A significant number (up to 2 percent of patient of a dermatological hospital) have impairments caused by chronic itching and skin picking (psychogenic excoriation). This condition is not yet accepted as a psychiatric disorder according to the DSM, but can often be associated to other psychiatric disorders.
It may be related to problems of impulse control and obsessive traits because of the repetetive behaviour and the subjective feelings of reduced tension. So there are similarities to trichtotillomania. The main problem for people with this symptoms is that the identify some sort of surface irregularities of their skin. The bad habit of picking or manipulation of the skin will cause additional pruritus (itching) and the skin surface has less chance of recovery. So the tendency to manipulate will increase.
Most of the patient use their fingernails for scratching or picking.
There are some psychiatric disorders that seem to be more or less related to these kind of problems. It is a spectrum of disorders, so you cannot identify one single diagnosis but a cluster of comorbid problems in most cases.
1. Obsessive compulsive spectrum disorders
One of the problems of psychogenic or neurotic itching is that you cannot stop this odd habit. You know that you should, but you cannot resist. For some people this would be rather similiar to obsessive compulsive syndroms. If the patient feels relief after the itching but would get anxious and nervous or have some obsessive thoughts related to this habit, we would consider a problem related to ocd.
2. Body dsymorphic disorder
Some patients have a wrong self-immage and think they have a problem associated to appearance. They may think their nose is to ugly or any other part of the human body. Even if anyone else would not see any special problem they would still believe they should change for a different look. This may also cause problems related to chronic itching.
For some people with ADHD or related disorders the itching and scratching of the skin is a way of self-stimulation. The pain helps to activate the brain and to stay focussed for a while. Many people with ADHD are not aware of these habits.
4. Psychotic disorders
This is a rare condition. But for some people itching or sensations like small animals on / under your skind my be closely related to psychotic states.
Therapy will depend on the orign of the problem. There are some rather good experiences with the use of SSRI (e.g. fluvoxamine) for this kind of disorder spectrum. The good news is that SSRI do work for Obsessive spectrum disorders, Body dysmorphic conditions and depression. So there is a rather realistic chance that this kind of medication would have a positive influence on the severity of symptoms.