[Reconciliating neurology and psychiatry: The prototypical case of frontotemporal dementia].

Frontotemporal degeneration (FTD) in its behavioral variant (bvFTD) is probably one of the conditions that best illustrates the links between psychiatry and neurology. It is indeed admitted that between a third and half of patients with this condition, especially in early-onset forms, receive an initial diagnosis of psychiatric disorder (depression, schizophrenia, bipolar disorder) and are then referred to a psychiatric ward. BvFTD can thus be considered a neurological disorder with a psychiatric presentation. Among psychiatric symptoms reported in this disease, psychotic symptoms (hallucinations, delusions, especially of persecution), which have long been underestimated in bvFTD and are not part of the current diagnostic criteria, are present in about 20% of cases and may be inaugural. They are particularly common in the genetic forms related to a mutation in the C9orf72 gene (up to 50%), and to a lesser extent in the GRN gene (up to 25%). C9orf72 gene mutation is often associated with a family history of dementia or motor neuron disease but also of psychiatric disorders. It has also been described in sporadic presentation forms. Sometimes, the moderate degree of brain atrophy on MRI described in patients carrying this mutation may complicate the differential diagnosis with late-onset psychiatric diseases. In the present article, we underline the importance of considering that psychiatric - especially psychotic - symptoms are not rare in bvFTD, which should lead to a revision of the diagnostic criteria of this disease by taking greater account of this fact. We also propose a diagnostic chart, based on concerted evaluation by neurologists and psychiatrists for cases of atypical psychiatric symptoms (late-onset or pharmacoresistant troubles) leading to consider the possibility of a neurological disorder, in order to shed a new light on these difficult clinical situations. In the field of research, bvFTD may constitute a model to explore the neural basis of certain psychiatric disorders, and a possible molecular link between bvFTD and psychoses, which could eventually lead to new therapeutic approaches, has been recently suggested. Thus, bvFTD illustrates how the links between neurology and psychiatry are close and tend to evolve with the progress of scientific knowledge. It is necessary to strengthen collaboration between the two disciplines both to improve the care - diagnosis and management of these patients - and to promote the emergence of innovative clinical research.