Complex regional pain syndrome (CRPS) is a set of signs and symptoms characterized by continuing regional pain that is seemingly disproportionate in time or degree to the usual course of pain after trauma or other injury [1]. The lesion in CRPS usually has a distal predominance of abnormal sensory, motor, sudomotor, vasomotor edema, and/or trophic findings. It significantly affects the quality of life and daily functioning of the patient as it causes severe pain with resultant psychosocial and socioeconomic adverse effects [2].

In general, CRPS has been more common in female, and the incidence increases with age [3,4]. However, reports of previous epidemiological studies of CRPS in Korea have been controversial. One previous single-center retrospective study that included 150 CRPS patients reported that in Korea, the condition was more prevalent in male than in female, and was especially common in the third and fifth decades [5]. Among those patients, 73% of patients in the third decade (16/22 patients) were obligatory military service soldiers. However, it may be difficult to generalize the results since only 22 subjects in the third decade were included in the study. Another study utilizing electronic population health data demonstrated that the incidence rate of CRPS in Korea was higher in females, which was in agreement with the demographic distribution in other countries, and peak incidence was observed in the 70s age group [4,6–8]. Although this study analyzed a large population using the National Health Insurance Service, the results might be not be exact since diagnosis of CRPS was not made in a homogeneous manner with consensus.

Epidemiology of a disease may be influenced by a special social condition. Korea is the only divided country in the world, which is currently in a state of armistice with North Korea, and all healthy young adult male are required to serve in the military. According to the Korean Military Manpower Administration, a total of 227,115 soldiers were enlisted in the army in 2017 [9]. These soldiers have a duty of national defense, and further, after the 2-year military service, they form an important part of the work force in Korean society. Therefore, if a specific disease, such as CRPS, is more prevalent in this young male population, and is especially associated with military service in South Korea, it can be an important social issue, which increases not only the individual but also the national health burden. However, to the best of our knowledge, the potential association between the incidence of CRPS and military service in the young male population has never been analyzed in detail.

Therefore, the aim of this study was to investigate the clinical characteristics of CRPS in young male patients in South Korea, especially focusing on the association with their military service status.

A total of 430 patients who visited Seoul National University Hospital Pain Center between January 2007 and May 2017 were suspected to have CRPS. Among them, 294 patients (68.4%) were male and 136 (31.6%) were female (male to female ratio, 2.2:1.0). Of these, 200 males (68.0%) and only 45 females (33.1%) were diagnosed with CRPS (Fig. 1).

The demographics of the 245 patients who were diagnosed with CRPS are described in Table 1. The age at onset was significantly lower in male patients than in female patients (P = 0.015). Lesions in the lower extremities were more frequent than those in the upper extremities in both male and female patients; the lesions were mostly unilateral. In male CRPS patients, 95 cases (47.5%) were associated with military service, whereas there was no female who was involved in military service. In terms of etiology, surgical procedure was the most common etiology (27.1%), followed by sprain/strain (24.1%), and contusion/crush injury (22.6%) in males, whereas in females, traffic accident was the most frequent etiology (31.1%), followed by surgical procedure (26.7%) and contusion/crush injury (15.6%).

In terms of the characteristics of male CRPS patients according to their military service associations, of the 200 male patients diagnosed with CRPS, 95 cases (47.5%) were associated with CRPS-related events during their military service period (Table 2). All patients were soldiers who were on duty for 2-year military service, not professional soldiers. The age at first visit was significantly lower and the duration of illness was shorter in the military service group than in the non-military service group (P = 0.024 and P < 0.001, respectively). With regard to the type of CRPS, CRPS type I was more common in both groups; however, it was significantly more frequent in the military service group than in the non-military service group (94.7% vs. 82.9%; P = 0.016). Etiology relevant to CRPS was different between the groups. In the non-military service group, surgical procedure was the most common cause (34.6%), followed by contusion/crush injury (20.2%) and traffic accident (18.1%). However, sprain/strain was the most common etiology in the military service group (40.0%) and traffic accidents were not a cause in any of the patients in this group (0.0%). Although the incidence was higher in the lower extremities than in the upper extremities in both service groups, it was significantly more frequent in the military service group than in the non-military service group (84.2% vs. 60.0%; P = 0.001). When symptoms and signs relevant to CRPS, such as sensory abnormality, skin color difference and temperature difference between the affected and contralateral extremities, edema, motor dysfunction, trophic change, and sweating were explored, there was no difference between the military service and non-military service groups with regard to any of these parameters (Fig. 2). Sensory abnormality was the most commonly observed symptom and sign in the young male CRPS population. Interestingly, the second most common symptom that patients complained of was motor dysfunction, whereas the second most common objective sign was temperature difference.

