Seasonal Variation in the Incidence of Transient Global Amnesia in South Korea.

Dear Editor,

Transient global amnesia (TGA) is a syndrome characterized by sudden-onset transient amnesia that is not associated with other neurological deficits.1 Although the exact pathophysiology is not completely understood, emotional stress, physical exertion, temperature change, and sexual intercourse are common precipitating events of TGA.2

Keret et al.3 recently hypothesized that the incidence of TGA exhibits seasonal variations, based on their findings that the monthly incidence of TGA in Israel was maximal during December and March, and with seasonal peaks in winter and spring. The present study compared the seasonal changes in the incidence of TGA between South Korea and Israel.

A retrospective analysis was performed using a TGA registry database. We diagnosed TGA patients who visited Seoul National University Bundang Hospital from 2011 to 2015 using the following accepted criteria:1 1) presence of an anterograde amnesia event witnessed by an observer, 2) cognitive impairment limited to amnesia, 3) no focal neurological signs, 4) no recent history of head trauma or seizures, and 5) resolution of symptoms within 24 hours.

We conducted our study using the methods described by Keret et al.3 A logistic regression was used to analyze the monthly or seasonal incidence of TGA. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated as estimates of relative risk. Spring and October were used as the reference categories for the seasonal and monthly analyses, respectively.

During the 5-year analysis period, 1,751,322 patients were admitted to the hospital, and TGA was the final diagnosis in 386 cases. The demographic characteristics of the patients were as follows: mean age, 60.74 years; proportion of males, 34%; and proportion of patients with recurrent episodes of TGA, 4.73%.

The monthly incidence of TGA peaked three times during each year (Table 1): in December (OR=2.01, 95% CI=1.22–3.30), September (OR=1.85, 95% CI=1.11–3.08), and June (OR=1.71, 95% CI=1.02–2.87). A seasonal peak in the incidence of TGA occurred in winter (OR=1.35, 95% CI=1.01–1.80).

Table 1

ORs and 95% CIs for the incidence of transient global amnesia according to month and season

	OR	95% CI
Month*
January	1.4026	0.8236–2.3887
February	1.1978	0.6799–2.1105
March	1.0990	0.6271–1.9260
April	1.2449	0.7202–2.1518
May	1.1047	0.6304–1.9361
June	1.7167	1.0254–2.8742
July	1.4811	0.8751–2.5066
August	1.1936	0.6876–2.0722
September	1.8512	1.1109–3.0849
November	1.4396	0.8453–2.4517
December	2.0124	1.2241–3.3082
Season†
Winter	1.3516	1.0121–1.8050
Autumn	1.2401	0.9232–1.6656
Summer	1.2716	0.9498–1.7023

*October was the reference month, †Spring was the reference season.

CI: confidence interval, OR: odds ratio.

Our analyses revealed seasonal but not monthly variations in the incidence of TGA. Akkawi et al.4 described an association between TGA occurrence and low ambient temperature, with TGA occurring most and least often when the outdoor temperature was less than 6.9℃, and greater than 24℃, respectively (p<0.0001). Israel has a typical Mediterranean climate, with cool rainy winter and long hot summer. The lower temperatures observed in Israel during the winter and early spring also occur in South Korea. Indeed, the seasonal incidence determined in our study indicates that TGA occurrence is related to a low ambient temperature in South Korea, similar to the findings of Keret et al.3 In contrast, the monthly incidence pattern determined in our study differed from that reported by Keret et al.3 We understand that TGA is associated with various pathophysiologies,2 and so the cause of TGA is almost certainly not related to temperature alone; instead, diverse pathophysiologies probably underlie monthly variations in TGA incidence.

In conclusion, our findings suggest that TGA shows a seasonal variation in incidence, which supports the findings of Keret et al.3