Testing cognition may contribute to the diagnosis of movement disorders

Diseases with movement disorders may be difficult to diagnose, both at the onset, when motor symptoms are mild and not sufficiently specific (e.g., difficulty in manipulating objects), or at end stage when distinctive motor signs are diluted in a severe and polymorphous clinical picture (eg, gait disorder with postural instability and cognitive decline in aged patients). Erroneous diagnoses are not infrequent, even in the most common disorders such as Parkinson's disease (PD). ^[1] Some of these errors may be avoided if the cognitive dysfunctions that often accompany these diseases are included in the diagnostic criteria.

In general, the cognitive disturbances found in movement disorders are in the category of ``subcortical dementia,'' consisting of a frontal lobe-like syndrome, without genuine amnesia or impairment of instrumental functions (aphasia, apraxia, or agnosia). Except for a few illnesses, such as essential tremor and idiopathic dystonia, in which intellectual deterioration is absent, subtle but specific cognitive deficits can frequently be detected in patients with a variety of diseases accompanied by movement disorders. ^[2] This is explained by the neuronal pathways connecting the basal ganglia to the cortex projecting not only to regions involved in the control of movements (motor, premotor, supplementary motor areas) but also to cortical areas contributing to cognitive functions (prefrontal cortex). In each of these diseases, the loss of different specific populations of neurons in the basal ganglia produces not only a characteristic motor syndrome but also a recognizable pattern of neuropsychological deficits. The use of appropriate neuropsychological tests to detect these deficits can therefore contribute to the diagnosis of such diseases.

This brief review will not deal with disorders caused by subcortical lesions of nondegenerative origin (tumors, sequelae of trauma or stroke, inflammatory and metabolic disorders, Creutzfeldt-Jakob disease, etc.) or with easily identified neurodegenerative diseases, i.e., when motor disability is prominent …