Subarachnoid Hemorrhage Incidence in the United States Does Not Vary with Season or Temperature

R.J. McDonald; J.S. McDonald; J.P. Bida; D.F. Kallmes; H.J. Cloft

doi:10.3174/ajnr.A3059

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Fig 1.
USDA hardiness zone map. Used with permission from the Arbor Day Foundation (http://www.arborday.org/media/zones.cfm).
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Fig 2.
Monthly stratified relative rates of (A) admission frequency and (B) in-hospital mortality after admission for SAH. Relative rates of SAH admission and in-hospital mortality, stratified by month, were determined from regression models. RRs greater than 1.0 suggest that the event (admission or death) is more likely to occur relative to the reference month (Jan), whereas RRs less than 1.0 suggest the event is less likely to occur relative to the reference month. Significant differences relative to the reference month are represented by an asterisk (*).
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Fig 3.
Temperature (A, C) and climate (B, D) stratified relative rates for (A, B) admission frequency and (D, E) in-hospital mortality after admission for SAH. Relative rates of SAH admission and in-hospital mortality, stratified by temperature and climate, were determined from regression models. RRs greater than 1.0 suggest that the event (admission or death) is more likely to occur relative to the reference variable (temperature strata [<30°F]; climate strata [USDA zone 3]), whereas RRs less than 1.0 suggest the event is less likely to occur relative to the reference variable. Significant differences relative to the reference temperature stratum or zone are represented by an asterisk (*).

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Table 1:

Distribution of SAH admissions, deaths, and hospital characteristics by USDA zone

Zone	Hospitals (n)	Admissions (n)	Deaths (n [%])	Hospital Characteristics by Admission Counts^a
Zone	Hospitals (n)	Admissions (n)	Deaths (n [%])	Teaching (n [%])^b	Large (n [%])^c	Urban (n [%])^d
3	8	266	64 (24.1)	255 (95.9)	184 (69.2)	259 (97.4)
4	118	1656	378 (22.8)	1219 (73.6)	1028 (62.1)	1458 (88.0)
5	374	6967	1737 (25.0)	5079 (72.9)	4973 (71.4)	6339 (90.9)
6	671	13,251	3429 (25.9)	10,416 (78.6)	10,385 (78.4)	12,653 (95.5)
7	414	9829	2517 (25.6)	7619 (77.5)	7388 (75.2)	9338 (95.0)
8	351	8677	2126 (24.5)	5356 (61.7)	6711 (77.4)	8328 (96.0)
9	368	7971	2008 (25.2)	4598 (57.8)	5633 (70.8)	7769 (97.6)
10	131	4087	1013 (24.8)	2372 (58.0)	3270 (80.0)	4061 (99.5)
11	16	192	57 (29.7)	106 (55.2)	72 (37.5)	158 (82.2)

↵a Among hospitals with recorded SAH admissions.
↵b Compared with nonteaching hospitals.
↵c Compared with small and medium-sized hospitals.
↵d Compared with rural hospital locations.

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Table 2:

Effects of climate, patient demographics, and hospital characteristics on the incidence of SAH and SAH-related mortality

Variable	SAH Incidence Model (χ² = 3483, df = 20, P < .0001)		SAH Death Model (χ² = 896, df = 20, P < .0001)
Variable	RR^b	P value	RR^b	P value
Temperature stratum (df = 6)^c	see Figure 3A	.1453	see Figure 3B	.3005
USDA zone (df = 8)^c	see Figure 3c^a	.0009^a	see Figure 3D	.0863
Age (df = 1)^d	0.99 (0.98–1.01)	.5787	1.18 (1.16–1.19)^a	<.0001^a
Female gender (df = 1)^e	1.03 (1.01–1.06)^a	.0375^a	1.01 (0.98–1.03)	.5579
Teaching hospitals (df = 1)^f	1.25 (1.20–1.32)^a	<.0001^a	0.90 (0.85–0.97)^a	<.0001^a
Large hospital bed size (df = 1)^g	1.07 (1.02–1.11)^a	.0017^a	0.99 (0.94–1.01)	.1124
Urban location (df = 1)^h	1.09 (1.04–1.15)^a	.0003^a	0.97 (0.92–1.02)	.1358

↵a Indicates significant effects.
↵b RR with 95% confidence interval as defined in Materials and Methods.
↵c See Materials and Methods for a description of stratifications and groupings.
↵d Change in RR for every 10-year increase in age.
↵e Compared to males.
↵f Compared with nonteaching hospitals.
↵g Compared with small and medium-sized hospitals.
↵h Compared with rural hospital locations.