Somatoform disorders could be considered clusters or syndromes of physical symptoms which have a psychological cause.
Here are some examples:
1) somatization disorder -- a syndrome of multiple physical symptoms--typically pain symptoms-- which have a psychological cause
2) conversion disorder -- typically there is a complaint of paralysis or loss of sensation (including blindness) despite an absence of neurological signs; the symptoms may be generated without conscious intent, but may be profoundly disabling. With modern examination techniques and tests, these symptoms are easily demonstrated to be of non-neurological origin.
3) somatic delusions, in the context of psychotic depression or schizophreniform disorders. These have a wide variety of manifestations, though are most commonly bizarre in nature. Arguably, cases of somatization or conversion could be treated as somatic delusions.
4) somatic manifestations of anxiety -- this is extremely familiar to us all: tremor, sweating, bowel problems, etc. can all occur as a direct obvious consequence of anxiety. At times this physical component becomes the dominant feature, leading to behaviours intended to relieve the physical complaint, leading in turn to worsened avoidance, withdrawal, and exacerbation of the underlying problem.
This whole subject requires a lot of care, in my opinion. I believe that somatization is very common, and exists in a wide range of extremity--from minor symptoms to syndromes that can be almost totally disabling--yet it is also true that undiagnosed medical ailments of non-psychological origin can often be misdiagnosed as psychosomatic or somatoform. Therefore, thorough physical medical assessment and care is needed as a multidisciplinary strategy to manage these problems. These types of problems do indeed tend to be handled poorly by the conventional medical system--either through excessive and harmful medical interventions (e.g. in Munchausen's Syndrome), or through the dismissive neglect of a frustrated caregiver.
I think it is fair to say--and an observation I certainly find consistently in my experience--that physical symptoms of any cause ALWAYS have a psychological component as well. Often times, the psychological component is simple and direct: recurrent migraine headaches, malignant chronic pain, recurrent seizures, etc. (among hundreds of different causes of physical symptoms) cause a disruption to daily life & function, and their unpredictable patterns can leave one in a nearly constant state of anxiety. It can be hard to plan activities, time for relationships, work schedules, etc. when symptoms may come at any moment. So there is obvious direct psychological stress. This stress understandably can cause a feedback loop which may exacerbate the underlying medical condition.
Other times, I believe that the psychological effects of medical conditions can be more subtle or indirect. Chronic conditions can come to have a lot of power to redefine one's sense of self, often in a way which pronounces one to be more disabled than the medical problems necessitate. Some types of symptom clusters may be sufficiently common as to allow a community of fellow sufferers to form. While this may permit the supportive care of a community, it may also consolidate or entrench the aspects of the phenomenon which have to do with identity. The relief that one may find in a group of people experiencing something similar may sometimes be so compelling that entrenched factitious beliefs about disability are deepened, at the expense of therapeutic growth.
Some currently unexplained diagnostic entities, such as chronic fatigue syndrome (CFS), may in some cases be examples of complex somatoform illness. I acknowledge that in other cases--perhaps even in the majority--there may well be some as yet unexplained physical pathology driving the symptoms. A physiologic disposition towards fatigue may cause a cascade of behavioural changes (including withdrawal from activities), leading to a further cascade of cognitions about illness, mood change (which can often present itself, for many people, in a further somatized set of symptoms), and perpetuating of underlying symptoms. The worldwide network of fellow sufferers may lead to perpetuation of symptoms, rather than relief, because the group consolidates some of the beliefs and identity formation which individuals may have about the condition, and also may agitate against what is seen as a dismissive or ineffectual medical system. The group dynamics may also foster the spread of various spurious alternative therapies, whose evidence base would often consist of glowing testimonial accounts rather than careful randomized data. Factitious therapies could sometimes be quite effective for factitious illnesses, since the therapeutic effort would permit the sufferer a psychological opportunity to move away from the illness symptoms, and attribute the improvement to something external, rather than to psychological change. Such is, in my opinion, the basis for most stories of so-called "faith healing" which have been around for millenia.
It is helpful to have observed extreme examples of somatoform illness. Case examples include individuals who have had recurrent factitious seizures (pseudoseizures), often leading to dangerous and harmfully inappropriate medical interventions. Many persons with a history of pseudoseizures also have neurologically-based epilepsy as well: somatized, factitious, or conversion symptoms often co-exist with their non-psychiatric counterparts. Other case examples include situations where individuals are delusionally convinced that they are paralyzed (due to a conversion disorder) causing them to have lived in a wheelchair for years. Such individuals often have networks of people in their lives who support them in their paralyzed role; such supporters often include physicians and other caregivers. Yet, it has been an amazing experience for me to witness cases of this type--cases where there has never been any objective sign of neurologic disease, but where the impact of the problem has been extreme; if a very careful neuropsychiatric evaluation is done, with strongly structured psychiatric and rehabilitative therapy, I have seen situations where a person experiencing paralysis is able to walk home after a hospital stay.
But cases like these are inevitably complex. If a person has lived in a certain way for years, the behaviours themselves, and the associated thoughts, become integrated into identity. If you live as a paralyzed person for many years, it will not be so easy to get up and walk, even if you are neurologically healthy. There are physical barriers, but obvious psychological and social ones as well.
I believe this is a theme which epitomizes our understanding of brain function: repeated behaviour entrenches neural pathways. If "illness behaviour" exists despite "no illness", the brain learns to function "as if" a physical injury were present. It is just like language learning--with immersive experience over a course of months or years, the brain will speak the new language with ever greater fluency. It is a difficult task for the brain to "unlearn" such experience.
But this suggests a therapeutic imperative: for all cases of this type, immersive physical rehabilitation is necessary. In every single case I have ever seen of severe conversion, for example, the cure required intensive, prolonged, structured involvement of physiotherapists, in addition to whatever medications (typically antidepressants and antipsychotics) and psychotherapeutic work the person needed.
I believe this theme crosses over into the realm of ALL chronic disease, regardless of cause. Management of chronic disability or chronic diseases is greatly assisted by physical rehabilitation. In the language of narrative therapy, if we consider the illness or symptom to be like a negative character in our lives, that character is constantly telling us to do less and less--part of the therapy to challenge this is to find a structured and safe manner in which to do more and more, or to optimize our fitness so that we can do the most despite the limitations imposed by the disease.
Another interesting modality of therapy for conversion, one which can illustrate very compellingly the existence of a structure of drives and defenses first suggested by Freud, is the so-called "amytal interview." In the version I have seen, a patient with a conversion syndrome (following informed consent, of course) is given a dose of ritalin (which allows more amytal to be given without loss of consciousness), followed by intravenous sodium amytal (a barbiturate), with the supervision of an anesthetist in a well-equipped medical setting. The dose is titrated just to the point before the patient loses consciousness. The effect of the medication is to cause disinhibition. In this condition, the psychological forces necessary to continue the conversion symptom are weakened, so for example a person describing paralysis of an arm can be guided to raise the paralyzed arm in the air, and flex it, etc. This event can be videotaped. When the effects of the drug wear off, the person may not remember the scene, but when presented with the video footage (of the non-paralyzed limb in action), the person's psychological defense of conversion will be substantially weakened. As a result, often times a strong emotional reaction takes place, usually the overt emotions or affects consistent with a severe underlying depression which had previously shown itself through "paralysis." In this way, "conversion" operates as a psychological defense, a way in which the brain deals emotionally or behaviourally with a painful symptom. These defenses can be vital ways to survive in the world, but sometimes--as in conversion disorders--the defense system goes awry, and becomes the core problem.