How Similar Are the Disorders Included Under the Umbrella of Obsessive-Compulsive Disorder and Related Disorders?-Reply

In Reply Costa and colleagues raise important issues regarding how grooming disorders (GDs) should be optimally treated and classified.

N-acetylcysteine (NAC) showed superiority over placebo as monotherapy in excoriation disorder,¹ similar to findings in trichotillomania. However, NAC’s findings in obsessive-compulsive disorder (OCD) are mixed. Two clinical trials reported significant benefits of NAC augmentation over placebo for OCD,^2,3 while a third study did not show benefits over placebo.⁴ As Costa and colleagues suggest, differential pharmacological response across disorders is an important factor when considering diagnostic classification. This is also complicated by differences in study designs, as results may vary when using NAC as augmentation to a selective serotonin reuptake inhibitor rather than a primary intervention.

A common link between GD and OCD is an appealing notion based on overt symptoms: these conditions are characterized by repetitive acts that are difficult to suppress. There is evidence for familial and comorbid overlap as well, but whether this overlap is stronger than between GD and other disorders (eg, alcoholism or attention-deficit/hyperactivity disorder) is less clear. Neuropsychological deficits in OCD span a broader range of domains than those identified in GD. Even without these differences, the neurobiological underpinnings may differ between disorders. Also, cognitive dysfunction is heterogeneous even within a given disorder: while patient groups may manifest general cognitive deficits relative to control groups, these problems may not always persist when assessed on an individual basis. The logical question then is whether cognitive, imaging, or other markers could be used to individualize treatment based on unique clinical features, possibly transdiagnostically.

We agree with the caveats by Costa et al regarding animal models of GD and OCD. In our experience, some animal models of OCD are in fact measuring excessive grooming, while other models of excessive grooming bear little face validity with human disorders, such as animals who excessively groom cage-mates, as opposed to themselves. While these modalities may be beneficial in assessing wider trends, their utility in optimizing treatment based on individual traits may be more limited.

We suggest that current diagnostic classification of GD is arbitrary but pragmatic given available evidence. Classifying GD with OCD in the DSM-5 encourages clinicians to screen for relevant comorbidities and encourages research to support or refute this conceptualization. Whether the International Statistical Classification of Diseases and Related Health Problems, Eleventh Revision will adopt a similar classification remains unclear because GDs are currently listed in a separate category to OCD in the International Statistical Classification of Diseases and Related Health Problems, Tenth Revision. Despite possible neurobiological or cognitive distinctions, it may be beneficial to classify GD and OCD under a common heading based on similar symptoms, given that not all clinicians have experience with GD and may conflate symptoms with OCD unless distinct guidelines are provided, as is often the case with disorders falling under a common classification.

Also of relevance here is the concept of behavioral addiction. In an animal model, NAC dampened the effects of cocaine on glutamate transporters in the nucleus accumbens and dorsal striatum.⁵ Perhaps the beneficial effects of NAC in GD occur at least partly via a similar mechanism, an issue that could be explored further.