Table 1 Evidence tables of data extracted of included studies

Althbuaity et al., 2021

Saudia Arabia

Cross-sectional study

Population(s)

Type 2 diabetes

Control(s)

Not applicable (NA)

Numbers: 179

Mean Age: 58.6 ± 9.6 yrs.

Male: Reported as “almost equally distributed according to gender”

Diabetes type: 179 (100%) T2

Diabetes duration: 11–16 yrs. -84 (47.0%)

Diabetes-related lower extremity complications (DRLECs):

PN: 123 (68.7%)

Peripheral arterial disease (PAD): Not reported (NR)

DFU History: NR

DFU: 75 (47.5%)

AMP: 7 (3.9%)

Other: NR

Diabetic polyneuropathy (DPN)

Montreal Cognitive Assessment (MoCA)-Arabic version

No association found

Blanquisco et al., 2017

Philippine

Cross-sectional study

Population(s)

Type 2 diabetes

Control(s) - NA

Numbers: 179 (With MCI – n = 60; Without MCI – n = 73)

Mean Age: 67 ± 4.8 yrs.

Male: 40 (29.1%)

Diabetes type: 179 (100%) T2

Diabetes duration (IQR) 12 ± 12 yrs.

DRLECs:

PN: n = 22 (16.5%)

PAD: NR

DFU History: NR

DFU: NR

AMP: NR

Other: NR

Peripheral neuropathy (PN)

MoCA – Philippine version

No association found

Brismar et al., 2007

Sweden

Cross-sectional study

Population(s)

Type 1 diabetes

Control(s) - NA

Numbers: 150

Mean Age: 43 ± 7.8 yrs.

Male: 44.0%

Diabetes type: n = 150 (100%) T1

Diabetes duration − 26.6 ± 11.4 yrs.

DRLECs:

PN – Neurological impairment assessment (NIA score) > 7 (70%)

PAD: NR

DFU History: NR

DFU: NR

AMP: NR

Other: NR

NA

The 10 cognitive sub-domains domains and global cognition score

Correlation found [Neurological impairment assessment (NIA - PN) vs. Cognition scores]

de Bresser et al., 2010

Netherland

Not reported (NR) (Cohort study with 4-year follow-up)

Population(s)

Type 2 diabetes

Control(s) - NA

Numbers: 68

Mean Age: 65.6 ± 5.6 yrs.

Male: 32 (47.0%)

Diabetes type: 68 (100%) T2

DRLECs:

PN = 24 (36%)

PAD: NR

DFU History: NR

DFU: NR

AMP: NR

Other: NR

PN

Composite cognition score:

11 verbal and nonverbal tasks that cover five cognitive sub-domains

No association found at baseline &

No difference found between baseline & follow-up.

Dejgaard et al., 1991

Denmark

NR

(Case-control study)

Population(s)

Type 1 diabetes (Long duration with PN vs. Short duration without PN)

Control(s) - Healthy non-diabetes subjects

Numbers: 159

Ca: LD − 20

SD − 19

Co: 120

Mean Age:

Ca: LD − 44.0 ± 9.0 yrs.

SD − 23.0 ± 8.0 yrs.

p = NR

Co: NR

Male:

Ca: NR for both LD and SD but was reported as sex-matched

Co: NR

Diabetes type:

Ca: LD − 20 (100%) T1

SD − 19 (100%) T1

Diabetes duration

Ca: LD – 26.0 ± 9.0 yrs.

SD – 2.0 ± 2.0 yrs.

DRLECs: (Ca only reported)

PN –

Ca: LD = 20 (100%)

SD = 0

PAD: NR

DFU History: NR

DFU: NR

AMP: NR

Other: NR

PN

17 Neuropsychological examination for intelligence and cognition

Association found

Ding et al., 2019

China

Case-control study

Population(s)

Type 1 diabetes

Control(s) - Healthy volunteers

Numbers: 118

Ca: 70

Co: 48

Mean Age:

Ca: 32.1 ± 9.5 yrs.

Co: 31.5 ± 7.1 yrs.

p = 0.108

Male:

Ca: 37 (52.9%)

Co: 25 (52.1%)

p = 0.934

Diabetes type:

Ca: 70 (100%) T1

Co: 0

DRLECs: (Ca)

PN = NR

PAD: NR

DFU History: NR

DFU: NR

AMP: NR

Other: NR

NA

Mini-Mental State Examination (MMSE) and MoCA

Association found nerve conduction velocity (NCV) of Sural Nerves for PN and cognitive impairment

El-Tamawy et al., 2022

Egypt

Case-control study

Population:

People with T 2 diabetes

Control(s):

Age, sex and education level matched people with T2 diabetes without DPN

Numbers: 40

Ca: 20

Co: 20

Mean Age:

Ca: NR

Co: NR

p = NR

Male:

Ca: 11 (55.0%)

Co: 13 (65.0%)

p > 0.05

Diabetes type:

Ca: 20 (100%) T2

Co: 20 (100%) T2

DRLECs: Ca

PN – 20 (100%)

PAD: NR

DFU History: NR

DFU: Excluded

AMP: Excluded

Other: NR

DPN

RehaCom procedure

Difference found

Moreira et al., 2015

Brazil

Case-control study

Population:

People with T 2 diabetes

Control(s):

Age, sex and education level matched healthy cohort

Numbers: 148

Ca: 94 (DPN⁺ – 45/ DPN^- – 48)

Co: 54

Median Age:

Ca: 65.8 (61–84) yrs.

