5 anxiety disorders

PATHOPHYSIOLOGY: STUDY OF WHAT GOES WRONG 21transmitters. Examples of this problem include the abnormal movements seenin Parkinson’s disease and Huntington’s disease. Treatment with certain antipsychoticmedications may also produce abnormal movements by causingimbalanced neurotransmission in this same circuit.2.3.2 Why It Goes WrongIf it has been difficult to prove that abnormalities of neurotransmission cause certainmental illnesses, you can imagine how difficult it is to show what causes suchabnormalities to occur in the first place. However, we have made some progress inthis area.Genetic Factors. Some mental illnesses are well known to run in families, whichof course raises the question of genetic inheritance. For many major mental disordersincluding bipolar disorder and schizophrenia, twin studies have been particularlyrevealing. For example, if an identical twin has schizophrenia, the chancethat the other identical twin also develops schizophrenia is 50%. This is known asthe concordance rate for the illness. Studies with identical twins adopted into differentfamilies confirm that there is a genetic contribution to certain mental illnessessuch as bipolar disorder, schizophrenia, and depression. Many medical illnessessuch as cystic fibrosis, hemophilia, and sickle cell disease are the direct result of asingle abnormal gene. Except for Huntington’s disease, major psychiatric illnessescannot be explained in this simple fashion.Nevertheless, there is strong evidence that some mental illnesses are partlyinherited. One prevailing theory is the “two hit hypothesis.” The first of these twohits is a genetic trait that leaves one vulnerable to the illness. The second hit is somestressful life event or environmental insult (e.g., infection or toxic exposure) thattriggers the onset of the illness in the vulnerable individual.Brain Deterioration (Neurodegeneration). Other mental illnesses are calledneurodegenerative because they result from the death of nerve cells in the brain.Neurodegenerative illnesses steadily worsen over time as more and more nerve cellsprogressively die. These illnesses are most common in the elderly, and the bestknownexamples are Alzheimer’s disease and Parkinson’s disease, though othersincluding Huntington’s disease and Pick’s disease have been described. However,neurodegenerative diseases can strike at any time. For example, Tay–Sachs diseaseis a relatively rare inherited neurodegenerative disease that most commonly affectsJews of eastern European descent (Ashkenazi Jews). The illness strikes duringinfancy and quickly leads to death. There are also neurodegenerative disorders ofthe spinal cord such as amyotrophic lateral sclerosis (ALS) that result in progressiveparalysis and ultimately death.Abnormal Brain Development. In a certain sense, neurodegenerative disordersreside at the opposite end of the spectrum from the so-called neurodevelopmentaldiseases. These result from abnormal development of brain circuitry. The causes of