Functional movement disorder comorbidity in Parkinson's disease: Unraveling the web

Functional movement disorders are commonly seen in neurology services and may coexist with other neurological diseases. This combination is known as "functional overlay" and an increasing interest on this topic has emerged in the past decade as the field of functional neurological disorders has moved forward. Some neurological diseases may be more prone to develop "functional overlay" than others, and within the field of movement disorders, most studies have focused on patients with Parkinson's disease. This review comprehensively summarizes the current body of knowledge on this topic and provides an expert opinion to equip clinicians with a pragmatic approach to recognize functional movement disorders in patients with Parkinson's disease, to communicate the diagnosis and to become familiar with potential therapies in this complex clinical scenario. Potential underlying mechanisms and risk factors that may play a role in increasing the vulnerability of Parkinson's disease patients to develop functional movement disorder comorbidity are also discussed within the framework of modern neurobiological theories of brain functioning.