J Neurol Res

Journal of Neurology Research, ISSN 1923-2845 print, 1923-2853 online, Open Access

Article copyright, the authors; Journal compilation copyright, J Neurol Res and Elmer Press Inc

Journal website https://www.neurores.org

Original Article

Volume 13, Number 1, September 2023, pages 33-42

Seasonal Variation in Ischemic Stroke Hospitalization: Results From a Large Health System in Six Western States of the United States

Figure 2. Seasonal pattern of daily acute ischemic stroke hospitalization in six western states of the USA. AIS: acute ischemic stroke.

**Table 1.** Patient Characteristics by the Season of Admission for Ischemic Stroke
Variables	LVO	Non-LVO	Total population	Spring	Summer	Fall	Winter	P
^aMultiple comorbidities allowed. IQR: interquartile range; LVO: large vessel occlusion.
Age (years), median (IQR)	74.0 (20, 103) (21)	73.0 (18, 108) (21)	73.0 (18, 108) (21)	73.0 (18, 106) (20)	72.0 (21, 108) (20)	73.0 (19, 103) (21)	73.0 (19, 107) (21)	0.25
Sex (female), n (%)	2,182 (49.8%)	4,585 (47.9%)	9,192 (48.7%)	2,207 (48.3%)	2,145 (47.9%)	2,291 (48.8%)	2,549 (49.5%)	0.40
Race, n (%)
Native American/Alaska Native	39 (0.9%)	79 (0.8%)	166 (0.9%)	52 (1.1%)	28 (0.6%)	42 (0.9%)	44 (0.9%)	0.22
Asian	285 (6.5%)	523 (5.5%)	1,069 (5.7%)	267 (5.8%)	274 (6.1%)	237 (5.1%)	291 (5.7%)
Black/African American	168 (3.8%)	397 (4.1%)	771 (4.1%)	178 (3.9%)	189 (4.2%)	192 (4.1%)	212 (4.1%)
Native Hawaiian/Pacific Islander	24 (0.5%)	52 (0.5%)	108 (0.6%)	23 (0.5%)	29 (0.6%)	28 (0.6%)	28 (0.5%)
Unable to determine	430 (9.8%)	783 (8.2%)	1,628 (8.6%)	362 (7.9%)	378 (8.4%)	435 (9.3%)	453 (8.8%)
White	3,436 (78.4%)	7,737 (80.8%)	15,144 (80.2%)	3,683 (80.7%)	3,584 (80.0%)	3,758 (80.1%)	4,119 (80.0%)
Center certification, n (%)
None	340 (7.8%)	687 (7.2%)	1,214 (6.4%)	289 (6.3%)	255 (5.7%)	305 (6.5%)	365 (7.1%)	0.27
Comprehensive stroke center	1,804 (41.2%)	3,317 (34.7%)	6,594 (34.9%)	1,617 (35.4%)	1,553 (34.6%)	1,624 (34.6%)	1,800 (35.0%)
Primary stroke center	1,217 (27.8%)	3,495 (36.5%)	6,864 (36.3%)	1,627 (35.6%)	1,677 (37.4%)	1,701 (36.3%)	1,859 (36.1%)
Thrombectomy capable center	1,021 (23.3%)	2,072 (21.6%)	4,214 (22.3%)	1,032 (22.6%)	997 (22.2%)	1,062 (22.6%)	1,123 (21.8%)
Region, n (%)
Alaska	168 (3.8%)	436 (4.6%)	923 (4.9%)	202 (4.4%)	275 (6.1%)	233 (5.0%)	213 (4.1%)	< 0.001
Montana/Washington/Oregon	2,261 (51.6%)	4,721 (49.3%)	9,939 (52.6%)	2,499 (54.7%)	2,383 (53.2%)	2,484 (52.9%)	2,573 (50.0%)
California/Texas	1,953 (44.6%)	4,414 (46.1%)	8,024 (42.5%)	1,864 (40.8%)	1,824 (40.7%)	1,975 (42.1%)	2,361 (45.9%)
Comorbidity^a, n (%)
Atrial fibrillation	1,422 (32.5%)	1,950 (20.4%)	4,241 (22.5%)	1,042 (22.8%)	941 (21.0%)	1,085 (23.1%)	1,173 (22.8%)	0.06
Diabetes mellitus	1,122 (25.6%)	3,029 (31.6%)	5,910 (31.3%)	1,394 (30.5%)	1,443 (32.2%)	1,481 (31.6%)	1,592 (30.9%)	0.34
Hypertension	3,137 (71.6%)	7,134 (74.5%)	14,039 (74.3%)	3,382 (74.1%)	3,327 (74.2%)	3,506 (74.7%)	3,824 (74.3%)	0.91
Dyslipidemia	2,161 (49.3%)	5,002 (52.3%)	9,828 (52.0%)	2,394 (52.4%)	2,325 (51.9%)	2,401 (51.2%)	2,708 (52.6%)	0.49
Previous stroke	974 (22.2%)	2,666 (27.9%)	4,986 (26.4%)	1,224 (26.8%)	1,144 (25.5%)	1,265 (27.0%)	1,353 (26.3%)	0.40
Obesity/overweight	2,175 (49.6%)	4,842 (50.6%)	9,606 (50.9%)	2,287 (50.1%)	2,274 (50.7%)	2,391 (51.0%)	2,654 (51.6%)	0.55

