Assessment of cognitive functions

Cognitive functions can be examined either by using formal, validated scales or by administering several tests for each cognitive domain. For each purpose one can use either simple scales or a limited number of easy to administer bedside tests for screening purposes, or more elaborate scales and more complex neuropsychological tests for a detailed and quantitative evaluation. A two-level approach, simpler screening scales and tests for routine clinical practice, versus a more detailed assessment for complex cases or for research purposes, was also proposed in the recommendations for the diagnosis of PD-D published by a Task Force of the Movement Disorder Society [2]. A validation study revealed that the proposed screening checklist showed 100% specificity but only 46.7% sensitivity, the most problematic items for lack of sensitivity were absence of depression and a Mini-Mental State Examination (MMSE) score of less than 26 [3]. Another validation study also found a high specificity but low sensitivity and suggested that replacing MMSE by a more specific screening scale such as Mini-Mental Parkinson with a cut-off value of 27 points may increase the sensitivity [4].

Composite cognitive scales

Simple, easy to administer, and less time-consuming scales include those which are not specific for PD as well as those developed specifically for screening patients with PD (Table 20.2). General-purpose scales, such as the most widely known MMSE, can be used for brief screening. The Montreal Cognitive Assessment (MoCA) is probably more sensitive for detecting cognitive deficits in PD as it includes a more detailed assessment of executive functions [5]. It has a high test-retest and inter-rater reliability and a cut-off score of 21/30, yielding a sensitivity of 90% in PD-D patients [6]. Addenbrookes Cognitive Examination (ACE) scale takes longer to complete than the other screening batteries [7]. However, it offers the advantages that a more complete and extensive cognitive evaluation can be achieved, and a MMSE score can be extracted from this scale. In a validation study in which the Mattis Dementia Rating Scale (DRS) was used as a reference it was found to be a valid tool for assessing dementia in PD, displaying good correlation with both the PD-specific Scales for Outcomes in Parkinson’s Disease-Cognition (SCOPA-COG) scale (see later in this section), as well as with the less specific MMSE; a cut-off score of 83 points was proposed for the PD population [8].

Screening batteries specifically developed for PD include the Parkinson Neuropsychometric Dementia Assessment (PANDA) [9] and the Mini-Mental Parkinson [10]. These scales specifically include tests assessing common deficits seen in patients with PD. Another screening instrument which focuses on impairment in frontal-executive functions, one of the core deficits in PD-D, and also including assessment of some behavioural symptoms, is the Frontal Assessment Battery (FAB) [11].

More elaborate scales are reserved for a more detailed and quantitative assessment, and these are usually administered in the context of research studies or in early stage patients with a high level of education who can show a normal performance on simple screening scales. These also include two categories: those not specific for PD, and those specifically developed to assess typical deficits in PD. Among the established and validated scales in the former group are the most widely used composite cognitive scale, the Alzheimer Disease Assessment Scale-Cognitive Section (ADAS-COG) [12] and the Mattis DRS [13], which is more sensitive for PD as it includes an extensive executive functions component. In a study performed in a Hungarian PD population in which ACE, FAB, Mattis DRS, and MMSE were compared, the Mattis DRS was proposed to have the best clinicometric profile to detect PD-D, with a cut-off core of 125 points [14]. In a clinical trial to assess the long-term outcome of treatment with the cholinesterase inhibitor rivastigmine, the Mattis DRS was found to be sensitive to change induced by treatment as well detecting changes occurring in the course of the disease [15]. SCOPA-COG [16] and Parkinson Disease-Cognitive Rating Scale (PD-CRS) [17] were both specifically developed to assess cognitive functions in patients with PD, and can also be used for follow-up purposes to assess change over time.

When using composite scales and trying to interpret their scores it is important that one should consider not only the total score but also the profile of deficits. For example a score of 27 on the MMSE is nominally a normal score. This score, however, may be abnormal in a well-educated patient when the errors include difficulties copying the intersecting pentagons, failing to remember one out of three words, and one mistake on subtracting serial sevens. These show typical deficits in attention, visuospatial function, and free recall and may indicate an abnormal performance in this particular patient despite a nominally normal MMSE score.

Table 20.2 Composite scales which can be used to evaluate cognitive functions in Parkinson's disease (PD)

A: Screening scales

General-purpose scales:

• Mini-Mental State Examination (MMSE)

• Montreal Cognitive Assessment (MoCA)

• Addenbrooke's Cognitive Assessment (ACE)

• Frontal Assessment Battery (FAB)

PD-specific scales:

• Mini-Mental Parkinson

• Parkinson Neuropsychometric Dementia Assessment (PANDA)

B: More quantitative scales

General-purpose scales:

• Alzheimer Disease Assessment Scale-Cognitive Section (ADAS-COG)

• Mattis Dementia Rating Scale (DRS)

PD-specific scales:

• Scales for Outcomes in Parkinson's Disease-Cognition (SCOPA-COG)

• Parkinson Disease-Cognitive Rating Scale (PD-CRS)

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