No evidence for physical anhedonia as a candidate symptom or an endophenotype in bipolar affective disorder

Objectives: Bipolar affective disorder (BPAD) is clinically and genetically heterogeneous and the affected phenotype is poorly defined, hampering studies of its genetic basis. Studies of specific, familial, clinical indicators of BPAD may be useful for identifying heritable forms. Homogeneous forms of the disease may be identified in patients (candidate symptom approach) and some vulnerability markers may be sought in unaffected relatives of patients (intermediate traits or endophenotypes). Physical anhedonia (PA) is considered a possible candidate symptom and endophenotype in schizophrenia, but has never been specifically investigated in BPAD. Methods: Physical anhedonia scores (measured using Chapman's Physical Anhedonia Scale) were compared in 351 euthymic bipolar patients, 130 of their first-degree relatives and 170 healthy controls with no personal or familial history of schizophrenia, mood disorders or suicidal behavior. We investigated intrafamilial resemblance of PA and compared the progressive and clinical characteristics of hedonic and anhedonic bipolar probands. Results: Physical anhedonia was a stable trait in normothymic bipolar patients and significant intrafamilial correlation of PA scores was observed in bipolar families. However, PA scores were similar in unaffected relatives and controls and the clinical characteristics of anhedonic and hedonic patients did not differ significantly. Physical anhedonia was not associated with an increased familial risk for bipolar disorder. Conclusions: Physical anhedonia is a stable, familial dimension in BPAD families. It cannot be considered an endophenotype because unaffected relatives of bipolar patients and healthy controls have similar PA scores. It also cannot be considered a candidate symptom because it does not identify a homogeneous clinical and familial sub-group of bipolar patients. Given the results of previous studies, PA might be a specific candidate symptom (and endophenotype) to schizophrenia. However, the validation of this hypothesis requires replication studies in bipolar disorder and schizophrenia and further investigations in other psychiatric diseases (in particular across the mood disorder spectrum).