Table 1. Patient Characteristics by the Season of Admission for Ischemic Stroke

Variables

LVO

Non-LVO

Total population

Spring

Summer

Fall

Winter

4,382

9,571

18,886 (100)

4,565 (24.2)

4,482 (23.7)

4,692 (24.8)

5,247 (27.3)

^aMultiple comorbidities allowed. IQR: interquartile range; LVO: large vessel occlusion.

Age (years), median (IQR)

74.0 (20, 103) (21)

73.0 (18, 108) (21)

73.0 (18, 106) (20)

72.0 (21, 108) (20)

73.0 (19, 103) (21)

73.0 (19, 107) (21)

0.25

Sex (female), n (%)

2,182 (49.8%)

4,585 (47.9%)

9,192 (48.7%)

2,207 (48.3%)

2,145 (47.9%)

2,291 (48.8%)

2,549 (49.5%)

0.40

Race, n (%)

Native American/Alaska Native

39 (0.9%)

79 (0.8%)

166 (0.9%)

52 (1.1%)

28 (0.6%)

42 (0.9%)

44 (0.9%)

0.22

Asian

285 (6.5%)

523 (5.5%)

1,069 (5.7%)

267 (5.8%)

274 (6.1%)

237 (5.1%)

291 (5.7%)

Black/African American

168 (3.8%)

397 (4.1%)

771 (4.1%)

178 (3.9%)

189 (4.2%)

192 (4.1%)

212 (4.1%)

Native Hawaiian/Pacific Islander

24 (0.5%)

52 (0.5%)

108 (0.6%)

23 (0.5%)

29 (0.6%)

28 (0.6%)

28 (0.5%)

Unable to determine

430 (9.8%)

783 (8.2%)

1,628 (8.6%)

362 (7.9%)

378 (8.4%)

435 (9.3%)

453 (8.8%)

White

3,436 (78.4%)

7,737 (80.8%)

15,144 (80.2%)

3,683 (80.7%)

3,584 (80.0%)

3,758 (80.1%)

4,119 (80.0%)

Center certification, n (%)

None

340 (7.8%)

687 (7.2%)

1,214 (6.4%)

289 (6.3%)

255 (5.7%)

305 (6.5%)

365 (7.1%)

0.27

Comprehensive stroke center

1,804 (41.2%)

3,317 (34.7%)

6,594 (34.9%)

1,617 (35.4%)

1,553 (34.6%)

1,624 (34.6%)

1,800 (35.0%)

Primary stroke center

1,217 (27.8%)

3,495 (36.5%)

6,864 (36.3%)

1,627 (35.6%)

1,677 (37.4%)

1,701 (36.3%)

1,859 (36.1%)

Thrombectomy capable center

1,021 (23.3%)

2,072 (21.6%)

4,214 (22.3%)

1,032 (22.6%)

997 (22.2%)

1,062 (22.6%)

1,123 (21.8%)

Region, n (%)

Alaska

168 (3.8%)

436 (4.6%)

923 (4.9%)

202 (4.4%)

275 (6.1%)

233 (5.0%)

213 (4.1%)

