Cortical thinning of the right anterior cingulate cortex in spider phobia: A magnetic resonance imaging and spectroscopy study

There a lack of consistent neuroimaging data on specific phobia (SP) and a need to assess volumetric and metabolic differences in structures implicated in this condition. The aim of this study is investigate possible metabolic (via 1H MRS) and cortical thickness abnormalities in spider-phobic patients compared to healthy volunteers. Participants were recruited via public advertisement and underwent clinical evaluations and MRI scans. The study started in 2010 and the investigators involved were not blind in respect to patient groupings. The study was conducted at the Ribeirão Preto Medical School University Hospital of the University of São Paulo, Brazil. Patients with spider phobia (n = 19) were matched to 17 healthy volunteers with respect to age, education and socio-economic status. The spider SP group fulfilled the diagnostic criteria for spider phobia according to the Structured Clinical Interview for DSM-IV. None of the participants had a history of neurological, psychiatric or other relevant organic diseases, use of prescribed psychotropic medication or substance abuse. All imaging and spectroscopy data were collected with a 3 T MRI scanner equipped with 25 mT gradient coils in 30-minute scans. The Freesurfer image analysis package and LC Model software were used to analyze data. The hypothesis being tested was formulated before the data collection (neural correlates of SP would include the amygdala, insula, anterior cingulate gyrus and others). The results indicated the absence of metabolic alterations, but thinning of the right anterior cingulate cortex (ACC) in the SP group when compared to the healthy control group (mean cortical thickness±SD: SP=2.11±0.45 mm; HC = 2.16±0.42 mm; t (34)=3.19, p = 0.001 [-35.45, 71.00, -23.82]). In spectroscopy, the ratios between N-acetylaspartate and creatine and choline levels were measured. No significant effect or correlation was found between MRS metabolites and scores in the Spider Phobia Questionnaire and Beck Anxiety Inventory (p > 0.05). The ACC is known to be related to the cognitive processing of fear and anxiety and to be linked with the conditioning circuit. The MRS findings are preliminary and need more studies. The finding of reduced ACC thickness in SP is in agreement with evidence from previous functional neuroimaging studies and highlights the importance of this brain area in the pathophysiology of SP.