In this study, pure tone audiometry electricity was determined in order to assess hearing loss in patients with AD and vascular dementia. The two different types of dementia patients had different clinical characteristics, but both had a certain degree of hearing impairment. The degree of the hearing impairment was positively correlated with cognitive damage. Thus, hearing impairment can be used for evaluation of cognitive impairment in AD and VaD. This study also found that the hearing loss in AD patients occurred at high frequency hearing, but hearing loss in patients with vascular dementia were at most frequency bands of hearing. The hearing loss in patients with vascular dementia at most frequency bands, was more significant than that of patients with AD.
Alzheimer's disease and vascular dementia are two important types of dementia, and they account for over 50 % of cognitive impairments in the crowd[15]. The “amyloid cascade hypothesis’’ is the main theoretical construct for AD[16-18]. In AD, αβ-peptides accumulate in neurons and trigger series of reactions such as inflammatory response, apoptosis, and blood-brain barrier injury[19-21]. Vascular dementia caused directly by stroke arises as a direct consequence of arterial pathology which triggers profound changes in the neurovascular unit and the normal physiology of neurovascular coupling[22, 23]. There are differences in the pathogenesis of the two kinds of dementia. There are also some differences in the clinical manifestations of these types of dementia. In this study, blood glucose control was significant for AD patients. Obesity index, history of smoking and drinking habits, and blood glucose control were more significant as high-risk factors of stroke in patients with vascular dementia than in AD patients. These suggest that although the clinical characteristics of the two types of dementia are different, there are some correlations between them. In the pure tone hearing threshold test, it was found that AD patients and VaD patients had hearing impairment, but there was also a certain degree of difference between two groups. The degree of hearing loss in VaD group was greater than that in AD group. Moreover, VaD patients had both high-frequency and low-frequency HI, but the degree of impairment was more severe in AD patients at most frequency bands. These characteristics might be related to nerve injury and ear lesions.
Several cross-sectional studies have shown that accelerated rate of decrease in brain volume in temporal lobe in AD is not only important in cognitive impairment, but also in auditory function[24, 25]. Research has also shown that difficult auditory perception may be accompanied by extra burden of severe cognitive load that exhausts cognitive reserve[26-28]. At the same time, AD-related pathology which has been observed in the retina and cochlea, might be associated with cochlear pathology and hearing loss. This pathological mechanism of AD is closely related to age[29]. Some studies have reported age-related hearing loss in AD patients[30]. In the present study, it was also found that the hearing impairment in AD patients was mainly high-frequency hearing loss, and it was more significant with increase in audio frequency. This is consistent with the characteristics of age-related hearing loss. In vascular dementia, there are multiple-infarct lesions and white matter injury in the brain. In cerebral infarction, the internal carotid artery is the most easily blocked vessel. Acute ischemia of the internal carotid artery system often leads to basal ganglia infarction or temporal cortex infarction[31]. Similarly, in some patients with chronic cerebral ischemia, there is damage to the communication system in white matter[32]. Lesions in these functional structures cause decline in processing of spoken language, leading to impairment of cognitive function[33]. In some studies, it was found that there was an association between hearing impairment and speech function in patients with vascular dementia[34, 35]. There is a significant relationship between speech disorder and low-frequency hearing loss. These findings indicate that the damage to brain structure had an impact on hearing function. Furthermore, in some studies, the degree of sclerosis or occlusion of vertebrobasilar artery or internal auditory artery was higher in vascular dementia; this may affect the blood supply of the ear[36]. Therefore, vascular pathology may also occur in the cochlea as a potential contributor to the hearing loss associated with vascular dementia[37]. In this study, patients with vascular dementia had hearing impairment at low sound frequencies. At the same time, compared with AD patients, the degree of hearing damage was more serious in low-frequency and high-frequency ranges. The hearing loss of AD patients was mainly mild high-frequency hearing loss, but in VaD patients, it was mainly moderate or above hearing loss in full frequency. Therefore, hearing loss exists in AD and vascular dementia, but there might be differences in their pathogenesis and pathological mechanisms, leading to different clinical manifestations of hearing impairment.
In this study, there was a correlation between the degree of hearing loss and severity of dementia in AD patients. In results of logistic regression analysis, hearing impairment significantly contributed to the prognosis of severity of dementia in AD group. Furthermore, in VaD group, body mass index, history of cerebral infarction, history of smoking, HbA1c, and the degree of hearing loss were correlated with dementia severity[38, 39]. However, logistic regression analysis showed that hearing impairment was the only significant risk factor for prognosis of severity of dementia in VaD group. These results indicated that hearing impairment was important for cognitive function. Mounting evidence suggests a link between hearing loss and dementia. Some potential causal pathways would explain the relationship between hearing loss and dementia. The mechanism involved in the occurrence of dementia may be similar to the corresponding pathological mechanism in the cochlea, such as formation of α,βamyloid in the internal auditory artery, resulting in decline of auditory nerve function[40]. This means that dementia might be the cause of hearing loss[41]. In patients with vascular dementia, the occlusion of internal auditory artery or the aggravation of arteriosclerosis in the inner ear decreases auditory conduction function[36]. Some animal studies on rodents have demonstrated that decline of auditory function led to the simplification of exogenous stimuli, which in turn led to the degeneration of cortex-related memory formation and storage. A third mechanism suggests that hearing impairment uses greater cognitive resources for listening; this negatively affects cognitive tasks such as attention, working memory, or language processing[42].
At present, it has been found that many factors affect cognitive function. These include uncontrollable factors such as age, gene and gender, as well as acquired controllable factors such as chronic disease history, obesity, bad habits, and levels of blood lipids and blood glucose[43]. Hitherto, there were no known factors that may be used for early detection of the existence of cognitive impairment and improve cognitive impairment after intervention. However, in this study, there was a close relationship between hearing and the degree of cognitive impairment, such that hearing impairment may be an index of mild cognitive impairment. There are different characteristics of pure-tone audiometry in patients with different types of dementia. Pure-tone audiometry might be an important method for evaluating cognitive impairment and for distinguishing between the two types of cognitive impairment. Furthermore, the issue of the effect of hearing interventions on the development of dementia has aroused interest amongst researchers. Some prospective randomized controlled trials provided the existing evidence related to the effect of hearing aid on recovery of cognitive function. Hearing intervention could reduce the degree of brain atrophy and play a role in nerve remodeling in auditory cortex. Therefore, it is of great significance to further study the relationship between hearing and cognitive function.
Limitations of the study
This investigation has some limitations. Expansion of sample size or a multi-center clinical trial would help to avoid the possibility of bias in case selection. The relationship amongst hearing function, speech function and cognitive function can be further elucidated using speech audiometry. There are also potential ethical issues such as economic burden and physical tolerance in patients with dementia with implanted hearing aid.