What Is Comorbid?

The word "comorbid" is used to describe secondary or tertiary illnesses or disorders that exist in a person in addition to a primary illness. For example, drug addiction is often said to be comorbid with other mental illnesses. These secondary or tertiary illnesses are believed to interact with each other and the primary illness, and are thought to have an effect on the prognosis or course of each condition, according to the National Institute on Drug Abuse. Comorbidity is not to be confused with multimorbidity, which refers to multiple illnesses existing in the same person without a primary, or "index" disorder.

Models of Comorbidity

• There are several different ways in which multiple disorders may be considered comorbid. The simplest is direct causation, in which one disease causes the other. In the associated risk factor model, the risk factors for the two diseases are closely related -- for example, smoking and alcohol abuse may cause emphysema and liver cirrhosis. In the heterogeneous model, the risk factors for the two diseases are not related to each other, but either risk factor could be related to either disease -- for example, smoking and age being present in a person with lung cancer and ischemic heart disease. The final model of comorbidity is independence, in which two secondary diseases turn out to be symptoms of a third disease -- for example, tension headaches and hypertension being symptoms of pheochromocytoma.

Charlson Index

• The Charlson index is used to predict the likelihood of one-year mortality (that is, death within one year) in a patient with comorbid disorders. It uses 19 categories of comorbidity to calculate a patient's score; the higher the score, the more likely it is that the patient will not survive the year. Though the Charlson index is the more widely used comorbidity measurement tool, many health providers and researchers are now using the Elixhauser index, which measures comorbidity using 30 conditions.

Mental Illness

• The National Institute on Drug Abuse estimates that 45 percent of American prison inmates have a mental illness comorbid with drug addiction.

  One of the most common types of comorbidity is that between drug addiction and other mental disorders. In some users, drug use uncovers latent symptoms of mental illnesses such as schizophrenia, bipolar disorder and depression. Others become addicted to drugs while self-medicating a mental illness. For example, there is a strong connection between schizophrenia and nicotine addiction. Research has shown that the particular brain abnormalities associated with schizophrenia increase the rewarding effects (for example, the release of dopamine) associated with nicotine. In addition, drug addiction and mental disorders are often related to the same underlying factors, such as genetic vulnerabilities and early childhood trauma.

Treatment

• Though many medications have been proven effective in the treatment of opioid, nicotine and alcohol addiction, there has been little research into these medications as treatments for comorbid mental illnesses. Many drug users with comorbid conditions have reached breakthroughs with behavioral therapies such as dialectical behavior therapy, assertive community treatment and even exposure therapy. Since physical comorbid conditions often share aspects of disease management, they are often treated together by specialists. For example, a patient with diabetes and ischemic heart disease as well as various cardiovascular risk factors such as hypertension will be treated primarily at a cardiovascular clinic.