Major Depressive Disorder (MDD), colloquially referred to as "clinical depression" or just "depression" or "unipolar depression", is a mental disorder that affects between 7.8 to 10.4% of the U.S. adult population each year and between 16.9 to 20.6% of the U.S. adult population across the lifetime. ­Major Depressive Disorder, as defined in the Diagnostic and Statistical Manual of Mental Disorders 5th Edition (DSM 5) shows considerable phenotypic variation, due possibly to the presence of other, co-occuring conditions. Thus, as defined presently, it is likely a heterogenous group of conditions that is characterized by depressed mood, decreased motivation, changes in energy level, reduced ability to have pleasure, sleep disturbances, appetitive disturbances, hopelessness, suicidality and cognitive changes (e.g. preoccupying, negative ruminations, decreased focus/concentration and short-term memory impairments). Different forms of depression may present with a different constellation of these symptoms and may also include significant anxiety, obsessive ruminations and even psychotic features.

As the name implies, Persistent Depressive Disorder (PDD) is a depressive disorder that is characterized by its chronicity. Individuals may suffer from "low grade" chronic depression (previously called Dysthymic Disorder) or a more "high grade" chronic Major Depressive Disorder. They need to be symptomatic for at least 2 years continuously to meet diagnostic criteria and not have features of Bipolar Disorder. Initial treatment approaches for PDD and MDD are nearly identical. However, one particular form of psychotherapy, known as Cognitive Behavioral Assessment of Symptoms (CBAS), has demonstrated efficacy in chronic depression.

Premenstrual Dysphoric Disorder (PMDD) is a hormonally-mediated mood disorder that affects 3-8% of of women during their menstrual years. In order to meet diagnostic criteria for PMDD, a woman must experience a majority of menstrual cycles with luteal phase mood symptoms that are severe enough to cause functional impairments or clinically significant distress. These symptoms then improve significantly with menstruation.

The mood symptoms of PMDD can include any of the following features:

In addition to these mood symptoms, women with PMDD also experience other accompanying symptoms, including:

The best-studied and most effective treatments for PMDD are certain selective serotonin receptor inhibitor (SSRI) antidepressants. FDA-approved antidepressants for PMD include fluoxetine, sertraline, and paroxetine. However, data also support the off-label use of other SSRIs, such as citalopram and escitalopram and one serotonin-norepinephrine reuptake inhibitor (SNRI): venlafaxine.

Bipolar Disorders are a group of mood disorders that collectively affect almost 5% of U.S. adults over the lifetime. In addition to the presence of depression (Major Depressive Episode), Bipolar Disorders are also defined by the presence of abnormally elevated mood states referred to as hypomania (Hypomanic Episode) or mania (Manic Episode). Individuals with Bipolar Disorders may also experience a mood state known as a Mixed Episode, in which features of depression and hypomania/mania co-occur.

Hypomanic Episodes and Manic Episodes, themselves, can come in different "flavors", with some individuals exhibiting prominent euphoria, unbridled optimism and excitability and others extreme irritability and aggression.

Distinguishing between Bipolar Disorders and unipolar disorder (or Major Depressive Disorder) is among the most important decisions a psychiatrist faces in the evaluation of mood disorders, as the evidence-based treatment of these conditions has diverged significantly.

Conventional antidepressants, for example, are discouraged in the treatment of Bipolar Disorders, as they appear to be ineffective and have the potential to destabilize mood in Bipolar Disorders. Instead, mood stabilizing medications such as lamotrigine, lithium, sodium valproate, carbamazepine and oxcarbazepine are favored with episodic - or sometimes, chronic - treatment with atypical antipsychotic medications that are widely effective for mania and sometimes depression (e.g. quetiapine, lurasidone, cariprazine and olanzapine/fluoxetine combination).

As the name implies, Seasonal Affective Disorder, now called Major Depressive Disorder with Seasonal Pattern is a mood disorder that is characterized by seasonal onset or worsening of depressive symptoms. The most common variant appears to be fall/winter-onset depression. Moreover, the particular phenotype of depression that occurs in Seasonal Affective Disorder tends to be characterized by hypersomnia (increased sleep need), hyperphagia (increased appetite), and leaden paralysis (heaviness in limbs). Typically, mood and energy states are both low in this variation of depression.

That this variant of depression is somehow related to the quantity of daylight was suggested by a researcher’s personal experience of worsening depression in the winter months in New York City, after moving from South Africa for residency training, and subsequent epidemiological data that suggested that the condition was uncommon in regions of the world near the equator and more prevalent in regions above and below 30 degrees latitude. This hypothesis led to the development of bright light therapy, which is an evidence-based treatment for Major Depressive Disorder with Seasonal Pattern. (As it happens, bright light therapy also appears to be effective for non-seasonal Major Depressive Disorder and Bipolar Depression).

The peak burden of winter-onset depression tends to be in the months of January and February.

In the U.S., the annual prevalence of MDD with Seasonal Pattern is 5%, but varies significantly by geographic region, with northern latitudes in the U.S. having a higher prevalence. Individuals with other mood disorders have a higher likelihood of having seasonal depression, with individuals with Bipolar II Disorder being particularly vulnerable to seasonality.

