Claims data from n = 37 hospitals of the largest private German hospital group (i.e., Helios) fulfilling the inclusion criteria defined below were used for this retrospective analysis.

Patient characteristics considered in our analysis were age, age group, gender, Elixhauser weighted score (AHRQ algorithm was applied (27, 28)) and corresponding comorbidities. Uncomplicated and complicated diabetes were lumped into one single category, as were uncomplicated and complicated hypertension.

Comparisons of discharged patients, discharged cases (hospital stays), and discharge rates were made for different time periods according to the following definitions:

Annual comparison: Discharge rates were analyzed for the years 2019, 2020, 2021, and 2022. For 2022, only the first six months (January to June) were included due to limited data availability.

Quarterly comparisons: Quarterly discharge rates in 2020, 2021 and 2022 were compared to the respective quarter in 2019 to minimize annual effects. As the first quarter of 2020 overlap both periods (prepandemic and pandemic), results for this comparison (Q1 2020 vs. Q1 2019) should be treated with caution.

Prepandemic vs. pandemic: Prepandemic period is the time interval between 2019-01-01 and the week including 2020-02-28 whereas the pandemic period is the time interval between the week starting after 2020-02-28 and 2022-06-30. We have defined the beginning of the pandemic period according to the date when the first patients with COVID-19 were detected in two large German federal states (i.e., Baden-Wuerttemberg, North Rhine-Westphalia) on the 26^th of February 2020.¹ The end date of the pandemic period was defined taking into account the distinct decrease of COVID-19 cases and associated healthcare utilization in the second quarter of 2022 (29).

Analyses were startified for regions (federal states of Germany) and pain units size. Pain unit size were defined as follows, (i) ‘high volume’: units with at least 300 cases in 2019 and (ii) ‘low volume’ with less than 300 cases according to the aforementioned DRG definitions.

For assessing the comorbidities of patients, the AHRQ Elixhauser Comorbidity Index was used (28). For this index, comorbidites were defined via patients’ ICD codes and entered the index with different weights. The index was evaluated with respect to in-hospital mortality and readmissions.

For the analysis of patient characteristics, categorical variables were analyzed using χ2 test, and two-sample t-test was used for analyzing continuous variables.

For the comparison of the discharge rate between different cohorts, a Poisson GLMMs with log-link function and the hospitals and a random intercept (30) was used. Hospitals entered the analyses as random factor. We analyzed weekly numbers for linear trends over time with Poisson regression. Discharge rates are reported as mean weekly discharge (SD), incidence rate ratios (IRR) together with 95% confidence intervals and p values. For weekly data, IRRs indicate average weekly changes. Stratified analyses were conducted for DRG and hospital regions. For all tests we applied a two-tailed 5% error criterion for significance. All the analyses are patient-based analyses (and not case-based). In case of multiple admissions for a patient (which appears to be rather rare), only the first case was included in our analysis.

Inferential statistics were performed in the R environment for statistical computing (version 4.0.2), 64-bit build (31) Effects of mixed models were estimated with the lme4 package (version 1.1–26) (30, 32).

Comparing the characteristics of patients discharged during the observed pandemic versus pre-pandemic period, only minor differences were found (Table 1). During the pandemic period, patients were slightly younger (mean age: 60.3 ± 13.8 yrs. vs. 61.0 ± 13.6 yrs., p < 0.01) and had a slightly higher mean Elixhauser score (−2.0 ± 5.8 vs. −2.2 ± 6.0, p < 0.05). With regard to comorbidities used for defining the Elixhauser score, a slightly increased proportion was found during the pandemic period for patients suffering from ‘other neurological disorders’ (3.4% vs. 2.8%, p < 0.05) whereas a decreased proportion of patients had a documented diagnosis of obesity (23% vs. 27%, p < 0.001), depression (46% vs. 48%, p < 0.05), and hypertension (50% vs. 52%, p < 0.01).

Starting from a total of 5,533 cases in 2019, the COVID-19 pandemic reduced the number of cases to 3,942 in 2020 and to 4,262 in 2021. Hence, in 2020 the most significant decrease was recorded in day-care, inpatient interdisciplinary multimodal pain therapy. In 2021, there was a slight increase which, however, did not reach the level of care in 2019. The same is true for the first half of 2022 (Table 2).

Regarding the whole study period, we found a trend for a decreased number of discharged patients for almost all DRGs (Table 3). By separating the two main period (pre-pandemic vs. pandemic) we found for the majority of DRGs a significant increase of patients during the pandemic period underlining the sharp decrease at the beginning of the pandemic followed by a slight increase of treated patients.

