by Prof. Yuliarni Syafrita, Sp.N(K); Harnavi Harun, Sp.PD-KGH, FINASIM; Restu Susanti, Sp.S(K) M. Biomed; and Syarif Indra, Sp.N(K)

Prof. Syafrita and Drs. Susanti Indra are with the Department of Neurology, Faculty of Medicine at Andalas University in Padang, Indonesia. Dr. Harun is with the Department of Internal Medicine, Faculty of Medicine at Andalas University in Padang, Indonesia.

Funding: No funding was provided for this article.

Disclosures: The authors have no conflicts of interest relevant to the content of this article.

Innov Clin Neurosci. 2024;21(10–12):44–47.

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Abstract

Objective: Cognitive impairment is a recurrent complication in people with chronic kidney disease (CKD), which includes those undergoing hemodialysis (HD). Researchers aimed to analyze vitamin D levels, beta-amyloid 42, indoxyl sulfate, and serum parathyroid hormone (PTH) in patients with cognitive impairment who underwent HD.

Design: This comparative, cross-sectional study was conducted at the HD unit of Dr. M Djamil Padang Hospital. This study enrolled 60 patients with CKD who underwent routine HD and 20 normal subjects as controls. In both groups, serum levels of vitamin D, beta-amyloid 42, indoxyl sulfate, and PTH were measured using the enzyme-linked immunosorbent assay method, and cognitive function was assessed using the Indonesian version of the Montreal Cognitive Assessment neuropsychological test.

Results: The mean±standard deviation age of the study subjects was 51.48±11.44 years, with 53.4 percent being male. Vitamin D levels were higher in the control group, compared to the case group (p<0.05). The case group had higher levels of beta-amyloid, indoxyl sulfate, and PTH, compared to the control group (p<0.05). Significant differences were found in vitamin D and indoxyl sulfate levels between the groups with and without cognitive impairment (p<0.05).

Conclusion: Lower levels of vitamin D and higher levels of indoxyl sulfate were observed in the group with cognitive impairment when compared to the group without cognitive impairment.

Keywords: Beta-amyloid, cognitive impairment, hemodialysis, indoxyl sulfate, parathyroid hormone, vitamin D

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Hemodialysis (HD) must be carried out in patients with advanced chronic kidney disease (CKD) when the kidneys can no longer remove various metabolic waste toxins from the body. HD is typically undertaken by patients with stage 5 CKD, wherein it is estimated that kidney function has been disrupted approximately 85 to 90 percent.¹ In this condition, patients typically receive HD two to three times per week. CKD has many complications that can arise because various toxic substances have already accumulated in the body; common diseases that contribute to kidney disease, such as hypertension and diabetes mellitus, also contribute to the occurrence of these complications. In addition, impaired cognitive function can worsen the quality of life of people with CKD.¹ 

The prevalence of cognitive impairment in patients with CKD is reported to be 30 to 60 percent, depending on the neurocognitive examination techniques employed and the degree of impaired kidney function.² The more severe the impairment of kidney function, the more severe the impairment of cognitive function will be.¹ Glomerular filtration and albuminuria are two independent factors that are associated with cognitive impairment in patients with CKD;^3,4 however, various toxic substances that circulate the blood due to kidney function failure and a body’s reaction to kidney function disorders also play significant roles in the occurrence of cognitive function disorders.

Vitamin D insufficiency often occurs in people with CKD, especially those who are undergoing HD, because the kidneys mainly produce the active form of vitamin D. Parenchyma damage and loss of renal mass leads to decreased 1,25(OH)₂D secretion.⁵ Current research suggests that the combination of hypovitaminosis D and CKD might cause impaired cognitive function to occur more quickly and with greater severity.⁵ 

Impaired cognitive function in patients with chronic renal impairment can also be attributed to other toxic substances, including beta-amyloid. The kidneys play a role in lowering beta-amyloid levels in the circulation, and previous studies reported that approximately 40 to 60 percent of beta-amyloid cleared from the brain is removed from the body through the kidneys.^6–8 High levels of beta-amyloid in the circulation due to renal failure can re-enter the brain and increase the likelihood of cognitive impairment.⁸  

Various uremic toxins, including parathyroid hormone (PTH) and indoxyl sulfate, have a role in the occurrence of cognitive disorders. Several reports state that uremic toxins play a role in cognitive impairment,⁹ but the role of toxins on the incidence of cognitive impairment in patients who received HD has not been widely explored. This study aimed to evaluate the relationship between levels of vitamin D, beta-amyloid, indoxyl sulfate, and serum PTH with cognitive function in patients who had undergone routine HD.