Generalized Anxiety Disorder (GAD) is an anxiety disorder that affects 0.9 to 2.7% of the U.S. adult population on an annual basis and has a lifetime prevalence in U.S. adults of 5.7 to 6.2%. It is characterized by excessive, difficult-to-control worrying, inability to relax, physical tension, irritability, concentration impairments, fatigue and episodic sleep disturbance. This condition is highly comorbid with Major Depressive Disorder (MDD), with approximately 43-60% of patients with GAD also meeting criteria for MDD across the lifetime.

Evidence-based treatments for GAD include Cognitive Behavioral Therapy (CBT) and various antidepressants in the SSRI and SNRI classes. The formally FDA-approved medications for GAD are the SSRIs paroxetine and escitalopram and the SNRIs venlafaxine and duloxetine. Buspirone, an older medication, may also be helpful in some patients with GAD. Moreover, the anticonvulsant medication pregabalin has efficacy data in GAD and can be used adjunctively with one of the FDA-approved medications or on its own.

Social anxiety disorder is the most prevalent anxiety disorder in the U.S. after Specific Phobias, affecting 7.1 to 7.9% of the U.S. adult population on an annual basis and 12.1 to 13% of the U.S. adult population over the lifetime. As the name implies, it is characterized by anxiety symptoms related to social situations or in anticipation of social situations. Individuals who suffer from Social Anxiety Disorder experience other human beings and social interactions as threatening to varying degrees. Trait rejection sensitivity likely plays a role in Social Anxiety Disorder, as it does in Borderline Personality Disorder, depression and Body Dysmorphic Disorder. Most often, individuals with Social Anxiety Disorder anticipate negative judgement (appraisal) by others. In severe cases, individuals avoid social/performance situations to such an extent that it has negative repercussions in their personal and professional lives.

Like GAD, social anxiety disorder can be treated with Cognitive Behavioral Therapy (CBT) and certain antidepressant medications. FDA-approved medications for Social Anxiety Disorder include 2 SSRIs, paroxetine and sertraline, and 1 SNRI, extended release venlafaxine. Off-label treatment with pregabalin and gabapentin can be considered for individuals who do not respond adequately to first-line pharmacotherapy options. Monamine oxidase inhibitor (MAOI) antidepressants, an older class of antidepressants, can be tried in more treatment-resistant cases of Social Anxiety Disorder.

Panic Disorder is an anxiety disorder that is characterized by recurrent, unexpected (not situationally bound) panic attacks accompanied by anticipatory anxiety about having panic attacks. The past year prevalence of Panic Disorder is between 1.9 to 2.7% of the U.S. adult population, with a lifetime prevalence in U.S adults of 4.7 to 5.2%.

A panic attack is a sudden bout of anxiety that typically peaks within 10 minutes and is accompanied by a variety of physical symptoms. These can include racing heartbeat, pounding heartbeat, shortness of breath/shallow breathing, shaking, sweating, tingling, lightheadedness, dizziness and physical discomfort that seems to be situated in the abdomen. People who experience a panic attack often feel like they are going to die or are having a heart attack. They may also fear that they are going to pass out (faint).

Panic attacks are highly distressing, particularly the first few times someone experiences them. It is not uncommon, for example, for individuals to go to the emergency department when they are experiencing such symptoms for the first time. Typically, the emergency department work up is negative for any cardiac or respiratory cause of the symptoms.

While panic attacks can occur in other anxiety disorders (e.g. Social Anxiety Disorder, Generalized Anxiety Disorder and Specific Phobias), the recurrent, unexpected nature of them accompanied by anticipatory anxiety about having panic attacks characterizes Panic Disorder.

Some individuals with Panic Disorder will have accompanying Agoraphobia (fear of public spaces). Most commonly, individuals fear that they will have an unexpected panic attack in public, away from the relative safety of their home, causing embarrassment or other emotional distress.

Evidence-based treatment for Panic Disorder include certain antidepressant medications in the SSRI and SNRI classes and Cognitive Behavioral Therapy (CBT). The formally FDA-approved antidepressants for Panic Disorder include fluoxetine, paroxetine, sertraline and extended release venlafaxine. The benzodiazepine medications, alprazolam and clonazepam, are also FDA-approved for the treatment of Panic Disorder. These are typically used for short-term treatment, to reduce acute emotional distress while the individual waits for antidepressant medication or Cognitive Behavioral Therapy to take effect.

As the name implies, Specific Phobias are characterized by phobic reactions to specific objects or situations, avoidance of those objects or situations, and clinically significant distress or functional impairments in sufferers. Moreover, Specific Phobias tend to be highly persistent. As a group, they are the most common anxiety disorder among U.S. adults, with annual prevalence estimates of between 9.1 to 16.3% and lifetime prevalence estimates of between 12.5 to 13.8%.

Fear of animals and fear of heights (Acrophobia) appear to be the most common Specific Phobias, followed by fear of closed spaces (Claustrophobia) and fear of flying (Aerophobia). Individuals with Specific Phobias may experience situationally-bound panic attacks when confronted by their trigger.

The most effective treatment for Specific Phobias is exposure therapy, a component of Cognitive Behavioral Therapy (CBT). Sometimes, benzodiazepines are prescribed as an adjunct to CBT for Specific Phobias, but there is some controversy regarding the potential for benzodiazepines to interfere with fear extinction, particularly when given before a course of CBT.