In our analysis, we found some regional differences at level of the federal states of Germany. Saxony-Anhalt has shown the smallest changes of discharged pain patients over the course of the pandemic (Table 4).

With regard to the total discharge rates, we found statistically significant differences between the pre-pandemic and pandemic period for both, low and high volume pain units (Table 5). In a combined analysis of all selected DRGs, a more pronounced decrease was revealed for high volume pain units [incidence rate ratio [IRR]: 0.63 (95% CI: 0.59 to 0.66)] compared to low volume units [IRR: 0.90 (95% CI: 0.86 to 0.94)] between the two periods for the DRGs (Table 5).

With regard to single DRGs, a more complex picture was found. There are three codes, one each for the musculoskeletal system, the nervous system, and psychological and somatoform disorders. Minimum lengths of hospital stay of at least 14 treatment days or under 14 treatment days are coded separately. These 6 codes allowing the number of IMMST treatments over time to be shown using the DRG coding. Whereas for most DRGs a decreased discharge rate was found for both, high and low volume units comparing the pre-pandemic and the pandemic period, discrepant results were found for B47A and B47B. For both unit types, an increased IRR (i.e., increased discharge rate comparing pre-pandemic and pandemic period) was found for U42 C.

Changes in hospital care for patients with chronic pain occurred as a result of the COVID-19 pandemic. There was a significant decrease in inpatient multidisciplinary multimodal pain care, from a total of 5,533 hospitalizations in 2019 before the pandemic to 3,942 in 2020 and to 4,262 in 2021. There was a slight increase in the first half of 2022. Overall, however, these figures are also lower than the pre-pandemic figures from 2019.

In our study, we found some small but statistically significant differences between the two cohorts. For example, patients of pandemic cohort were slightly younger and had a slightly higher Elixhauser Score than pre-pandemic patients. These results suggest a shift of treating severe pain under in-hospital conditions from elderly patients to younger, sicker patients (i.e., suffering from a higher number of comorbidities). However, these difference are small and of questionable clinical relevance.

Regarding regional variation, it’s worth mentioning that there are 16 federal states in Germany responsible for the regional hospital care. The measures to contain the pandemic were not the same in all of them. However, Table 4 provides an overview of the absolute care figures by federal state. Discussing all different regional COVID-19 related measures and their impact on our study results would be far beyond of our study topic.

The multimodal inpatient and day-care treatment of chronic pain patients is based on the bio-psycho-social disease model and is an important component of pain medicine care. However, because of patient-centered therapies and participation in treatment groups, IMMST was very limited during the different phases of the pandemic due to stricter hygiene measures. This was compounded by uncertainty and anxiety among patients and their families, which led to a decline in demand for this healthcare service.

Emergency treatments and treatments that could not be postponed in the hospital were prioritized (33). In addition to these medically sound care decisions, there were also economic incentives in Germany to keep bed capacities free for patients infected with COVID-19 or to use the remaining capacities as economically as possible (18). Staff reassignments were also necessary to care for wards with a focus on COVID-19 patients, intermediate care, and intensive care units.

The experience of psychosocial and spiritual aspects of palliative care can be used to evaluate pain care under COVID-19 conditions (34, 35). The pandemic had an impact on the components of the bio-psycho-social disease model of chronic pain patients. However, mood changes, social isolation, physical inactivity, and changes in dietary habits affected all populations to varying degrees. When pain patients have less contact with medical providers, it affects quality of life and pain severity. The risk of pain becoming chronic may be greater under the impact of a pandemic (35, 36). The pandemic has been and continues to be a psychosocial challenge for society and individuals. Coping strategies were different and should be the basis for reorienting medical action. The basis of pain management is the holistic approach, which is also required in psychosomatic treatment (1, 4, 24, 37). The pandemic has had a significant impact on patients with chronic pain. With increasing instability and chronification of pain and insufficient outpatient treatment options, an increase in multimodal inpatient treatment would have been logical. However, this did not happen because of the restrictive hygiene regulations and the focus of clinical treatment on COVID-19 patients.

Despite all the aforementioned legal restrictions and other challenges related to COVID-19, pain medicine care has taken place in hospitals. Even under these aggravated conditions, multimodal pain management has been implemented (38).

In our anlysis, a more pronounced decrease of patients receiving multimodal pain treatment was revealed for high volume pain units compared to low volume units. This might be related to the fact, that larger pain clinics or wards were more difficult to organize IMMST because these hospitals required more staff to care for critically ill COVID-19 infected patients and had to deal with staff absences due to sick leaves.