Methods

Study design. This study is cross-sectional comparative study conducted in the HD unit of Dr. M Djamil Padang Hospital. The study was carried out on patients with CKD who underwent HD regularly from March to September 2022. The Research Ethics Commission of the Faculty of Medicine, Andalas University, performed an ethical review of this study (number 888/UN.16.2/KEP-FK/2022).

Respondents. In this study, 60 patients met the inclusion criteria, and 20 healthy individuals were enrolled as controls. The inclusion criteria were as follows: patients who underwent routine HD twice per week for more than six months and were willing to be included in this study. The exclusion criteria were as follows: patients who had experienced cerebrovascular disease and those who had been diagnosed with dementia. All patients received health insurance coverage so that they did not bear material burden.

Cognitive function examination. Cognitive function was assessed using the Indonesian version of the Montreal Cognitive Assessment neuropsychological test (MoCA-Ina), administered one hour before HD to ensure the patients were in a fresh condition. The MoCA-Ina scores were categorized as follows: cognitively normal (≥26), mildly cognitively impaired (18–25), moderately cognitively impaired (10–17), and severely cognitively impaired (<10). The study sample was also examined for the presence or absence of depression using the Patient Health Questionnaire-9 (PHQ-9). This score ranged from 0 to 27; a score greater than 4 indicated presence of depression, and a score of 4 or less indicated that there was no depression. The presence or absence of depression is important for revealing the effect of depression on impaired cognitive function.

Examination of serum vitamin D, beta-amyloid 42, indoxyl sulfate, and parathyroid hormone. Approximately 5cc of patients’ blood serums from venous blood were obtained, inserted into a vacutainer tube, and centrifuged at 2,000 to 3,000rpm for 20 minutes. The serum formed was put into a microtube and stored at −80°C. Then, the samples were collected, and the levels of each marker were checked with an enzyme-linked immunosorbent assay (ELISA) reader using the ELISA Kit from Assay Genie.

Statistical analysis. The data were analyzed using SPSS Statistic 21. The relationship between categorical variables was determined using a Chi-squared test. The difference in levels between two variables was identified using a t-test when the data were normally distributed and using Mann–Whitney test when the data were not normally distributed. P-value less than 0.05 was used as the significance level.

Results

This study was conducted on patients who had undergone routine HD twice weekly for at least six months. The mean±standard deviation (SD) age of the study subjects was 51.48±11.44 years, with 53.4 percent being male. Based on the MoCA-Ina scores, the subjects were divided into three groups: 26 individuals (50%) with normal cognitive function (MoCA-Ina score ≥26), 16 individuals with mild cognitive impairment (MoCA-Ina score 18–25), and 10 individuals with moderate cognitive impairment (MoCA-Ina score 10–17). There were no subjects with severe cognitive impairment (MoCA-Ina score <10). 

Table 1 shows that most of the patient characteristic data was similar in the groups with and without cognitive impairment (p>0.05). It was found that the group with cognitive impairment experienced a higher prevalence of depression compared to the group without cognitive impairment; this difference was significant (p<0.05).

Table 2 shows significant differences in levels of vitamin D, PTH, beta-amyloid 42, and indoxyl sulfate between the case group and the control group (p<0.05). Vitamin D levels in the case group were lower than those in the control group. The levels of PTH, beta-amyloid 42, and indoxyl sulfate were higher in the case group, compared to the control group.

There were significant differences in vitamin D levels (p<0.025) and indoxyl sulfate levels (p<0.05) between the normal cognitive function group and the mild and moderate cognitive impairment groups (Table 3). However, no differences were found in beta-amyloid 42 (p>0.0125) and PTH levels (p>0.016) based on cognitive function. To determine which groups had significant differences, a post hoc test was conducted. The post hoc test revealed significant differences in vitamin D levels between the normal cognitive function group and the mild cognitive impairment group (p=0.01), as well as between the normal cognitive function group and the moderate cognitive impairment group (p=0.002). There was no significant difference in vitamin D levels between the mild and moderate cognitive impairment groups (p=0.186). For indoxyl sulfate levels, significant differences were found between the normal cognitive function group and the mild cognitive impairment group (p=0.005), between the normal cognitive function group and the moderate cognitive impairment group (p<0.001), and between the mild and moderate cognitive impairment groups (p=0.038). 

