Is There Differences in Cognitive and Motor Functioning between Hemodialysis and Peritoneal Dialysis Patients?

Abstract

Objective: Change in cognitive function is one of the well-known consequences of the end-stage renal disease (ESRD). The aim of this study was to determine the effect of hemodialysis (HD) and continuous ambulatory peritoneal dialysis (CAPD) on cognitive and motor functions. Methods: In this cross-sectional study, cognitive and motor functions were investigated in a selected population of 42 patients with ESRD (22 patients on chronic HD and 20 patients on CAPD, aged 50.31 ± 11.07 years). Assessment of cognitive and motor functions was performed by Symbol Digit Modalities Test (SDMT) and Complex Reactiometer Drenovac (CRD-series), a battery of computer-generated psychological tests to measure simple visual discrimination of signal location, short-term memory, simple convergent visual orientation, and convergent thinking. Results: The statistically significant difference in cognitive–motor functions between HD and CAPD patients was not found in any of the time-related parameters in all CRD-series tests or SDMT score. Higher serum levels of albumin, creatinine, and calcium were correlated with better cognitive–motor performance among all patients regardless of dialysis modality. The significant correlation between ultrafiltration rate per HD and short-term memory actualization test score (CRD-324 MT) among HD patients was found (r = 0.434, p = 0.025). Conclusion: This study has demonstrated that well-nourished and medically stable HD and CAPD patients without clinical signs of dementia or cognitive impairment and without significant difference in age and level of education performed all tests of cognitive–motor abilities without statistically significant difference.

• Uremia
• dialysis
• cognitive functions

INTRODUCTION

Chronic kidney disease (CKD) is the permanent loss of kidney functions and final stage of CKD is end-stage renal disease (ESRD). In ESRD, kidney functions can be replaced by hemodialysis (HD) and continuous ambulatory peritoneal dialysis (CAPD). Each dialysis type has its advantages and disadvantages and has a different impact on patient’s physical, psychological, and social health, and each places its own limitations on lifestyle.¹ Change in cognitive function is one of the well-known consequences of the ESRD.² It has been shown that cognitive impairment is associated with the severity of kidney disease^3,4 and that the prevalence of cognitive deficits is particularly high in subjects with ESRD.⁵

The accumulation of toxic substances resulting from significantly reduced metabolic rates, as well as chronic dialysis, has been shown to impair functions of the central nervous system. The patients with ESRD have been demonstrated to develop uremic or dialysis-related encephalopathy accompanied by frontal and basal ganglia abnormalities on neuroimaging.^6–9 The impairment of cognitive functioning is also attributed to the effect of uremic toxins on neurones. However, the persistence of cognitive impairment despite clinically adequate dialysis dose delivery indicates that other factors also contribute to the brain dysfunction.¹⁰

The diagnosis of cognitive impairment is important because it is associated with an increased risk of death in dialysis patients and with a decreased quality of life in this population.^11,12 Furthermore, cognitive impairment may impact decision-making and the ability to adhere to dialysis recommendations, such as dietary modification and medication compliance. Cognitive impairment is also associated with increased staff time in caring for the patient, greater utilization of healthcare resources, more frequent hospitalizations, and an increased number of days spent in hospital.⁵

The most frequently reported cognitive problems in this population include disturbances in memory, executive functioning, attention or speed of information processing, perceptual motor abilities or language.¹³

Although a considerable number of articles on cognitive function in ESRD have been published, there are a limited number of studies comparing cognitive function in HD and CAPD patients. The effect of dialysis modality on risk of cognitive impairment is unclear. Some data suggest that the prevalence of cognitive impairment may be different in patients treated with HD compared with patients treated with CAPD.^14–16 They also demonstrated that CAPD patients had consistently better cognitive function than HD patients, but the results from these studies may not reflect the dialysis procedure itself but selection bias as to who is receiving which modality.

