Severe, debilitating depression is to as chronic, low-grade depression is to

Major depression; dysthymic disorder

Explanation:

Major depression is a common, serious, highly debilitating illness that negatively affects how an individual feels, thinks, and acts. Depression causes feelings of sadness and / or loss of interest in activities that previously brought pleasure. It can lead to a variety of emotional and physical problems and can diminish a person's ability to carry on their normal activities at work and at home. It is a very serious disease that should be treated with a health professional.

Dysthymia, in turn, is a chronic form of depression, but less severe than the better known form of the disease. With dysthymia, symptoms of depression can last a long time - often two years or more. The patient with dysthymia may lose interest in normal daily activities, feel hopeless, have low productivity, low self-esteem, and a general feeling of inadequacy. It can also be debilitating, but not as severe as major depression.

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