Additionally, based on the results shown in Table 1, the incidence of depression was associated with impaired cognition. Table 4 shows the relationship between levels of vitamin D, beta-amyloid 42, indoxyl sulfate, and PTH and the incidence of depression. There were no significant differences in levels of vitamin D, beta-amyloid 42, indoxyl sulfate, and PTH in the groups with and without depression (p<0.05).

Discussion

Vitamin D levels were lower in the group with cognitive impairment than in the group without cognitive impairment. Vitamin D is a fat-soluble steroid hormone that plays a role in various physiological processes. In the central nervous system, vitamin D acts as a neuroprotector, increases neurotransmitter synthesis, acts as an anti-inflammatory, maintains intracellular calcium homeostasis, mediates gene expression so that neuron function runs optimally, prevents cell damage caused by oxidative stress, and prevents the accumulation of beta-amyloid in brain tissue.^10,11  Vitamin D receptors and enzymes responsible for the synthesis of the active form of vitamin D are widespread in brain tissue. This fact supports the role of vitamin D in nerve function.^12,13 However, its neuroprotective effect is associated with the ability of vitamin D to modulate excessive proinflammatory states and reduce levels of beta-amyloid oligomers that are the forerunners of amyloid plaque formation.¹⁴ Animal studies have also shown that vitamin D deficiency leads to reduced vitamin D receptor genes, and a decreased amount of nerve growth factor in the hippocampus, in comparison to controls.¹⁵ 

Indoxyl sulfate levels were higher in the group with cognitive impairment, compared to the group without cognitive impairment. Indoxyl sulfate is derived from the breakdown of the amino acid tryptophan by microbes in the large intestine in the form of small molecules, with more than 90 percent binding to plasma proteins.¹⁶ Under healthy conditions, indoxyl sulfate will be secreted in the renal tubules, but HD machines cannot fully perform this function. Clearance through HD is limited to indoxyl sulfate that dissolves and does not bind to plasma proteins so that it can pass through the dialysis membrane. Considering that the clearance power of dialysis is much lower than clearance by healthy kidneys, indoxyl sulfate accumulates in considerable quantities in plasma.^16–18 Uremic toxins, including indoxyl sulfate, play a role in cognitive impairment,¹⁹ but the role of indoxyl sulfate on the incidence of cognitive impairment in patients who have been on HD is not fully known. The filtration mechanism does not work in patients with CKD, leading to quick accumulation of indoxyl sulfate in the brain. Indoxyl sulfate causes neurotoxic effects, mainly through endothelial injury improving neuroinflammation.²⁰ Indoxyl sulfate stimulates vascular calcification and has been shown to induce aging in hypertensive mice, which is a potentially detrimental effect on cognitive processes.²¹

Levels of beta-amyloid and PTH were higher in the group with cognitive impairment than the group without cognitive impairment; however, the difference was not statistically significant (p>0.016). The kidneys are the main organs of beta-amyloid secretion from the systemic; hence, impaired kidney function has the potential to increase beta-amyloid levels in the blood.²² It was found that beta-amyloid levels in patients with CKD who had undergone HD were twice as high as in the healthy control group (Table 2). High levels of beta-amyloid in the blood are at risk of entering cerebral circulation, leading to increased amyloid plaque formation in the brain. There might be a relationship between CKD and increased incidence of microhemorrhagic and beta-amyloid deposits in the brain.^23,24 However, studies involving humans are still nonexistent. In this study, there was no significant difference in beta-amyloid levels between the groups with cognitive impairment and those without cognitive impairment (p>0.125). Elevated levels of PTH are often found in patients with CKD in response to low calcium levels owing to reduced vitamin D in the active form. Hyperparathyroidism has been associated with various chronic conditions, such as impaired cognitive function and dementia.^24–28 

Hyperparathyroidism causes neuronal calcium dysregulation, hypoperfusion, and impaired neuronal signaling. Research results concerning this have not been consistent; some studies support a significant association between PTH levels and dementia, whereas others do not.^28,29 In this study, there was no difference in PTH levels in the groups with and without cognitive impairment. However, PTH levels increased in patients with CKD who had undergone HD compared to the healthy control group (Table 2).

In this study, the incidence of cognitive impairment following HD in patients with CKD undergoing HD could not be assessed because cognitive function data before HD was not available. Future studies to determine if cognitive function deteriorates in patients with CKD who receiveHD for a long period of time would be of clinical interest.

Conclusion

In patients with CKD who underwent routine HD, lower levels of vitamin D and higher levels of indoxyl sulfate were found in the group with cognitive impairment compared to the group without cognitive impairment.

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