In general, confirming a diagnosis of mania or endogenous depression is not difficult, particularly if emphasis is laid on a history of previous mood swings, a family history of the disorder, the patients psychologic constitution, and exclusion of somatic disease.
The most common diagnostic difficultly, with implications for treatment and prognosis, is distinguishing between the depressive phase of manic-depressive illness and neurotic depressive reactions.  Often, but not invariably, before a neurotic depression the patient's personality has been unstable and lacking in resilience, while manic-depressive patients tend to be warm, energetic, and outgoing.  A clearly defined loss such as bereavement, failure to achieve promotion, or marital breakdown, or merely a setting of chronic frustration and dissatisfaction usually precedes the onset of neurotic illness.  Manic-depressive episodes may occasionally be preceded by a significant life event, but they often develop for no clear reason.  The mood in neurotic depression differs only in intensity and duration from normal experience, and the reaction is understandable in the light of the circumstances that precipitated it.  The patient tends to feel worse toward evening, but may be temporarily diverted by stimulating company or events.  Anxiety, weepiness, a tendency to self-pity, and insomnia are also characteristic, but psychomotor retardation, persistent ideas of guilt, and delusional convictions are absent.
In middle-aged and elderly patients with depressive illness. Special care must be taken to exclude coexistent physical diseases, particularly carcinoma, myxedema, or cardiac failure.  Depressive states may also herald the onset of cerebral arteriosclerosis, parkinsonism, and cerebral tumor; and depression may sometimes develop in patients taking hypertensive drugs, oral contraceptives, and steroids.  Heaving drinking may lead to severe depression or be caused by it.
A severe depressive swing may be the initial symptom of schizophrenic illness, but the characteristic schizophrenic symptoms rapidly overtake the mood change.  In differentiating mania from acute schizophrenia, incoherence of thought, emotional incongruity, withdrawal from reality, and bizarre behavior favor diagnoses of schizophrenia, while heightened activity, infectious gaiety, and thought content congruent with an elevated mood suggest mania.  Mania presenting with irritability and aggressiveness, without obvious elevation of mood, and with persecutory delusions is commonly mistaken for schizophrenia.

Prognosis

The duration and course of the illness vary considerably with the individual patient.  The average duration of a manic episode is 3 to 6 mo and that of a depressive phase 6 to 9 mo.  Unipolar depressive episodes tend to be longer than bipolar depressions.  Complete recovery is the rule, but in bipolar psychoses rapidly fluctuating states of mania and depression or states of chronic depression or mania occasionally occur, practically in the elderly.  One or the other phase occurs in four fifths of the patients, and the frequency and duration of attacks increase with age.  Depression is more common than mania rarer among older patients.  One fifth of those afflicted with the disease commit suicide.  Death is also due to CVA or intercurrent disease.