638 Neurocognitive Disorders
Lewy body–related dementias (e.g., Alzheimer’s disease) and can sometimes be helpful in
the evaluation of major or mild  NCD due to Parkinson’s disease.
Differential Diagnosis
Major or mild neurocognitive disorder with Lewy bodies. This distinction is based sub-
stantially on the timing and sequence of mo tor and cognitive symptoms. For NCD to be at-
tributed to Parkinson’s diseas e, the motor and other symptoms  of Parkinson’s disease must
be present well before (by convention, at leas t 1 year prior) cognit ive decline has reached
the level of major NCD, whereas in major or mild NCD with Lewy bodies, cognitive symp-
toms begin shortly before, or concurrent wi th, motor symptoms. For mild NCD, the timing
is harder to establish because the diagnosis itse lf is less clear and th e two disorders exist on
a continuum. Unless Parkinson’s disease has been established for some time prior to the
onset of cognitive decline, or typical features  of major or mild NCD with Lewy bodies are
present, it is preferable to diagnose unspecified mild NCD.
Major or mild neurocognitive disorder due to Alzheimer’s disease. The motor features
are the key to distinguishing major or mild NCD due to Parkinson’s disease from major or
mild NCD due to Alzheimer’s disease. However, the two disorders can co-occur.
Major or mild vascular neurocognitive disorder. Major or mild vascular NCD may pre-
sent with parkinsonian features such as psychomotor slowing that may occur as a conse-
quence of subcortical small vessel disease. However, the parkinsonian features typically
are not sufficient for a diagnosis of Parkinson’s disease, and the course of the NCD usually
has a clear association with cerebrovascular changes.