Our results confirm that iNPH patients are severely affected in all cognitive domains measured. Moreover, we found that verbal memory and executive functions are one of the most affected cognitive domains in this diagnosis. These findings concur with previous research [11, 18]. We found attention/psychomotor speed to be less impaired than executive functions and verbal memory. Additionally, we found non-verbal memory and visuospatial abilities to be the least impaired of the measured cognitive domains in iNPH patients. However, it is important to note that all of the measured cognitive domains were considered in regard to educational and age correlated norms and all of them were severely impaired in that respect.
As pointed out in the introduction of this article, all of these cognitive domains are associated with the iNPH diagnosis. There is no common understanding however, of how much the individual domains are impaired when viewed in relation to each other. Thus, we offer our results as an invitation to further scrutiny. We found that verbal memory is much more impaired than non-verbal. These results suggest that iNPH damages memory unevenly. It is an interesting result, nevertheless, it is important to note that we measured non-verbal memory with only one neuropsychological test (ROCFT), and thus we recommend using more neuropsychological tests in the future which measure non-verbal memory in more detail, such as for example Brief Visual Memory Test - Revised. Also, the results of verbal memory in our observed group relies heavily on AVLT, which is a difficult task to perform. Considering the progress of cognitive decline, it can be suggested that in later stages the AVLT test has a smaller differentiating ability and may thus lead to potentially biased results. On the other hand, we used Fonemic and Categorical Verbal Fluency tests to assess the same cognitive domain – verbal memory in iNPH patients, which should improve the results and provide a more comprehensive picture of iNPH patient’s memory. The strong correlation appeared between executive functioning and verbal memory, while not between executive functioning and non-verbal memory supporting the idea that memory is impaired unevenly based on the presented material.
Our results additionally confirmed that executive functions are severely impaired in iNPH. This is in accordance with previous research [11, 18, 25]. It seems that inhibition and shifting ability of executive functions are impaired [11, 26, 27], however data on ability to maintain the activity are missing and the data on updating ability measured by Digit span backward test are mixed, impaired [26] and not significant [28]. Detailed description of executive functioning might help better diagnostic quality of cognitive tests in the future.
From our study, it seems that visuospatial abilities are the least affected cognitive domain in iNPH patients, even though it is still impaired compared to healthy individuals. Possible explanation might be that visuospatial abilities are affected in the later stages of the illness and hence are less severely impaired in general. It might be interesting to observe the progress of visuospatial abilities in iNPH with respect to its potential diagnostic value. Lastly, we did not focus on language abilities in this study, however, from the previous research it is possible to assume that verbal fluency is impaired but naming ability mostly remains intact [14].
Compared to Alzheimer's disease (AD), iNPH cognitive profile is characterized by frontal lobe symptoms and disproportionately impaired memory function [7]. Spontaneous recall seems to be impaired in both, AD [18, 29] and iNPH, however, in iNPH the role of clue is crucial and should normalize the memory performance as it is theorized as subcortical type of cognitive decline [4]. It might be due to a fact that iNPH does not typically affect the hippocampus as AD does [30]. Saito et al. [18] highlight the episodic part of memory to be more impaired in AD than in iNPH and also depict visuospatial abilities as more damaged in iNPH than AD. Also, attentional functions seem to be less severely impaired in AD patients rather than memory [31], unlike in iNPH where the attention along with psychomotor speed is theorized to be impaired quite early in the process [14]. On the other hand, in Frontotemporal Lobar Degeneration (FTLD), the cognitive decline is manifested by the impairment in executive functions and slower psychomotor speed much like we observe in iNPH [30]. Nevertheless, for example Diehl and Kurz [32] found in their study that patients with frontotemporal dementia performed better in visuoconstructional abilities not only in comparison to AD group but also performed equally in comparison to healthy control group. This would suggest no visuoconstructional impairment in Frontotemporal Dementia (FTD) and might be a differentiating factor between FTD and iNPH. Also, along with FTLD and especially its most common subgroup behavioral variant FTD, there are often psychiatric symptoms and frequent changes in personalities [33]. This is not a well studied topic in iNPH and might be a very interesting field to research in the future. Lastly, compared to Parkinson's disease (PD), iNPH cognitive disabilities arise earlier in the stages of the illness and with more diffuse impairment. Only 25% of PD showed executive dysfunctions compared to 65% of iNPH [34].
Study Strengths and Limitations
One of the limitations is the comparison of iNPH patients to standardized norms from different neuropsychological tests rather than healthy adult control groups. However, comparison of different cognitive domains in this study might serve as a solid ground to build a profound cognitive profile of iNPH patients. Secondarily, we did not discriminate between different stages of the illness in our cohort, thus reflecting one cognitive profile instead of detailed description of the development of overall iNPH cognitive decline. Also, the results might be skewed due to lack of differentiation between progression of the illness as the cognitive decline in later stages is global and cannot be easily differentiated from other cognitive declines such as the one with AD or PD [14]. Some cognitive domains were not covered with more neuropsychological tests and it might be worth examining them in more depth in the future .