Differences in the clinical manifestations and short-term prognosis of acute cerebral infarction after exposure to Agent Orange

Agent orange (AO) is an organic chemical containing several toxic materialsused during the Vietnam War to improve visibility. Among these materials, 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD), also known as dioxin and included in a small quantity, has the highest toxicity. TCDD is thought to be the cause of contemporary AO toxicity. The half-life of TCDD is 7.1–11.3 years, and therefore, its adverse effects result from accumulation in the human body [1].Human experiments on the effects of AO cannot be performed, and therefore,only animal experiments or epidemiological investigationsare possible. Apart from the differences between animals and humans, the limitation of animal experiments is that the average age span of animals is 2–3 years compared to the long half-life of TCDD. With regards to epidemiological investigations, there is little data of Vietnam veterans who have been exposed to AO, and there is no data regarding the amount of exposure to AO, which makes quantitative analysis impossible. Therefore, we can only obtain data from occupational, environmental, and case-controlled studies using similar substances and draw indirect inferences [2]. Studies conducted thus far show a significant correlation between AO and the occurrence of cardiovascular diseases, such as ischemic heart disease (IHD) and atherosclerosis and risk factors, such as diabetes mellitus (DM) and hypertension (HTN). Alternatively, stroke is assumed to be associatedwith AO as it shares many risk factors and pathogenic mechanismsfor cardiovascular diseases, but this has not been clearly demonstrated. There is little data on the association between AO and stroke, and limited studies have targeted patient groups exposed to AO [3].In this study, we aim to compare stroke patients with and without a history of AO exposure and study the differences in the clinical manifestations and short-term prognosis.

AO and TCDD are not synonymous, but AO toxicity is thought to be caused mainly by TCDD [7].

The study of the mortality patterns in New York State Vietnam Veterans was the first study that discussed the correlation of cardiovascular diseases with TCDD, where the cross ratio of cardiovascular diseases including stroke was as high as 1.48(1.19–1.83) [8].In epidemiological studies published in the late 1990s, several authors reported that TCDD contributes to the occurrence of IHD [1],and previous animal studies have shown that TCDD leads to decreased heart function and deformation of the structure [9].In 2004, an animal study proved that dioxin increased hyperlipidemia, blood pressure, and heart muscle weight. [10]Many other studies have reported that TCDD causes DM [11].

According to the “Veterans and agent orange: update” published by the Institute of Medicine at the National Academy of Sciences, DM has been classified as a suggestive category that is possibly caused by AO since the 2000 update, and since the 2006 update, HTN has been classified as such, while IHD has been classified as such since the 2008 update [2, 12].

Stroke has been classified as a suggestive category since the 2014 update, based on the fact that its pathogenesis is similar to IHD, and it shares similarvascular risk factors, based on recent epidemiological studies on similar substances [3, 13, 14].However,there are limited studies that verify the correlation between stroke and AO, while no studies have been conducted in Vietnam veterans,which could provide sound evidence.

Further, strokes are divided into subtypes, such as hemorrhagic strokes, small vessel disease, and cardio-embolic strokes,according to the pathogenesis, and this is much more complicated than IHD,which mostly originates from artherosclerosis. We are yet to ascertain the correlation between AO and stroke.

In this study, various clinical manifestations were compared according to the history of AO exposure to identify any differences, and several differences were confirmed. The exposure group had higher DM and less hyperlipidemia; this was ascertained while comparing the vascular risk factors.

There was no difference in other risk factors such as HTN, smoking, and history of stroke. The exposure group was expected to have more atrial fibrillation, based on previous studies that showed that AO could induce structural changes in the heart, although the sample sizes were small. There was also some difference in the subtype distribution. The exposure group was expected to have a higher frequency of large vessel disease, which occurs from artherosclerosis similar to IHD, or cardio-embolic stroke, which occurs secondary to impaired heart function, butthe frequency of small vessel disease was the highest. As a result, the short-term prognosis of the exposure group was better when comparing NIHSS at discharge or mRS after 3 months. In exposure group, small vessel disease frequency is higher. Small vessel disease is known to low association with hyperlipidemia, Sothere can be little association with hyperlipidemia and AO exposure. If small vessel disease is more common in the AO exposure group, additional studies are necessary to investigate the possibility of toxicity affecting small vessels or toxicity specific to brain tissue, in addition to previously known mechanisms.

This study has many limitations. AO exposure was confirmed in Vietnam veteran patients, but accurate information of the amount or duration of exposurewas not obtained, as in other veteran-based studies. Patients with a history of AO exposure were all registered at the same institution. We were not able to determine whether the AO exposure directly affected the occurrence of stroke or the increasedrisk, asonlythe clinical manifestations in stroke patients were compared.

Despite the limitations, this study targeted patients who are Vietnam veterans, thereby confirming a difference in the distribution of vascular risk factors and short-term prognosis according to AO exposure. This difference in clinical manifestations suggests that AO exposure has contributed to a certain extent to the onset of stroke. Larger systematic studies will be necessary in the future.

This study targeted patients who are Vietnam veteran. There is some difference in vascular risk factors and clinical manifestations suggest AO exposure has contributed to a certain extent to the stroke. The exposure group frequency of small vessel disease was the highest and the short-term prognosis of the exposure group was better when comparing NIHSS at discharge or mRS after 3 months. It is needed to study more systematically and with larger scale in the future.