The aim of this study was to determine the effect of HD and CAPD on cognitive–motor functions in patients with ESRD. This study was based on the assumption that uremia could influence the speed of complex psychomotor performance. Assessment of cognitive–motor functions was performed by complex reactiometer Drenovac (CRD-series). A CRD test item series is a PC-based psychodiagnostic laboratory for the determination of mental and psychomotor functions. It is based on the chronometric approach to the examination of dynamic properties and functional features of the activity of the central nervous system. The basic assumption of chronometry in psychology is that the time (duration) needed for a certain psychological activity contains information about the complexity of its structure and indicates the structure and validity of the functioning of neuropsychological mechanisms through which such activity is realized. In several earlier studies CRD was able to detect even the slightest cognitive and psychomotor changes. A strong correlation was found between the results of CRD-series tests and cognitive and psychomotor functions.^17–26

PATIENTS AND METHODS

Patients and Dialysis Protocol

In this cross-sectional study, cognitive–motor abilities were investigated in selected population of 22 patients on chronic HD and 20 patients on CAPD (15 females and 27 males, aged 50.31 ± 11.07 years) from the dialysis unit of the Department of Nephrology and Dialysis at University Hospital Split, Croatia. Dialysis treatment durations were 7.05 ± 4.27 years and 3.8 ± 1.54 years for HD and CAPD patients, respectively.

The study inclusion criteria for HD patients include: patients aged 18 years and older, stabile duration of chronic HD more than 2 years on three times weekly dialysis program before study entry, continuity of dialysis regimen, and stability during dialysis session.

All CAPD patients were aged 18 years and older and had stabile duration of chronic dialysis therapy more than 2 years receiving standard 2 L solution four times a day.

No subject (HD or CAPD patients) had a history of hospitalization, clinically evident cerebrovascular disease, evident depression, unstable coronary vascular disease, uncontrolled hypertension, alcoholism, brain injury, dementia according to the medical records, psychosis, autoimmune diseases, malignancies, liver disease, or other metabolic diseases leading to encephalopathy during 3 months prior to the study. None of the patients was receiving antibiotics, cytotoxic drugs, antihistamines, blood transfusions, corticosteroids, or other medications with known effects on neuropsychological functions during 3 months before including in the study. The target dialysis doses (Kt/V) were 1.2 and 2.0 for HD and CAPD patients, respectively.

The patients were informed about the purpose and nature of the study and gave written consent. The study protocol was accepted by the local University Hospital Ethics Committee according to the Declaration of Helsinki.

Dialysis Protocol

All HD patients were receiving conventional 4 h treatment, three times weekly, with bicarbonate solution and low-molecular-weight heparin as standard anticoagulation using low-flux polysulfone membranes F6 and F8 (Fresenius, Bad Homburg, Germany) with a blood flow rate of 250–300 mL/min. The dialysis bath consisted of bicarbonate 32–35 mmol/L, sodium 138–145 mmol/L, potassium 2 mmol/L, and calcium 1.25–1.5 mmol/L. Ultrafiltration (UF) was measured volumetrically on the dialysis machine.

CAPD patients were receiving standard 2 L solution four times a day.

Neuropsychological Tests

The neuropsychological tests used for this study were CRD-series tests and symbol digit modalities test (SDMT).

CRD-series consists of software and four computer-supported work instruments. The test procedure is noted online and guarantees an unbiased documentation. Among 34 standard tests available on the CRD-series, four representative tests (CRD-11, CRD-21, CRD-311, and CRD-324) covering a broad spectrum of mental and psychomotor processing were selected for the study. Test CRD-311 was used to measure simple visual discrimination of signal location, test CRD-324 was used to measure short-term memory actualization, test CRD-21 was used to measure simple convergent visual orientation, and test CRD-11 was used to measure convergent thinking. The sequence of testing was from the most simple to more complicated, that is, 311, 324, 21, and 11. The goal was to complete each test quickly and with as few errors as possible. If the subject gave an incorrect response, an error was counted, and the test would not proceed until the correct response was given.