< 0.001

Montana/Washington/Oregon

2,261 (51.6%)

4,721 (49.3%)

9,939 (52.6%)

2,499 (54.7%)

2,383 (53.2%)

2,484 (52.9%)

2,573 (50.0%)

California/Texas

1,953 (44.6%)

4,414 (46.1%)

8,024 (42.5%)

1,864 (40.8%)

1,824 (40.7%)

1,975 (42.1%)

2,361 (45.9%)

Comorbidity^a, n (%)

Atrial fibrillation

1,422 (32.5%)

1,950 (20.4%)

4,241 (22.5%)

1,042 (22.8%)

941 (21.0%)

1,085 (23.1%)

1,173 (22.8%)

0.06

Diabetes mellitus

1,122 (25.6%)

3,029 (31.6%)

5,910 (31.3%)

1,394 (30.5%)

1,443 (32.2%)

1,481 (31.6%)

1,592 (30.9%)

0.34

Hypertension

3,137 (71.6%)

7,134 (74.5%)

14,039 (74.3%)

3,382 (74.1%)

3,327 (74.2%)

3,506 (74.7%)

3,824 (74.3%)

0.91

Dyslipidemia

2,161 (49.3%)

5,002 (52.3%)

9,828 (52.0%)

2,394 (52.4%)

2,325 (51.9%)

2,401 (51.2%)

2,708 (52.6%)

0.49

Previous stroke

974 (22.2%)

2,666 (27.9%)

4,986 (26.4%)

1,224 (26.8%)

1,144 (25.5%)

1,265 (27.0%)

1,353 (26.3%)

0.40

Obesity/overweight

2,175 (49.6%)

4,842 (50.6%)

9,606 (50.9%)

2,287 (50.1%)

2,274 (50.7%)

2,391 (51.0%)

2,654 (51.6%)

0.55

**Table 2.** Estimation Results From the Generalized Estimating Equations (GEE) Model, the Association of Season With the Number of Acute Ischemic Stroke Admission
Variables	β	95% CI	P
^aMultiple comorbidities allowed. CI: confidence interval; β: beta coefficient.
Unadjusted model
Spring	Reference
Summer	-0.05	-0.86, 0.76	0.94
Fall	0.84	0.05, 1.6	0.04
Winter	3.60	2.7, 4.4	< 0.001
Adjusted model
Age (years)	0.00	-0.01, 0.01	0.91
Sex
Male	Reference	-0.16, 0.12	0.80
Female	-0.02
Race
Native American/Alaska Native	Reference	-1.3, 0.25	0.20
Asian	-0.52	-1.0, 0.49	0.50
Black/African American	-0.27	-1.9, 0.48	0.20
Native Hawaiian/Pacific Islander	-0.69	-0.69, 0.74	0.92
Unable to determine	0.02	-1.1, 0.32	0.30
White	-0.37	-1.3, 0.25	0.20
Center certification
None	Reference
Comprehensive stroke center	0.11	-0.21, 0.43	0.50
Primary stroke center	0.94	0.60, 1.3	< 0.001
Thrombectomy capable center	0.59	0.23, 1.0	0.001
Region
Alaska	Reference
Montana/Washington/Oregon	0.41	0.05, 0.77	0.03
California/Texas	1.30	0.92, 1.7	< 0.001
Comorbidity^a
Atrial fibrillation	-0.14	-0.34, 0.05	0.20
Diabetes mellitus	0.02	-0.13, 0.17	0.80
Hypertension	-0.14	-0.32, 0.03	0.11
Dyslipidemia	0.29	0.13, 0.46	< 0.001
Previous stroke	-0.24	-0.40, -0.08	0.004
Obesity/overweight	0.53	0.36, 0.69	< 0.001
Seasons
Spring	Reference
Summer	-0.10	-0.92, 0.71	0.80
Fall	0.66	-0.14, 1.5	0.10
Winter	3.30	2.4, 4.1	< 0.001

Table 2. Estimation Results From the Generalized Estimating Equations (GEE) Model, the Association of Season With the Number of Acute Ischemic Stroke Admission

Variables

95% CI

^aMultiple comorbidities allowed. CI: confidence interval; β: beta coefficient.