Co: 66.4 (60–75) yrs.

p = 0.210

Male:

Ca: 2 (3.7%)

Co: 7 (7.4%)

p = 0.480

Diabetes type:

Ca: 94 (100%) T2

Co: 0

DRLECs: Ca

PN – 45 (47.8%) -

PAD: NR

DFU History: NR

DFU: NR

AMP: NR

Other: NR

DPN

MMSE - Portuguese Version.

Trail marker test - TMT (A & B)

Verbal Fluency Test (VFT - Animals)

No difference found

Natovich et al., 2016

Israel

Case-control study

Population:

People with T2 diabetes

Control(s): Sex and duration of diabetes matched

people with T2 diabetes without DFUs

Numbers: 194

Ca: 99

Co: 95

Mean Age:

Ca: 58.0 ± 6.9 yrs.

Co: 61.3 ± 7.0 yrs.

p < 0.001

Male:

Ca: 76 (77%)

Co: 76 (80%)

p > 0.05

Diabetes type:

Ca: 99 (100%) T2

Co: 95 (100%) T2

DRLECs:

PN –

Ca − 88.9%

Co − 15.8%

PAD: NR

DFU History: NR

DFU:

Ca – 100%

Co – 0%

AMP: NR

Other: Macrovascular -

Ca − 88.9%

Co − 51.6%

DFU

Computerised neuropsychological battery of tests (Neuro-Trax) and paper-and-pencil tests

GCS (Global cognitive score)

Difference found

Perlmuter et al., 1984

America

Cross-sectional study

Population:

People with T2 diabetes

Control(s):

People without diabetes with normal fasting glucose, normal random blood sugar and normal HbA1c levels

Numbers: 178

Ca: 140

Co: 38

Mean Age:

Ca: 64.3 ± 5.4

Co: 63.1 ± 5.2

Male:

Ca: 54%

Co: 43%

Diabetes type:

Ca: 140 (100%) T2

Co: 0

DRLECs:

PN - NR

PAD: NR

DFU History: NR

DFU: NR

AMP: NR

Other: NR

NR

Serial Learning Task

Digit Span Test

Correlation found (Percentage words correct) vs. PN

Roman de et al., 2013

Belgium

Cross-sectional study

Population:

People with T2 diabetes (including with and without PN)

Control(s):

Age and sex-matched healthy older adults (without T2 diabetes)

Numbers: 101

Ca: 56 (PN⁺= 28; PN^-= 28)

Co: 45

Mean Age:

Ca: PN⁺= 74.8 ± 7.5 yrs.

PN^- = 74.1 ± 8.2 yrs.

Co: 71.3 ± 8.1 yrs.

p > 0.05

Male:

Ca: PN⁺ − 15 (53.6%)

PN^- − 10 (35.7%)

Co: 20 (44.4%)

p > 0.05

Diabetes type:

Ca: 101 (100%) T2

Co: 0

DRLECs: Ca

PN – 28 (50%)

PAD: NR

DFU History: NR

DFU: NR

AMP: NR

Other: NR

PN

MMSE

The clock drawing test (CDT)

MMSE-CDT

Difference found

Zhang et al., 2021

China

Case-control study

Population:

People with T2 diabetes

Control(s):

Demographically matched healthy control

Numbers: 108

Ca: 67 (T2_C: 37; T2_NC: 30)

Co: 41

Mean Age:

Ca:

T2_C: 53.8 ± 1.1 yrs.

T2_NC: 55.0 ± 1.0 yrs.

Co: 53.9 ± 0.8 yrs.

p = 0.467

Male:

Ca:

T2_C: 19 (51%)

T2_NC: 17 (56%)

Co: 22 (53%)

p = 0.442

Diabetes type:

Ca: 67 (100%) T2

Co: 0

DRLECs: Ca

PN – T2_C: 37 (100%)

PAD: NR

DFU History: NR

DFU: NR

AMP: NR

Other: NR

With and without diabetes-related Complications (PN)

MMSE

MoCA

CDT

TMT

Difference found

Zhao et al., 2021

China

Case-control study

Population:

People with T 2 diabetes

Control(s):

Age and sex-matched healthy control

Numbers: 106

Ca: 66 (DPN⁺= 29; DPN^-= 37)

Co: 40

Mean Age:

Ca: DPN⁺= 56.7 ± 11.3 yrs.

DPN^- = 56.1 ± 9.4 yrs.

Co: 52.4 ± 1.3 yrs.

p > 0.05

Male:

Ca: 19 (46.9%)

Co: 31 (47.5%)

p > 0.05

Diabetes type:

Ca: 101 (100%) T2

Co: 0

DRLECs: Ca

PN: 29 (100%)

PAD: NR

DFU History: NR

DFU: NR

AMP: NR

Other: NR

DPN

MMSE

No difference found