CRD-series measures the time needed to perform and solve single task in the test, as well as the complete test. Total test solving time (TT) and minimal single task solving time (MT) were measured. TT and MT are descriptors of speed, reliability (accuracy), and mental endurance. In each of the four tests subjects were required to provide the correct answer by pressing the corresponding light-emitting diodes using their dominant hand as quickly as possible. The correct answer would automatically start the next single task. Higher CRD-series tests scores (TT and MT measured in seconds) indicate poorer cognitive and motor performance (tests took longer to complete). A more detailed description of CRD-series tests could be found elsewhere.^17,24

SDMT was selected on the basis of previous reports of sensitivity and also for its extensive use in the general population and renal patients.²⁷ The test was used in many previous studies of uremic patients.^28–30 This is a task that requires visual attention–concentration, scanning, and visual shifting for successful completion. Occulomotor abilities and hand–eye coordination are also involved. It consists of matching numbers and symbols as fast as possible within a time frame of 90 s with the number of correct matches being the score. The written form of SDMT was used in this study.

Laboratory Methods

Blood samples from subjects were taken under fasting conditions and for HD patients in a midweek predialysis state. Blood was taken just prior to connecting the subjects to the dialysis machine and before giving the heparin. Post-dialysis blood sample (for post-dialysis blood urea concentration) was taken with a blood pump reduced to 50 mL/min and then after 2 min blood specimen was taken from the arterial part of the dialysis system.

For patients treated with CAPD blood samples were taken under fasting conditions, after clinical visit, and cognitive testing.

Concentrations of serum urea, creatinine, calcium, phosphorus, albumin, total cholesterol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, hemoglobin, and post-dialysis urea were measured by standard laboratory techniques using an automatic analyzer (Olympus AU560, Hamburg, Germany). Intact parathyroid hormone was determined by immunoradiometric assay. C-reactive protein concentration was measured by tubulodimetric method. Plasma total homocysteine concentration was determined by the enzyme-linked immunosorbent assay method. Low-density lipoprotein cholesterol concentration was calculated using the Friedewald formula.

Study Design

In this cross-sectional design study, the cognitive tests were administered on a midweek dialysis day before the HD session for HD patients to minimize the effect of fluctuation in uremic symptoms or blood pressure. For CAPD patients the tests were administered during a clinical visit.

The efficiency of HD was assessed from the urea reduction rate, based on the formula (1−post-blood urea concentration/pre-blood urea concentration) × 100, and the delivered dose of dialysis (Kt/V urea) using a Daugirdas second generation model. The efficiency of CAPD was assessed using the program “Patient on line” v. 4.2 (Fresenius Medical Care, Bad Homburg, Germany).

Data of UF and erythropoietin administration usage per HD session were assessed from patient’s records 3 months retrospectively from the testing of cognitive functions (36 HD sessions per subject were assessed).

Statistical Analysis

Data were expressed as arithmetic mean ± SD with range. The significance of differences in means between the two groups was assessed by unpaired Student’s t-test. Correlations between variables were tested by Pearson correlation coefficients in the linear regression analysis. Statistical analysis was performed using SPSS 12.0 software for Windows (SPSS Software, Chicago, IL, USA). A one-tailed p-value of <0.05 was considered statistically significant.

RESULTS

Characteristics of all study subjects are shown in Table 1. Differences between HD and CAPD groups are shown in Table 2. Selected groups of HD and CAPD patients were not significantly different for important demographic variables, including age and level of education.

Table 1. Characteristics of all study subjects (N = 42).

