Nail biting

Onychophagia (also onychophagy) or nail biting, is an oral compulsive habit (sometimes described as a parafunctional activity; the common use of the mouth for an activity other than speaking, eating or drinking) in children and adults.

Classification
Nail biting is considered an impulse control disorder in the DSM-IV-R, and is classified under obsessive-compulsive and related disorders in the DSM-5. The ICD-10 classifies it as "other specified behavioral and emotional disorders with onset usually occurring in childhood and adolescence." Nevertheless, the frontier between normal and a pathological nail biting is not clear

Signs and symptoms
Nail biting usually leads to deleterious effects in fingers, but also mouth and more generally the digestive system. These consequences are directly derived from the physical damage of biting or from the hands becoming an infection vector. Moreover, it can also have a social impact. The ten fingernails are usually equally bitten to approximately the same degree. Biting nails can lead to broken skin on the cuticle. When cuticles are improperly removed, they are susceptible to microbial and viral infections such as paronychia. Saliva may then redden and infect the skin. In rare cases, fingernails may become severely deformed after years of nail biting due to the destruction of the nail bed.

Nail biting is also related to oral problems, such as gingival injury, and malocclusion of the anterior teeth. It can also transfer pinworms or bacteria buried under the surface of the nail from the anus region to the mouth. When the bitten-off nails are swallowed, stomach problems can develop.

Nail-biting is also associated to guilt and shame feelings in the nail biter, a reduced quality of life, and increased stigmatization in the inner family circles or at a more societal level.

Related disorders
Other body-focused repetitive behaviors include dermatillomania (skin picking), dermatophagia (skin biting), and trichotillomania (the urge to pull out hair), and all of them tend to coexist with nail biting. As an oral parafunctional activity, it is also associated with bruxism (tooth clenching and grinding), and other habits such as pen chewing and cheek biting.

In children nail biting most typically co-occurs with attention deficit hyperactivity disorder (75% of nail biting cases in a study), but also with a plethora of other psychiatric disorders including oppositional defiant disorder (36%), and separation anxiety disorder (21%). It is also more common among children and adolescents with obsessive–compulsive disorder. Nail biting appeared in a study to be more common in men with eating disorders than those without them.

Treatment
The most common treatment, which is cheap and widely available, is to apply a clear, bitter-tasting nail polish to the nails. Normally denatonium benzoate is used, the most bitter chemical compound known. The bitter flavor discourages the nail-biting habit.

Behavioral therapy is beneficial when simpler measures are not effective. Habit Reversal Training (HRT), which seeks to unlearn the habit of nail biting and possibly replace it with a more constructive habit, has shown its effectiveness versus placebo in children and adults. A study in children showed that results with HRT were superior to either no treatment at all or the manipulation of objects as an alternative behavior, which is another possible approach to treatment. In addition to HRT, stimulus control therapy is used to both identify and then eliminate the stimulus that frequently triggers biting urges. Other behavioral techniques that have been investigated with preliminary positive results are self-help techniques, and the use of wristbands as non-removable reminders.

Evidence on the efficacy of drugs is very limited and they are not routinely used. A small double-blind randomized clinical trial in children and adolescents indicated that N-acetylcysteine, a glutathione and glutamate modulator, could be more effective than placebo in decreasing the nail-biting behavior, albeit it was only useful in the short term.

Finally nail cosmetics can help to ameliorate nail biting social effects.

Independently of the method used, parental education is useful in the case of young nail biters to maximize the efficacy of the treatment programs, as some conducts by the parents or other family members may be helping to perpetuate the problem. For example, punishments have been shown to be not better than placebo, and in some cases may even increase the nail biting frequency.

Epidemiology
While it is rare before the age of 3, about 30 percent of children between 7 and 10 years of age and 45 percent of teenagers engage in nail biting. Finally, prevalence decreases in adults. Figures may vary between studies, and could be related to geographic and cultural differences. The proportion of subjects that have ever had the habit (lifetime prevalence) may be much higher than the proportion of current nail-biters (time-point prevalence). Although it does not seem to be more common in either sex, results of epidemiological studies on this issue are not fully consistent. It may be underrecognized since individuals tend to deny or be ignorant of its negative consequences, complicating its diagnosis. Having a parent with a mental disorder is also a risk factor.