Comment: should we diagnose MCI in Parkinson disease?

Abstract

Both Rektorova and Martinez-Horta, and Kulisevsky agreethat Parkinson disease (PD) patients have frequent cogni-tive impairment already at disease onset, which progressesat a varying rate, terminating in dementia in a substantialnumber of patients. Cognitive deterioration in PD followsan indolent course, like that seen in other neurodegenera-tive diseases, and it is an arbitrary decision to term anysection of this process as mild cognitive impairment (MCI),or by any other name. It will be difficult to justify a‘‘starting point’’ of MCI (since deterioration preceded iteven if not fulfilling criteria), and even the conversion todementia is more complicated to define than in Alzheimerdisease (AD), since activities of daily living are impaired inPD by the motor disability, as well as by apathy, depres-sion, etc. which precludes a clear-cut determination as itapplies, for example, to AD.The term MCI has been coined to denote the prede-mentia cognitive impairment stage of AD and has attractedobjections (Ash and Korczyn 2011). Borrowing it to otherdisorders should not be automatic. Several difficulties arestressed in both papers. In addition, it must be recalled thatmany of the PD patients have a unique premorbid per-sonality, they frequently have depression, apathy andsomnolence, are treated by drugs and of course have motorimpairment—each of which could interfere with the exe-cution and interpretation of neuropsychological tests.The common definition of MCI depends on complaintsby the patient (or a close informant). This is particularlytrue for memory impairment, and it is critical to employthis parameter since a deterioration from a previous func-tional level is implied and there are usually no premorbidscores of cognitive testing in AD. However, accepting thatin PD the first cognitive manifestation may be executivedysfunction, what should be the complaints? Clearly, in PDpatients these cannot be just slowness! At present, thisissue has not been addressed.Both Rektorova and Martinez-Horta, and Kulisevskyemphasize the variability of cognitive deterioration. Atpresent, there are few data on factors influencing the rate ofdecline. One of the important factors may be comorbidities,such as co-existent AD or vascular changes (Korczyn2002). These, of course, will complicate further anyinterpretation of cognitive testing.Another very important issue is why diagnose clinicallyinconspicuous or very mild cognitive impairment in PD?In AD, the reason given is that disease modifying therapiesare being developed which should be used early in thecourse of the disease. Since no such therapies are beingdeveloped against the cognitive deterioration in PD, thebenefits from diagnosing an incipient dementia state are notclear. PD patients know that they face deterioration, par-ticularly motor decline. They will not benefit from beingtold that they have been identified as being more likelythan other PD patients to develop dementia as well.References