	X ± SD	Minimum	Maximum
Education (years)	12.62 ± 2.19	8.00	16.00
Age (years)	50.31 ± 11.07	30.00	72.00
BMI (kg/m^2)	24.92 ± 3.78	19.05	37.58
Parathormone (μmol/L)	367.92 ± 340.13	30.90	1648.00
Calcium (mmol/L)	2.42 ± 0.18	1.83	2.80
Phosphorus (mmol/L)	2.08 ± 0.51	0.88	3.30
Ca × P (mmol^2/L^2)	5.04 ± 1.33	2.06	8.28
C-reactive protein (mg/L)	6.16 ± 8.47	0.30	32.90
Beta 2M (mg/L)	46.68 ± 15.93	1.60	88.60
Albumin (g/L)	39.60 ± 3.68	25.00	46.00
Urea (mmol/L)	19.87 ± 5.13	7.53	30.52
Creatinine (μmol/L)	1112.57 ± 215.25	574.00	1647.00
Triglycerides (mmol/L)	2.23 ± 0.99	0.50	5.30
Cholesterol (mmol/L)	5.03 ± 1.27	2.54	8.20
LDL (mmol/L)	3.01 ± 0.99	1.09	5.63
HDL (mmol/L)	1.05 ± 0.23	0.65	1.56
Homocysteine (μmol/L)	29.02 ± 14.27	7.00	69.00
Vitamin B12 (pmol/L)	503.79 ± 221.61	168.00	1177.00
Ferritin (mg/L)	208.30 ± 200.03	12.90	919.30
EPO_week (IU)	5203.25 ± 3992.10	0.00	12,000.00
Hemoglobin (g/L)	111.19 ± 14.98	95.00	152.00

Note: X ± SD, arithmetic mean ± standard deviation; BMI, body mass index; Ca × P, calcium × phosphorus; beta 2M, beta2-microglobulin; LDL, low-density lipoprotein cholesterol; HDL, high-density lipoprotein cholesterol; EPO_week, weekly dosage of erythropoietin.

Table 2. Characteristics of study subjects treated with HD (N = 22) compared with patients treated with CAPD (N = 20) (Student’s t-test for independent samples, two-tailed).

	HD X ± SD	CAPD X ± SD	p
Number of cigarettes	5.00 ± 7.95	6.75 ± 10.92	0.566
Duration of dialysis (years)	7.05 ± 4.27	3.80 ± 1.54	0.003
Age (years)	49.59 ± 11.64	51.10 ± 10.66	0.665
Education (years)	13.00 ± 2.02	12.20 ± 2.33	0.241
Parathormone (μmol/L)	466.20 ± 383.72	259.80 ± 251.82	0.048
Calcium (mmol/L)	2.36 ± 0.20	2.48 ± 0.14	0.033
Phosphorus (mmol/L)	2.03 ± 0.32	2.13 ± 0.67	0.545
BMI (kg/m^2)	24.01 ± 3.10	25.92 ± 4.26	0.104
Vitamin B12 (pmol/L)	476.18 ± 215.52	534.15 ± 229.76	0.404
Homocysteine (μmol/L)	29.86 ± 8.40	28.10 ± 18.96	0.694
Ca × P (mmol^2/L^2)	4.79 ± 0.81	5.30 ± 1.72	0.217
C-reactive protein (mg/L)	4.92 ± 5.93	7.53 ± 10.59	0.325
Beta 2M (mg/L)	53.87 ± 14.23	38.76 ± 14.06	0.001
Albumin (g/L)	40.82 ± 2.68	38.25 ± 4.19	0.022
Urea (mmol/L)	19.84 ± 4.35	19.91 ± 5.93	0.964
Creatinine (μmol/L)	1083.14 ± 159.43	1144.95 ± 264.16	0.359
Triglycerides (mmol/L)	2.20 ± 0.93	2.27 ± 1.08	0.839
Cholesterol (mmol/L)	4.63 ± 1.16	5.48 ± 1.26	0.028
LDL (mmol/L)	2.70 ± 0.83	3.32 ± 1.07	0.045
HDL (mmol/L)	0.98 ± 0.17	1.13 ± 0.27	0.036
Homocysteine (μmol/L)	29.86 ± 8.40	28.10 ± 18.96	0.694
Ferritin (mmol/L)	281.42 ± 230.91	131.51 ± 126.45	0.014
Fe (mmol/L)	10.23 ± 3.73	13.82 ± 5.00	0.012
EPO_week (IU)	4920.63 ± 4031.34	5500.00 ± 4032.76	0.648
Hemoglobin (g/L)	110.41 ± 17.95	112.05 ± 11.26	0.728
CRD-311 TT (s)	41.96 ± 9.27	39.30 ± 5.09	0.261
CRD-311 MT (s)	0.49 ± 0.11	0.47 ± 0.08	0.369
CRD-324 TT (s)	83.41 ± 38.74	66.33 ± 19.24	0.083
CRD-324 MT (s)	0.65 ± 0.18	0.63 ± 0.15	0.629
CRD-21 TT (s)	86.31 ± 52.35	75.75 ± 35.90	0.455
CRD-21 MT (s)	1.39 ± 0.61	1.39 ± 0.41	0.982
CRD-11 TT (s)	201.22 ± 76.33	190.10 ± 53.51	0.591
CRD-11 MT (s)	3.14 ± 0.90	3.29 ± 0.89	0.600
SDMT score	29.04 ± 11.70	34.33 ± 14.90	0.088