Out of the 200 male CRPS patients, NRS pain scores from the initial to the last visits of 161 (80.5%) patients were available. These patients were divided into 2 groups according to their treatment outcome, which was predefined as a pain reduction of more than 50% or a decrease of more than 3 NRS points [13] or an NRS score ≤ 4 on the last outpatient visit. Among the patients, 61 patients (37.9%) were classified as the “Success group” and the remaining 100 patients (62.1%) as the “Fail group” (Table 3). The proportion of military service members was significantly higher in the Success group than in the Fail group (57.4% vs. 38.0%, P = 0.026). There were no significant differences in the age, duration of pain, CRPS type, pain severity at the initial visit, affected site, and treatment modality (except SCS implantation) between the outcome groups; more patients in the Fail group had undergone SCS implantation than those in the Success group (27.0% vs. 11.5%; P = 0.032). In terms of management of CRPS in young male patients, sympathetic block was the most frequent procedure (70.8%) and more than half (52.2%) the patients had taken opioid medication for pain management.

The results of multivariable logistic regression analysis for successful treatment outcome in male CRPS patients are shown in Table 4. In univariable analyses, age, duration of pain, the initial NRS pain score, relevance to military service, occurance in a lower extremity, opioid medication, and SCS implantation showed trends suggestive of statistical significance (P < 0.2). However, in multivariable analysis, moderate and severe initial pain scores (odds ratio [OR] = 0.14; 95% confidence interval [CI], 0.02–0.84; P = 0.032 and OR = 0.12; 95% CI, 0.02–0.74; P = 0.022, respectively) and relevance to military service (OR = 2.07; 95% CI, 1.04–4.13; P = 0.039) were identified as independent predictors of the treatment outcome.

In this study, we investigated the clinical characteristics of CRPS in the young population of South Korea and explored the characteristics of CRPS among young male patients, particularly with regard to their military service status. This is the first study to investigate the characteristics of young male patients under the age of 30 years diagnosed with CRPS using the Budapest research criteria and the use of a homogeneous standardized assessment protocol. The results of our study indicated that CRPS was more common in young adult male than in young adult female, with a male to female ratio of 2.2:1.0. Among these male patients, 47.5% were diagnosed with CRPS during their military service period. Pain from CRPS was more common in the lower extremity; this was particularly more prominent in military service members than in nonmilitary members. In terms of etiology, surgery was the most frequent cause of CRPS in male patients, whereas traffic accident was the most frequent cause in female patients. However, in the military service members, sprain/strain was the most common cause, accounting for 40% of the cases. A comparison of military and non-military service members showed that there were a higher number of younger patients with a shorter duration of disease in the military service members than in the non-military service members. In the overall population in this study, CRPS type I was more prevalent than CRPS type II, with a ratio of 6.4:1.0, and this was significantly higher in the military service members than in non-military service members. Multivariable analysis revealed independent associations of relevance to military service with successful treatment outcome.

Our results differ from those of previous studies conducted in other countries, particularly with regard to sex distribution. Previous studies have usually shown that CRPS is more common in females with the female to male ratio ranging from 2:1 to 4:1 [1]. In another recent large-scale national study of the Korean population, it was seen that CRPS was more frequent in female patients than in male patients, although the female to male ratio was not as pronounced as in the previous studies (1.3:1.0) [4]. However, in our study, where we have intensively investigated 245 young patients under the age of 30 years with CRPS, we found that the prevalence of CRPS in male patients was more than twice that in female patients. Although a similar finding was noted by Choi et al. [5], they included only 22 patients in the third decade and therefore, their study may not be representative of sex distribution in the young Korean population with CRPS. Therefore, the results of our epidemiological study may be the first to report a reverse sex distribution of CRPS in the young population of Korea.

One of the notable findings in this study was that the diagnosis rate of CRPS among subjects who underwent testing for CRPS was relatively higher in the younger population compared to the previous data suggested by the same clinic (68.8% vs. 49.5%, respectively) [5]. We used the Budapest research criteria for all cases from 2007 in a standardized manner through objective evaluation so that at least one symptom from all 4 categories (sensory, vasomotor, sudomotor/edema, and motor/trophic dysfunction) and at least 2 signs from the same 4 categories had to be satisfied for a diagnosis of CRPS [14]. When we compared our results to those of the previous study [5], the presence of CRPS signs was much higher in the current study than that in the previous one in all sign categories, such as skin color change (70.4% in this study vs. 45.3% in the former study), edema (61.4% vs. 37.9%), asymmetry in sweating (43.8% vs. 22.1%), etc. Although symptoms and signs may change over time and vary from person to person, pain, swelling, redness, noticeable changes in temperature and hypersensitivity usually occur in the earlier phase of CRPS [15]. Although further studies are required to investigate this, we think that the relatively short duration of CRPS in the younger population in this study may be associated with the high prevalence of symptoms and signs, which accounts for the high rate of CRPS diagnosis.

