The recent approval of psychosomatic medicine (PM) as the newest psychiatric subspecialty by the American Board of Medical Specialties¹ was predicated, in significant part, on the public health significance of the field and on the existence of a well-developed body of knowledge to support its validity and activities. The continued growth of the evidence base is crucial to the future of the field and to improving the care of the patients who are its focus, thus leading to a reduction of the public health burden associated with their condition. In this article, we present a brief overview of the existing evidence base of PM in the context of its public health significance; we also provide some suggestions for its future development.

Examples of the evidence Because the PM evidence base is rich and varied, it cannot be fully covered in an article of this length. Nevertheless, the following examples serve to illustrate the current status and depth of the research. Psychiatric comorbidity is very common in patients with medical conditions, with a prevalence ranging from 20% to 67%. Patients in the general hospital have a higher rate of psychiatric disorders than community samples or patients in ambulatory primary care. Compared with community samples, depressive disorders in hospitalized medical patients are more than twice as common, major depression 2 to 3 times as common, substance abuse 2 to 3 times as common, and somatization disorder more than 10 times as common. Delirium occurs in 18% of hospitalized patients, a rate much higher than one would expect in the community. Similarly increased rates are seen in primary care and long-term care.

Psychiatric comorbidity has serious effects on physically ill patients and is often a risk factor for their medical conditions. It is well established that depression is both a risk factor and a poor prognostic factor in coronary artery disease. Psychiatric illness worsens cardiac morbidity and mortality in patients with a history of myocardial infarction, diminishes glycemic control in patients with diabetes, and decreases return to functioning in patients who have had a stroke. Mood and anxiety disorders compound the disability associated with stroke. In the context of neurodegenerative disease (eg, Parkinson or Alzheimer disease), depression, psychosis, and behavioral disturbances are significant predictors of functional decline, institutionalization, and caregiver burden.

Hospitalized patients with delirium are significantly less likely to improve in function than patients without delirium. Delirium is associated with worse outcomes after surgery, even after controlling for severity of physical illness.¹ Dialysis patients with psychiatric conditions experience more difficulties in adjusting to dialysis, such as placement of the shunt, dependence on a machine, multiple needle-sticks, and accepting the reality of blood circulating outside their bodies. Psychopathology adversely affects patients during the long wait for an organ transplant. After organ transplantation, relatively minor disruptions in compliance with immunosuppressant medication such as may be associated with depression, may result in graft rejection and death.

In addition, depression and other psychiatric disorders significantly impact quality of life and the ability of patients to adhere to treatment regiregimens. For example, psychiatric disorders are linked to nonadherence with antiretroviral therapy, adversely affecting the survival of patients with HIV infection. Psychiatric disorders worsen the prognosis and quality of life of patients with cancer. Psychiatric disorders are also linked to nonadherence to safe sex practices and to the use of sterile needles in HIV-infected injection drug users, leading to major public health implications.

With regard to clinical services, failure to identify, evaluate, diagnose, treat, or achieve symptom resolution of psychiatric morbidity in medical care settings results in significant adverse outcomes. Depression, dementia, and delirium,³ common psychiatric disorders found in hospitalized medical patients, are associated with higher medical care use, both in the hospital (more than 30% longer duration of hospitalization) and after discharge.

Although it is the treatment of choice, fewer than 1 in 5 patients with agitated delirium receive neuroleptic treatment in the hospital. When delirium is unrecognized and untreated in the hospital, patients are needlessly placed in nursing homes instead of being sent home.

Physically ill patients with depression whose condition is not diagnosed and who therefore do not start treatment while hospitalized have only an 11% chance of receiving treatment for depression in the next year. Untreated depression is associated with a higher need for medical services after hospital discharge and with higher morbidity and mortality in coronary disease, hypertension, diabetes, and stroke.²

Based on existing research findings, the presence of co-occurring physical and psychiatric disorders can be explained by an adverse bidirectional interaction.³ This model proposes that the seemingly disparate medical and psychiatric disease states are, in fact, integrally related in their causation and pathophysiology and, ultimately, should be treated together. Underlying risk factors, including genetic vulnerability, childhood adversity, maladaptive attachment, and adverse life events are associated with depression and biobehavioral risk of chronic disease. A recent editorial summarizing findings of a conference on depression in medical illness called for more research "to understand this bidirectional relationship and identify possible common pathogenic, mechanistic pathways that link depression and serious medical illness."⁴