Unadjusted model

Spring

Reference

Summer

-0.05

-0.86, 0.76

0.94

Fall

0.84

0.05, 1.6

0.04

Winter

3.60

2.7, 4.4

< 0.001

Adjusted model

Age (years)

0.00

-0.01, 0.01

0.91

Sex

Male

Reference

-0.16, 0.12

0.80

Female

-0.02

Race

Native American/Alaska Native

Reference

-1.3, 0.25

0.20

Asian

-0.52

-1.0, 0.49

0.50

Black/African American

-0.27

-1.9, 0.48

0.20

Native Hawaiian/Pacific Islander

-0.69

-0.69, 0.74

0.92

Unable to determine

0.02

-1.1, 0.32

0.30

White

-0.37

-1.3, 0.25

0.20

Center certification

None

Reference

Comprehensive stroke center

0.11

-0.21, 0.43

0.50

Primary stroke center

0.94

0.60, 1.3

< 0.001

Thrombectomy capable center

0.59

0.23, 1.0

0.001

Region

Alaska

Reference

Montana/Washington/Oregon

0.41

0.05, 0.77

0.03

California/Texas

1.30

0.92, 1.7

< 0.001

Comorbidity^a

Atrial fibrillation

-0.14

-0.34, 0.05

0.20

Diabetes mellitus

0.02

-0.13, 0.17

0.80

Hypertension

-0.14

-0.32, 0.03

0.11

Dyslipidemia

0.29

0.13, 0.46

< 0.001

Previous stroke

-0.24

-0.40, -0.08

0.004

Obesity/overweight

0.53

0.36, 0.69

< 0.001

Seasons

Spring

Reference

Summer

-0.10

-0.92, 0.71

0.80

Fall

0.66

-0.14, 1.5

0.10

Winter

3.30

2.4, 4.1

< 0.001

**Table 3.** Estimation Results From the Generalized Estimating Equations (GEE) Model, the Association of Season With the Number of Acute Ischemic Stroke Subtypes Admission
Variables	Large vessel occlusion	Non-large vessel occlusion
^aMultiple comorbidities allowed. CI: confidence interval; β: beta coefficient.
Unadjusted model
Spring	Reference			Reference
Summer	-0.11	-0.94, 0.71	0.80	-0.10	-0.88, 0.68	0.80
Fall	0.73	-0.10, 1.6	0.08	0.80	0.04, 1.6	0.04
Winter	3.40	2.6, 4.2	< 0.001	3.50	2.6, 4.3	< 0.001
Adjusted model
Age (years)	0.00	-0.01, 0.01	0.91	0.01	-0.00, 0.01	0.2
Sex
Male	Reference	-0.16, 0.12	0.80	Reference
Female	-0.02			0.10	-0.11, 0.30	0.40
Race, n (%)
Native American/Alaska Native	Reference	-1.3, 0.25	0.20	Reference
Asian	-0.52	-1.0, 0.49	0.50	-0.43	-1.45, 0.601	0.40
Black/African American	-0.27	-1.9, 0.48	0.20	0.06	-1.06, 1.17	0.92
Native Hawaiian/Pacific Islander	-0.69	-0.69, 0.74	0.92	-0.56	-2.33, 1.22	0.50
Unable to determine	0.02	-1.1, 0.32	0.30	0.42	-0.62, 1.46	0.40
White	-0.37	-1.3, 0.25	0.20	-0.23	-1.22, 0.758	0.60
Center certification
None	Reference			Reference
Comprehensive stroke center	0.11	-0.21, 0.43	0.50	-0.37	-0.81, 0.08	0.11
Primary stroke center	0.94	0.60, 1.3	< 0.001	0.62	0.16, 1.08	0.01
Thrombectomy capable center	0.59	0.23, 1.0	0.001	0.36	-0.14, 0.86	0.20
Region
Alaska	Reference			Reference
Montana/Washington/Oregon	0.41	0.05, 0.77	0.03	0.07	-0.43, 0.58	0.80
California/Texas	1.30	0.92, 1.7	< 0.001	1.25	0.72, 1.78	< 0.001
Comorbidity^a
Atrial fibrillation	-0.14	-0.34, 0.05	0.20	-0.26	-0.54, 0.02	0.06
Diabetes mellitus	0.02	-0.13, 0.17	0.80	-0.14	-0.36, 0.09	0.20
Hypertension	-0.14	-0.32, 0.03	0.11	-0.23	-0.48, 0.02	0.07
Dyslipidemia	0.29	0.13, 0.46	< 0.001	0.49	0.27, 0.72	< 0.001
Previous stroke	-0.24	-0.40, -0.08	0.004	-0.31	-0.53, -0.08	0.007
Obesity/overweight	0.53	0.36, 0.69	< 0.001	0.66	0.45, 0.87	< 0.001
Season
Spring	Reference			Reference
Summer	-0.13	-1.10, 0.81	0.80	-0.14	-0.95, 0.66	0.70
Fall	0.56	-0.41, 1.5	0.30	0.712	-0.084, 1.51	0.08
Winter	3.20	2.20, 4.10	< 0.001	3.25	2.40, 4.10	< 0.001

