Sensory Modulation Disorders

Many advances are being made into understanding the biological foundation of eating disorders, but the neural pathways and genetic variants involved are still largely unknown. Sensory modulation disorders (SMDs) may be one such example of a little-known but potentially important factor.

SMDs are a clinically defined entity, and include a variety of abnormal sensory responses to pain, taste, and smell and somatosensory sensations reported among people with eating disorders. Over-responsiveness may lead to behavioral and affective inhibition. In contrast, under-responsiveness may be associated with behavioral disinhibition. The sensory links to eating disorders are readily apparent.

Ayelet Brand-Gothelf, MD, and a team of Israeli researchers explored SMD in a small study of female adolescent and young adult inpatients between 15 and 25 years of age; the patients either had restrictive-type AN (AN-R; n=20) or bulimia nervosa (BN; n=20). All 40 patients were acutely ill. A group of 27 healthy age-matched female volunteers acted as controls (Int J Eat Disord. 2016; 49:59). The research team also followed a second group, 20 female inpatients with AN-R, from admission to discharge and weight recovery.

The researchers hypothesized that the participants with AN-R would show general sensory over-responsiveness, while those with BN would have sensory under-responsiveness. The authors used a series of instruments, including the Eating Disorders Inventory-2 and the Sensory Responsiveness Questionnaire, which measures the severity of SMD symptoms. This 58-item questionnaire poses typical scenarios found in daily life, each involving one of the senses, such as auditory, visual, olfactory, hearing, vestibular, and somatosensory stimuli (except pain) (Disabil Rehabil. 2009; 31:189).

The hypotheses were correct, with some surprises

As the team had hypothesized, the participants with AN-R had elevated overall sensory over-responsiveness as well as higher taste/gustatory, vestibular/kinesthetic, and somatosensory/tactile scores compared to the controls and also compared to patients with BN. Higher sensory over-responsiveness correlated with higher ED-related symptoms in acutely ill patients with AN-R. This trend was also seen, to a lesser degree, after weight restoration among patients with AN-R. The authors noted that these findings support growing evidence that body image distortion among patients with AN seems to involve a multisensory impairment of body perception beyond visual misperception.

In contrast to previous studies, the authors found sensory over-responsiveness in individuals on the SRQ vestibular/kinesthesiology subscale, which reflects motor dysregulation. This was compatible with motor slowing seen in cases of starvation or energy-deficient diets but contrasted with previous reports of increased drive for physical activity and restlessness commonly seen in AN.

The women with BN showed greater sensory under-responsiveness on the intensity subscale of the SRQ but not on other parts of the test. On the SRQ, patients with BN only showed greater sensory under-responsiveness than those with AN-R on the intensity subscale, but not the frequency subscale. The authors commented that this was probably not unusual since sensory under-responsiveness is relatively uncommon, affecting only about 20% of individuals with SMD. In addition, about half of BN patients in the study also had a history of AN.

The second study: course across recovery

In the second study described in the paper, in patients with ANR who were followed from admission to recovery, only the auditory subsection of the SRQ improved in the time from admission to discharge, despite significant weight gain (the mean BMI rose from 15.3 to 19.6).

Dr. Brand-Gothelf and colleagues comment that their findings of abnormal sensory processing in patients with AN-R could fit with functional magnetic resonance imaging studies showing abnormal brain circuitry, which may underlie the pathophysiology of AN. This is a promising area for future study.