Application of Flush Response to Niacin Skin Patch in Schizophrenia Research: Comparing Visual Rating with Infrared Thermography and Their Relations to Fatty Acids Levels in Blood

Background: Attenuated niacin-induced flush response, a potential endophenotype for schizophrenia, has been postulated to involve a prostaglandin-related microvasodilation pathway where arachidonic acid or other fatty acids may play important roles. However, the correlations between the flush response and the implicated biochemical indexes remained incoherent, either because of different ways in giving niacin challenge test or in the evaluation of response. Alternatively measuring the raising temperature accompanying the niacin-induced flush might serve as an tool to assess the niacin-induced flush response.
Objectives: This study aimed to: 1) evaluate the applicability of infrared thermography in measuring the niacin-induced flush response, and 2) examine the relations of the flush magnitude to fatty acids levels in blood.
Methods: For the first aim, a pilot study of two groups of subjects were used to clarify the correlation between objectively measured temperature and visually rated flush score, whereas for the second aim another independent sample of psychiatric inpatients as well as healthy controls were used to explore the correlation between temperature, flush and fatty acids levels in red blood cells (RBCs) as well as in plasma. Three concentrations of niacin skin test with forty minute observation by visual rating method and infrared thermography method was performed in the sample of pilot study (31 patients with schizophrenia and 32 normal controls). On the other hand, niacin skin test with fifteen minute observation using both methods and blood fatty acids analysis were performed in the sample of biochemical study (9 patients with schizophrenia, 6 patients with bipolar disorder, and 15 normal controls). Fatty acids compositions of RBC and plasma were analyzed using gas chromatography.
Results: Visual rating score method showed group difference in 0.001M while thermography method did not reveal group difference in all concentration and times. The correlations between these two methods were weak. Subjects of biochemical sample were re-grouped as non-flushers versus flushers on the basis of the flush score for 0.01M at 5 minute. The RBC and plasma levels of n-6 fatty acids in non-flushers tended to be lower than the corresponding ones of flushers and the RBC level of EPA was significantly higher in non-flushers than flushers.
Conclusions: This study demonstrated that in measuring the niacin-induced flush response the visual rating method is better in discriminating schizophrenia patients from normal controls than the infrared thermography-based temperature rating. Furthermore, our results indicate that non-flushers tended to have lower levels of n-6 fatty acids but higher EPA levels than flushers. Future investigation is warranted to further explore the physiological abnormalities associated with the attenuated flush response in schizophrenia patients.