Whereas for most multimodal pain therapy DRGs, a decrease of discharge rates were found comparing pre-pandemic and pandemic period, an increase was found for selected DRGs (B74A: low volume units, B74B: high volume units). In other words, at least for ‘Multimodal pain therapy for diseases and disorders of the nervous system’ we found an increase of ‘short term treatments’ (less than 14 days) in low volume units whereas in high volume units, an increase of ‘long term treatment’ (at least 14 days) was found. Hence, we hypothesize a “centralisation” of severe (and long term cases) in high volume units.

Interestingly, discharge rates for multimodal pain therapy for mental diseases and disorders were increased for both unit types underlining the treatment need of pain and psychiatric comorbidities during the COVID-19 pandemia.

In addition to the problems of inpatient care, options in outpatient care for pain patients were also limited (39). Patients visited primary care physicians and specialists less frequently. This resulted in a lack of the usual hospital admissions. Desired treatments were postponed, and there was a general reluctance on the part of patients to undergo inpatient therapies. Skepticism and fear of infection when in contact with medical facilities were major issues here. An expected larger number of patients suffering from chronic pain due to inadequate care, as well as pandemic-related health restrictions in the biological, psychological, and social spheres due to lock-related movement restrictions, generally depressed mood, and reduced social contacts, should actually have led to a significant increase in inpatient IMMST after the end of the pandemic care restrictions, according to our expectations. However, this was not the case in the clinics studied. Overall, IMMST occurred despite the pandemic. The continued decline could be a temporary phenomenon. Another interpretation is that the perception of this form of treatment by referring physician colleagues has decreased. The clinics studied care for patients in different ways, some caring for more patients than before the pandemic, but others caring for fewer. The high demands placed on the structure of an IMMST can be better met by larger units of care (7, 40, 41).

Data from orthopedic practices show an increase in new neck and back pain diagnosed during the pandemic (42). There are some data showing that delay or even failure to diagnose pain-related conditions may lead to an increased risk of developing chronic pain (42). Periods of crisis that affect the psychological stability of people with chronic pain lead to a higher risk of exacerbation of the disease (32). Our study does not provide evidence that a greater number of patients are hospitalized for multimodal pain management after the pandemic. However, because patients with complex pain syndromes often receive frustrating therapy for an extended period of time, a general pandemic-related increase in complex pain treatments, which may occur somewhat later, cannot be ruled out at this time. It is possible, however, that a shift of complex pain treatments to the outpatient setting could also lead to a situation in which any increased post-pandemic need for care is not reflected in the inpatient setting.

We were able to show that pain medicine care in hospitals took place and was possible during the COVID-19 pandemic. This is important because a combination of methods is also necessary for the individual components of multimodal pain therapy, as the example of psychotherapy shows (43). The DRG-based reimbursement system is also viewed critically in terms of utilization and efficiency, even in international comparison (44, 45). This and the differences in care limit international comparability. In addition to inpatient treatment of severe pain, outpatient interdisciplinary pain treatment is also possible under certain conditions (2). Promising results have been shown, for example, for patients with chronic nonspecific back pain (10). We did not investigate medical care in this area.

Even after recovery from an acute COVID-19 infection, several pain-related long-term complaints were reported in post-COVID and long-COVID patients. For example, testicular pain, headache, chronic pain, and chest pain were reported as long-term pain syndromes (46). Hence, preventive measures are necessary to prevent chronic concomitant diseases, such as the chronic pain of COVID-19 infection (47). Vaccinations leading to an activation of antiviral immunity are preventive measures, as are presumably also the improvement of vitamin D deficiency and the intake of other micronutrients (48). However, further research is needed to fully understand and optimize treatment new onset and long lasting pain in COVID-19 patients as well as deterioration of chronic pain syndromes in patients with a COVID-19 infection. Furthermore, there is also a huge need for research in view of the emergence of new COVID variants or future pandemics which may build on the knowledge gained during the first pandemic waves.

A particular strength of our study is the large number of included hospitals, which includes a variety of different departments in different regions of Germany and reflects the diversity of the German hospital landscape. Especially the avoidance of a one-sided focus on specialized pain centers and the large number of included patients underlines the high external validity of our results.

As with any observational study using electronic hospital data, undercoding and miscoding of diagnoses must be expected. In addition, changes in the population during the observation period (hospitals newly admitted to the Helios Group or department closures) may also influence the absolute number of patients and cases considered. However, the case numbers of Helios hospitals under consideration can also be influenced by decisions of other hospital operators, such as possible new openings/closures or changes in the department structure. Since not all hospitals in Germany are included in the data, this limitation exists.

Against this background, the use of incidence rates in our study is explained in order to minimize such fluctuations in the population as far as possible.

Due to the data structure and because of data protection regulations, linking patient data between different hospitals was not possible. This limitation implies the possibility of double-counting specific patients who received treatment at multiple hospitals.