Notes: X ± SD, arithmetic mean ± standard deviation; BMI, body mass index; Ca × P, calcium × phosphorus; beta 2M, beta2-microglobulin; LDL, low-density lipoprotein cholesterol; HDL, high-density lipoprotein cholesterol; EPO_week, weekly dosage of erythropoietin; TT, total time of test solving; MT, minimal time of item solving.

*Denotes p < 0.05.

Table 3. Correlations between psychological parameters and serum concentrations of albumin, creatinine, and calcium (Pearson’s test of correlation, two-tailed) among all subjects.

	Albumin		Creatinine		Calcium
	r	p	r	p	r	p
CRD-311 TT	−0.253	0.105	−0.388	0.011	−0.235	0.134
CRD-311 MT	−0.325	0.036	−0.329	0.033	−0.348	0.024
CRD-324 TT	−0.308	0.048	−0.352	0.022	−0.489	0.001
CRD-324 MT	−0.189	0.230	−0.272	0.082	−0.135	0.395
CRD-21 TT	−0.179	0.256	−0.170	0.281	−0.207	0.189
CRD-21 MT	−0.173	0.274	−0.075	0.637	−0.124	0.434
CRD-11 TT	−0.264	0.091	−0.368	0.016	−0.272	0.082
CRD-11 MT	−0.428	0.005	−0.382	0.013	−0.218	0.166

Notes: r, Pearson’s correlation coefficient; TT, total time of test solving; MT, minimal time of item solving.

*Denotes p < 0.05.

The statistically significant difference between HD and CAPD patients was not found in any of the time-related parameters in all CRD-series tests or SDMT score in this study.

Correlations between cognitive–motor performance and other clinical measures were tested among all patients. The patients with higher albumin, creatinine, and calcium levels showed better performance in all cognitive–motor tests in overall study population. Cognitive–motor abilities improved with increased levels of albumin, creatinine, and calcium (Table 3).

Correlations between psychological parameters and serum concentrations of albumin and creatinine were tested in CAPD group. Among CAPD patients cognitive and motor performance improved with increased levels of albumin and creatinine (Table 4).

Table 4. Correlations between psychological parameters and serum concentrations of albumin, creatinine, and urea (Pearson’s test of correlation, two-tailed) in CAPD group.

	Albumin		Creatinine
	r	p	r	p
CRD-311 TT	−0.547	0.013	−0.460	0.041
CRD-311 MT	−0.510	0.022	−0.533	0.016
CRD-324 TT	−0.565	0.009	−0.591	0.006
CRD-324 MT	−0.126	0.597	−0.321	0.168
CRD-21 TT	−0.285	0.223	−0.277	0.237
CRD-21 MT	−0.219	0.353	−0.251	0.286
CRD-11 TT	−0.415	0.069	−0.522	0.018
CRD-11 MT	−0.498	0.026	−0.474	0.035

Notes: r, Pearson’s correlation coefficient; TT, total time of test solving; MT, minimal time of item solving.

*Denotes p < 0.05.