Table 3. Estimation Results From the Generalized Estimating Equations (GEE) Model, the Association of Season With the Number of Acute Ischemic Stroke Subtypes Admission

Variables

Large vessel occlusion

Non-large vessel occlusion

Adjusted model

95% CI

P value

95% CI

P value

^aMultiple comorbidities allowed. CI: confidence interval; β: beta coefficient.

Unadjusted model

Spring

Reference

Summer

-0.11

-0.94, 0.71

0.80

-0.10

-0.88, 0.68

0.80

Fall

0.73

-0.10, 1.6

0.08

0.80

0.04, 1.6

0.04

Winter

3.40

2.6, 4.2

< 0.001

3.50

2.6, 4.3

< 0.001

Adjusted model

Age (years)

0.00

-0.01, 0.01

0.91

0.01

-0.00, 0.01

0.2

Sex

Male

Reference

-0.16, 0.12

0.80

Reference

Female

-0.02

0.10

-0.11, 0.30

0.40

Race, n (%)

Native American/Alaska Native

Reference

-1.3, 0.25

0.20

Reference

Asian

-0.52

-1.0, 0.49

0.50

-0.43

-1.45, 0.601

0.40

Black/African American

-0.27

-1.9, 0.48

0.20

0.06

-1.06, 1.17

0.92

Native Hawaiian/Pacific Islander

-0.69

-0.69, 0.74

0.92

-0.56

-2.33, 1.22

0.50

Unable to determine

0.02

-1.1, 0.32

0.30

0.42

-0.62, 1.46

0.40

White

-0.37

-1.3, 0.25

0.20

-0.23

-1.22, 0.758

0.60

Center certification

None

Reference

Comprehensive stroke center

0.11

-0.21, 0.43

0.50

-0.37

-0.81, 0.08

0.11

Primary stroke center

0.94

0.60, 1.3

< 0.001

0.62

0.16, 1.08

0.01

Thrombectomy capable center

0.59

0.23, 1.0

0.001

0.36

-0.14, 0.86

0.20

Region

Alaska

Reference

Montana/Washington/Oregon

0.41

0.05, 0.77

0.03

0.07

-0.43, 0.58

0.80

California/Texas

1.30

0.92, 1.7

< 0.001

1.25

0.72, 1.78

< 0.001

Comorbidity^a

Atrial fibrillation

-0.14

-0.34, 0.05

0.20

-0.26

-0.54, 0.02

0.06

Diabetes mellitus

0.02

-0.13, 0.17

0.80

-0.14

-0.36, 0.09

0.20

Hypertension

-0.14

-0.32, 0.03

0.11

-0.23

-0.48, 0.02

0.07

Dyslipidemia

0.29

0.13, 0.46

< 0.001

0.49

0.27, 0.72

< 0.001

Previous stroke

-0.24

-0.40, -0.08

0.004

-0.31

-0.53, -0.08

0.007

Obesity/overweight

0.53

0.36, 0.69

< 0.001

0.66

0.45, 0.87

< 0.001

Season

Spring

Reference

Summer

-0.13

-1.10, 0.81

0.80

-0.14

-0.95, 0.66

0.70

Fall

0.56

-0.41, 1.5

0.30

0.712

-0.084, 1.51

0.08

Winter

3.20

2.20, 4.10

< 0.001

3.25

2.40, 4.10

< 0.001