The most noteworthy aspect of this study is that nearly half of all male patients in the younger age group were diagnosed with CRPS during their military service period. In the previous Korean epidemiological study [5], it was suggested that CRPS was common in the third decade because of the high risk of trauma during military service. However, there has been no study to date that has investigated the association between CRPS and military service status. A study that investigated the relationship between the incidence of CRPS and the patient’s occupation in 134 patients in tertiary hospitals in the USA reported that CRPS was almost twice as prevalent in restaurant workers and police officers as in patients with other occupations [7]. They suggested that occupational physical activity might be related to the development of CRPS [7]. Soldiers face a higher risk of trauma due to greater exposure to intense training, exercise, and work, as compared to the general population [16–18]. We suspect that vigorous activity during military service may have led to the higher incidence of CRPS in this subgroup.

In the previous Korean epidemiologic study, it was suggested that CRPS was common in the third decade because of the high risk of trauma during military service [5]. The most common etiology of CRPS in our military service group was the sprain/strain of soft tissue, which was different from that in the young female patients in this study and in patients in a previous study (traffic accident was most common in both) [5,19]. In another epidemiological study of 303 young male in the US army, 37% of the subjects experienced lower extremity injury during their service period and the most common cause was strain and sprain [20]. In another study that evaluated the prevalence of injury in Greek army officer cadets, ankle/foot strain and sprain were reported to be the most common injuries during basic combat training [21]. The military service members in the current study had a higher incidence of CRPS in their lower extremities compared to the non-military service group, which can be attributed to the sprain/strain of soft tissues. Therefore, we suggest that relatively common injuries in the lower extremity caused by sprain/strain during military service may have affected the epidemiology of CRPS in this study.

The high risk of trauma in the army may not be the only reason why CRPS occurs commonly in soldiers. Another factor to be addressed in military service is social stress due to the obligation to enlist. In Korea, it has been reported that military soldiers who have to compulsorily enlist due to the conscription system have a high psychological burden [22]. In a study conducted in Korean conscripts with lumbar disc herniation, there were more psychological problems, such as depression and anxiety, in conscripts compared to healthy controls, which were associated with functional availability [23]. Another study has reported that social stress around the time of the manifestation is a risk factor for the development of CRPS [24]. In addition, CRPS patients experienced more stressful life events prior to being affected by the condition [25]. Although psychological co-morbidity may not have a role in poor prognosis in CRPS, it is widely accepted that it plays a role in the development of the disease [26,27]. However, when we compared the differences in NRS pain scores from the initial to the final visit in male CRPS patients, military service members showed better treatment outcomes than non-military service members. This result may not be easily explained; however, we presume that the shorter duration of CRPS from the symptomatic manifestation to the hospital visit might have led to the better treatment outcomes in the military service group in this study. Besides, it is carefully suggested that psychological release alongside the wind-up of the military obligation may yield a better treatment outcome in this exceptional group. Although no studies have reported the association between the psychological stress from obligatory military service and the prevalence or clinical outcomes of CRPS, a previous study conducted in Korean soldiers showed that depression was inversely proportional to the remaining service period [28]. Therefore, further study is warranted regarding the effects of psychological stress from obligatory military service on the development of CRPS.

This study has several limitations. First is the inherent limitation of the retrospective study design; all data were not available due to insufficient records or loss of follow-up. Second, although only a limited number of hospitals professionally evaluate and manage CRPS in Korea, our results were obtained from a single tertiary university hospital, and therefore, it is difficult to fully generalize the findings. Third, we could not include variables that would be relevant to the outcome of CRPS, such as the results of psychological evaluations, litigation status, existence of secondary gain, any type of reimbursement, etc. In addition, this study would have been more sophisticated if the details of the military service could be analyzed, for example, the status of hierarchy in the army (e.g., status as a private or an officer), whether the soldier’s enlistment was voluntary or if it was enforced compulsory service, the soldier’s tour of duty, whether the patient was discharged from the army due to CRPS itself, etc.

In conclusion, our results demonstrated that the manifestation of CRPS in the young Korean population was more common in male and among these male CRPS patients, about half the cases manifested during the military service period. CRPS in the lower extremity due to sprain/strain was the most common type in the military service members. Therefore, various measures are needed to reduce sprain/strain in these populations, and if CRPS is suspected, a system for early diagnosis and treatment during the military service would be triggered. Further investigation with a large-scale epidemiological study of CRPS in the young Korean population, with particular consideration of unique groups, such as military service members, should be carried out to better interpret the results.