Table 5. Correlations between psychological parameters and serum concentrations of albumin, creatinine, and urea (Pearson’s test of correlation, two-tailed) in HD group.

	Creatinine		Albumin
	r	p	r	p
CRD-311 TT	−0.404	0.031	−0.269	0.113
CRD-311 MT	−0.139	0.269	−0.362	0.049
CRD-324 TT	−0.237	0.144	−0.491	0.012
CRD-324 MT	−0.225	0.157	−0.383	0.039
CRD-21 TT	−0.063	0.391	−0.232	0.149
CRD-21 MT	0.087	0.351	−0.187	0.202
CRD-11 TT	−0.262	0.120	−0.269	0.113
CRD-11 MT	−0.322	0.072	−0.355	0.052

Notes: r, Pearson’s correlation coefficient; TT, total time of test solving; MT, minimal time of item solving.

*Denotes p < 0.05.

In addition, among HD patients cognitive and motor abilities improved with increased serum levels of creatinine and albumin (Table 5).

Regardless of dialysis modality treatment, the statistically significant correlation between serum albumin level and cognitive and motor performance (convergent thinking test CRD-11 MT) was found. Cognitive and motor abilities improved with the increased level of albumin (Figure 1).

Figure 1. Correlation between serum albumin concentration and convergent thinking test scores (CRD-11 MT) among all patients (r = –0.428, p = 0.002) depicted as plot of linear regression.

The results from this study showed the influence of UF rate on cognitive–motor functions in HD patients. Therefore, HD patients with higher UF rate showed worse performance in cognitive and motor test (short-term memory actualization test, CRD-324 MT), and cognitive–motor function was impaired with the increased level of UF (Figure 2).

Figure 2. Correlation between ultrafiltration (UF) per HD and short-term memory actualization test score (CRD-324 MT) among HD patients (r = 0.434, p = 0.025) depicted as plot of linear regression.

DISCUSSION

While it is sufficiently well documented that the ESRD has been linked with change in cognitive function ^2,31 and that the cognitive impairment is associated with severity of CKD^3,4 little is known about the influence of different dialysis modalities on cognitive function. Previous investigations of cognitive function among ESRD patients demonstrated that CAPD patient had consistently better cognitive function than HD patients.^14–16

In contrast to previous studies, the subjects in this study consisted of patients with ESRD with no clinically evident occurrence of cognitive impairment or dementia. Therefore, cognitive and motor functions were assessed in detail using measures of simple visual discrimination of signal location, short-term memory actualization, simple convergent visual orientation, and convergent thinking. Importantly, selected groups of HD and CAPD patients were not significantly different for important demographic variables, including age and level of education. In addition, those patients who used psychoactive drugs were excluded from the study.

In this study CRD-series was used for the first time in clinical practice for evaluation of cognitive–motor functions in dialysis patients. Until now, CRD has been used in professional medicine, medical research, and occupational health and in clinical settings with anemic patients.²⁶

In contrast to previous studies that compared cognitive function in HD versus CAPD patients, the statistically significant difference between HD and CAPD patients was not found in any of the time-related parameters in all CRD-series tests or SDMT score in this study.

Wolcott et al.¹⁴ reported that CAPD patients had consistently more efficient cognitive function than HD patients. No cognitive function differences were found in groups categorized by sex or duration of dialysis and higher creatinine levels were associated with better cognitive function. Serum calcium, CO₂, total protein, albumin, and SGOT levels were also correlated with neuropsychological scores. In a study with HD patients, higher mini mental state exam score was associated with higher serum albumin level, protein catabolic rate, and interdialysis weight gain.³² Similarly, results from this study showed significant inverse correlations between serum concentrations of albumin, creatinine, calcium and cognitive and motor performance among all study subjects. These inverse correlations with CRD-series test scores indicate improving cognitive and motor performance (lower tests scores in seconds) with increase in laboratory values of albumin, creatinine, and calcium.

