Our results revealed that patients with aMCI exhibited a higher prevalence rate of neurological abnormalities than normal control participants. The preliminary results also suggested NSS and neuropsychological tests might reflect somewhat similar information for the brain functioning. The present study indicated that NSS may play an important role and serve as a tool to assist in the early detection of aMCI.
To our knowledge, this is the first study to investigate the NSS in older adults with aMCI. Our results demonstrated that aMCI individuals displayed significantly more neurological abnormalities than normal controls on motor coordination, disinhibition, and the total NSS. The effect sizes of group comparisons were large for motor coordination and total score and moderate for disinhibition. The findings were similar to previous studies focused on normal elderly people and people with pathological aging disease. With the same scale, Chan et al. (2011) found that NSS was common among elderly people, and the prevalence rate of soft signs increased with advancing age. In a neurological examination including few soft signs (etc., saccadic eye movement), the older adults who were with cognitively impaired but without dementia were found to produce a higher prevalence of signs than the normal controls. Patients with AD and other several forms of dementia were also found to have higher prevalence of NSS than those without dementia.
The current study indicated that older adults with aMCI showed more motor dysfunctions than cognitively normal older people. Previous findings indicated that people with MCI and mild AD demonstrated dysfunctions in equilibrium and limb coordination. Other studies confirmed that people with aMCI performed worse on tasks involving fine and complex motor functions than normal older adults[40, 41]. Lam et al. (2005) also found motor coordination signs were very sensitive in discriminating patients with or without dementia. Signs such as primitive reflexes and mirror movements were classified as disinhibition, which included the signs of spurious movements in a time and place where it was not expected to occur. In the present study, patients with aMCI demonstrated significantly more signs than normal controls in the disinhibition subscale. Similar findings were also reported in previous studies. Franssen et al. (1991) found participants in an early stage of AD showed higher mean score of deep tendon reflexes than normal elderly people, while patients with a later stage of AD demonstrated significantly increased prevalence of sucking reflexes compared with normal older adults and patients with the early stage of AD. Furthermore, more primitive reflexes were found in the more terminal stages of AD. Previous studies also found that MCI patients showed inhibition impairments in some neuropsychological tasks, such as go/no-go task, Stroop task[43, 44], Hayling test, and Flanker test.
The correlations between NSS and neuropsychological functions have significant implications. Our results indicated that the total NSS score was negatively correlated with the combined Z-score of neuropsychological tests in aMCI group. The current results were consistent with our recent findings in normal older adults that NSS had moderate associations with neurocognition function. The present study provided further evidence that the two measures were more or less statistically equivalent to capture the similar brain functions. These results support in part the assumption that motor coordination might be an indicator of the prefrontal lobe function[7, 8]. However, the correlation analysis also indicated that sensory integration and disinhibition did not have significant correlations with the neuropsychological tests. The reason might be as mentioned previously, sensory integration and disinhibition subscales were considered to reflect parietal and frontal lobe functions respectively[22, 23], and aMCI patients showed relatively less impairment in these two subscales. Whereas, there were no sensitive neuropsychological tasks to reflect frontal lobe function and no tasks specialized to measure the parietal lobe function in current study. To better understand whether there are same neural substrates between NSS and neuropsychological functions, future studies with larger samples and more comprehensive neuropsychological tasks are needed.
The present study has several limitations. First, the sample size of the study is relatively small. More studies with larger sample sizes are needed in order to confirm the neurological dysfunctions in persons with aMCI, which could help to further clarify the early neurological abnormalities of aMCI. Second, some of the aMCI participants were from a memory clinic, and these participants were not scored by blinded raters, which might bring some bias during assessment. Future studies should overcome the difficulties and assess the NSS with blind raters. Third, aMCI participants were recruited from both community and the memory disorder clinic, while the health elderly were only selected from the community residents. This may lead to differential selection bias. Fourth, due to the mean age of participants in both groups were older than 70 years, 40 out of the 57 participants suffered from one or more chronic diseases such as hypertension, diabetes mellitus, coronary heart disease and other diseases. Also, unfortunately quite a number of the elderly could not recall the exact type and dose of medicines they have been taking during the assessment and we did not further follow up with these; therefore, we are uncertain whether their medication will influence their neuropsychological or NSS performances. Moreover, the present study adopted the traditional diagnosis criteria of aMCI proposed by Petersen and colleagues[2, 26], which was called core clinical criteria by the National Institute on Aging and Alzheimer’s Association workgroup. The workgroup also recommended new diagnosis criteria: research criteria, which incorporated the biomarker based on imaging or cerebrospinal fluid measures into the core clinical criteria. Studies adopting more stringent research criteria for screening aMCI are needed to confirm the current findings.
Our current findings have shown that aMCI patients demonstrated significantly higher prevalence of NSS than healthy older adults. The total scores of NSS were significantly correlated with the combined Z-score of neuropsychological tests in aMCI group. NSS has been found to be indicative of the cognitive decline and brain dysfunction[4, 23]. The observed impairment of NSS in aMCI contributes further evidence to the literature on neurological deficits in pathological aging diseases. Given the assessment is simple, non-invasive and time-saving, neurological soft sign test may be used as an assistant tool for the bedside clinical examination of mild cognitive impairment.