotopes are produced in a cyclotron), and it does carry some risk (the injection of radioisotopes into a blood vessel).
In the future almost certainly positron-emitters will be attached to psychoactive drugs or known neurotransmitt substances and their activities traced in the brains of patients with mental illness, thereby helping us map areas of drug action and of abnormalities in neurochemical transmission.
- Dexamethasone-suppression test
- "Dexamethasone-suppression test" (DST) helps to illuminate neuroendocrine abnormalities. Many depressive patients (about 50%) produce abnormally large amount of cortisol, although it was not clear whether this was a cause or an effect of the illness. These patients fail to suppress the hypothalamic-pituitary-adrenal axis when taking dexamethasone, and DST test is abnormal. This test can be useful in clarifying the diagnosis between schizophrenia and a depression with many psychotic features. This test does seem to suggest that part of the cause of the illness may reside in the hypothalamus or the higher brain centers that govern it.
- 5-HIAA excreation test
The principal breakdown product of serotonin is 5-hydroxyindoleacetic acid (5-HIAA). Urinary 5-HIAA is a poor index of brain serotonin activity, but its presence in the spinal fluid can be used to explore the possibility of a serotonin deficit in depression. This is doing by collecting samples of spinal fluid from depressed patients through a "spinal tap". Much as in the case of norepinephrine, a subgroup of depressed patients (approximately 45 percent) have been found to have low 5-HIAA.
MHPG excreation test
MHPG excretion test is a test of neurotransmitter function. MHPG (3-methoxy-4hydroxy-phenylethylenglycol) is an important breakdown product of norepinephrine, one of the major transmitters in the central nervous system. According to "catecholamine hypothesis", depressive patients have a deficit of norepinephrine in the brain, leading to a generalized slowing and impairment of function that expresses itself as depressive symptoms. Patients with low MHPG and low norepinephrine should be placed on one particular type of antidepressant drugs that appear to stimulate the norepinephrine system in the brain. Patients with high MHPG should be given an entirely different drug because they tended to respond better to antidepressant drug that affect the serotonin system. This test has provided doctors with some information about selecting a particular type of medication.
The electrical activity of the Brain
The technique of electroencephalography (EEG), the measurement of "brain waves" is a relatively simple and painless laboratory test. The 70 - 80 percent of depressed patients tend to have abnormal EEG patterns during sleep. EEG studies of sleep in depression confirm that the depressed patient's subjective complain of insomnia is supported by objective research indicating that indeed the sleep patterns are disturbed-the depressed patient usually does not sleep as deeply as long.
1. The broken brain, Nancy C. Andersen, M.D., Ph.D., Harper and Row, Publishers, New York 1984