It is well known that as patients progress through the stages of CKD nutritional requirements are altered and metabolism of protein is affected.³³ Serum albumin is the most extensively studied nutritional marker in these patients due to its easy availability and strong association with hospitalization and risk of death.³⁴ According to these studies serum albumin is strongly associated with cognitive performance in patient with ESRD. A few population-based studies have suggested that low albumin may be an independent risk factor for poor cognitive status and dementia.^35,36 In addition, in this study inverse correlation of albumin serum level with CRD-series test scores indicates improving CRD-series test performance with increase in the laboratory value of albumin.

Buoncristiani et al.¹⁵ concluded that the most likely reason for the better preservation of the cognitive function in CAPD patients could be due to the continuity of action, the better anemia correction, more removal of the middle molecules, or lower levels of parathyroid hormone, which in some studies on HD patients have been correlated with alteration of cognitive function.^37,38 The mechanism underlying this association is not well understood. But, correlation between parathyroid hormone and cognitive–motor abilities was not found in this study.

In modern renal replacement treatment, recombinant human erythropoietin is an unavoidable drug for correcting anemia. Dialysis patients are almost uniformly anemic secondary to lack of erythropoietin production by the kidneys. The correction of anemia in ESRD patient has been shown to improve their quality of life and cognitive functions.^39,40 The discovery of the neuroprotective effects of erythropoietin in preclinical studies has been followed by studies in healthy volunteers where erythropoietin was reported to modulate cognitive function—independent of its hematopoietic effects—exerting beneficial effects on memory retrieval, emotional processing, mood, and verbal fluency.⁴¹ Kutlay et al.³² found no relationship between mini mental state exam score and years on dialysis, Kt/V value, hematocrit value, or erythropoietin use in HD patients. Similarly, no relationship between any of the time-related parameters in CRD-series tests and duration on dialysis, Kt/V, hemoglobin, or weekly dose of erythropoietin was found in HD or CAPD group.

The results from this study showed a relationship between UF per kilogram of body mass per single HD session and some of the psychological parameters among HD patients: simple visual discrimination of signal location test score (CRD-311 MT), short-term memory actualization test score (CRD-324 MT), and simple convergent visual orientation test score (CRD-21 TT).

This correlation of UF with CRD-series tests scores indicates worsening of cognitive and motor performance with increase in UF rates.

It is well known that HD patients are subject to acute hemodynamic changes and large fluid shifts. The rapid fluctuations in blood pressure, the removal of large fluid volumes, and hemoconcentration during HD increase the risk of inducing cerebral hypoperfusion.⁴² Significant decreases in blood velocity after HD compared with immediately before were found in studies using transcranial Doppler ultrasound of the carotid, basilar, and middle cerebral arteries.^43–45 These studies suggest that HD may induce acute recurrent cerebral ischemia, accompanying hypoxemia, and potentially secondary delirium. In addition, results from these studies are compatible with results from this study that large UF and large fluid shifts during HD may increase the risk of change in cognitive and motor functions.

In conclusion, as presumed earlier⁴⁶ this study has demonstrated that well-nourished and medically stable HD or CAPD patients without clinical signs of dementia or cognitive impairment performed all tests of CRD-series and SDMT without statistically significant difference between two modalities of treatment. It is also clear that removing a large amount of fluid may have influence on cognitive and motor performance in HD patients by probably inducing cerebral hypoperfusion.

The differences between HD and CAPD patients in previous studies might be found probably because of poorly matched CAPD and HD groups of patients and selection bias in the choice of dialysis modality.

Future investigations on cognitive–motor functions in uremic patients treated with HD and CAPD are needed, with larger number of participants in a prospective research model. Increased awareness of the effects of cognitive impairment on daily function, quality of life, medication, fluid, and dietary compliance among clinicians is needed.

This study also showed that CRD system is simple and sensitive test to assess cognitive–motor abilities in patients with ESRD. In addition, CRD system is not dependent on language and cultural or educational background and can therefore be used and compared with other neuropsychological tests used in previous work in different countries for investigating the complexity of cognitive–motor ability in uremic patients.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.