Depression and Comorbidity

Depression often occurs with anxiety, this is referred to as comorbidity. Comorbidity is the very common with depressive and anxiety disorders.  Such illnesses may precede the depression, cause it, and/or be a consequence of it. It is likely that the mechanics behind the intersection of depression and other illnesses (anxiety most often) differ for every person and situation. Regardless, these other co-occurring illnesses need to be diagnosed and treated. Determining which is the primary illness is very important in the treatment plan. The importance of establishing the primary illness versus the secondary illness is that if the practitioner focuses on the secondary illness such as Anxiety which might be driven by the primary illness viz. Depression; then the Depression will likely continue to cause the Anxiety attacks. 
Various Anxiety disorders, such as Post Traumatic Stress Disorder, Obsessive Compulsive Disorder, Panic Disorder, Social Phobia and Generalized Anxiety Disorder, often accompany depression. People experiencing PTSD are especially at risk to having comorbid depression. PTSD is a  condition that can result after a person experiences a terrifying event or ordeal, such as a violent assault, a natural disaster, an accident, or combat. Cormibidity is common with ADHD as well.
People with PTSD often replay the traumatic event in flashbacks, memories or nightmares. Other symptoms include irritability, hypervigilance, intense anger... A National Institute of Mental Health study found that more than 40% of those suffering from PTSD also had depression at one-month and four-month intervals after the traumatic event.
Depression is also comorbid with other serious medical illnesses such as heart disease, stroke, cancer, HIV/AIDS, Diabetes, and Parkinson’s disease. Depression is implicated in both the development and adverse outcomes of heart disease. Biologic pathways involving the sympathetic nervous system, the hypothalamic-pituitary axis, and the coagulation pathway are all implicated. Many often report severe depression after open heart surgery. Similar data seems to imply and demonstrate a link between anxiety, depression, and other common medical illnesses ranging from stroke and Parkinson disease to Irritable Bowel Syndrome, cancer, and Fibromyalgia. A recent study analyzing data from the US National Comorbidity Survey Part II concluded that anxiety disorders were positively associated with medical disorders; after adjusting for depression, substance-abuse disorders, and socio-demographics. This conclusion indicates a strong and unique association between anxiety disorders and medical disorders. Indeed, the study noted a stronger association of anxiety disorders and chronic pain syndromes than the association of depression and chronic pain. Thus, depression, anxiety, and chronic medical illness are closely related.
Separating the independent effect of depression and anxiety is often difficult given their common concurrence; unless the healthcare professional is well experienced in treating these conditions. Studies have shown that people who have depression in addition to another serious medical illness tend to have more severe symptoms of both depression and the medical illness, more difficulty adapting to their medical condition, and more medical costs than those who do not have comorbid depression. Moreover, comorbid anxiety and depression may increase greater chances of non-response to treatment, long-term poor outcome, and suicide. Research has yielded increasing evidence that treating the depression can also help improve the outcome of treating the comorbid illness. 
this article is for informational purposes only and not intended to diagnose or treat any mental illness