Synaesthesia,reflex hallucinations and Mitempfindung - one of the same or different entities?

To the Editor

Synaesthesia, first reported in the late 19^th century, consists of a stimulation in one sensory pathway leading to perceptual experiences in a second sensory pathway (Hubbard and Ramachandran, 2005). Another similar phenomenology previously identified yet poorly defined was reflex hallucinations. Cutting (1997) described reflex hallucinations as occurring when a precipitating stimulus was experienced in a different modality from that of the hallucination. A related phenomenology was Mitempfindung, which was described as a stimulus applied in one region of the body being felt as a tactile sensation in another region. A small study of 20 digit-colour synaesthetes found that Mitempfindung was reported in 40% of synaesthetes with only 10% reported incidence in two matched control groups. The authors of this study presented multiple similarities between synaesthesia and Mitempfindung (Burrack et al., 2006). The various terms used in the psychiatric literature to report such overlapping phenomenon was confusing because historical descriptions have not caught up with research findings.

We present a case highlighting these complexities and provide discussion about how these various terms can be better understood utilizing modern nomenclature. Ms AT is a 38-year-old woman with a schizoaffective disorder – bipolar type. Her recent admission was the result of an acute psychotic relapse. At admission she complained of an ongoing vibrating sensation in her genitalia caused by her mother opening and shutting doors and by the sound of her clicking her nails. Her psychosis improved with a short course of electroconvulsive therapy (ECT), in conjunction with lithium and risperidone. Ms AT’s symptoms were a type of reflex hallucination. Having similarities with synaesthesia, these two phenomena might share more in common rather than be separate entities. We propose that synaesthesia, reflex hallucinations and Mitempfindung share their pathophysiological basis and fall within the same dimensional framework, with synaesthesia representing a non-morbid phenotype.

Even though Ms AT experiences did not represent Mitempfindung, the nature of the phenomenology shared some common ground with reflex hallucinations and synaesthesia.

This case illustrates the potential confusion of various terms used in the literature; we propose that narrowing the definition may potentially lead to a further recognition and increased reporting of these unusual phenomena. It is possible that the presence of one might increase the probability of another in the same person. With further research in this area, a revised terminology and classification will emerge shedding new light on historical phenomenology that have been reported in clinical practice for over a century.