Malignant hyperthermia (MH) is an autosomal-dominant genetic disorder of the skeletal muscle, linked to ryanodine receptor (RYR1) gene mutations.1 This pharmacogenetic clinical syndrome is triggered by sensitivity to volatile inhaled anesthetic gases (e.g., sevoflurane, desflurane, isoflurane) and the depolarizing neuromuscular blocking drug succinylcholine, leading to skeletal muscle hypermetabolism.2 Epidemiologic data on MH are scant. The prevalence of MH due to anesthesia in surgical patients treated in New York State hospitals is estimated to be approximately 1 per 100,000.3 There is little information about the prevalence of MH in patients treated in the ambulatory care setting.
Since the early 1980s advances in medical technology, the changes in payment arrangements and increasing health care costs have contributed to the increase of ambulatory surgery centers (ASCs) in the United States.4 ASCs are health care facilities that provide same-day surgical care outside the hospital setting, including diagnostic and preventive procedures. They are considered to be more practical and generally less expensive than hospitals.4 According to the National Center for Health Statistics of the Centers for Disease Control and Prevention, in 2010, ambulatory surgery cases totaled 18.7 million, accounting for 63.6% of all surgical procedures.5 From 1996 to 2006, the rate of visits to freestanding ASCs tripled, whereas the rate in hospital-based ASCs remained unchanged.4 According to the Healthcare Cost and Utilization Project (HCUP), the number of ASCs increased from 336 in 1985 to 6167 in 2008.6 To ensure patient safety in ambulatory care settings, it is necessary to better understand the epidemiology of MH in ASCs.7 The purpose of this study was to estimate the prevalence of MH diagnosis and characterize factors associated with MH diagnosis recorded in a large sample of ASC discharges.
This study meets the criteria for the Protection of Human Subjects exemption 4 (research involving preexisting data) of the United States Code of Federal Regulations (45 CFR 46.101). The study was deemed exempt from review by the IRB’s Administrative Review Committee at the Columbia University Medical Center (New York, NY).
Data for this study came from HCUP State Ambulatory Surgery Databases (SASD) for New York State from 2002 to 2011. SASD contains the New York State’s ambulatory surgery discharge records, including abstracts from hospital-owned ASCs and nonhospital-owned freestanding ASC facilities.6 A facility is considered hospital based if any 1 of the 3 criteria is met6: (1) an American Hospital Association (AHA) hospital identifier can be determined; (2) the facility provides inpatient data to the HCUP project, but no AHA hospital identifier can be determined; and (3) documentation provided by the data source clearly indicates the facility is hospital based. An AHA hospital identifier cannot always be determined for hospital-based facilities because hospitals may not be matched to the AHA. The SASD database contains clinical and nonclinical information on all patients including patient diagnoses and procedures, admission and discharge status, patient demographics (e.g., gender, age, and race), payment source (Medicare, Medicaid, private insurance, and uninsured), and some hospital characteristics. HCUP state databases are discharge-level (not patient) files in which each record represents 1 ambulatory surgical visit. The Agency for Healthcare Research and Quality aggregates International Classification of Disease, Ninth Revision, Clinical Modification (ICD-9-CM) diagnostic coding into meaningful clinical groups: the Clinical Classification Software (CCS) codes.8 According to HCUP, the record selection criteria for ambulatory surgery discharges varied from 2009 to 2011, resulting in fluctuation in record counts in these years.a
The study sample consisted of all ASC discharges in New York State from 2002 through 2011. MH cases were identified by screening all the discharge diagnoses using the ICD-9-CM code 995.86 to indicate MH due to anesthesia.
The prevalence of MH due to anesthesia was calculated based on all ASC discharges in SASD from 2002 to 2011. MH prevalence was examined according to patient characteristics including age, sex, payment type, exposure to anesthesia, and comorbid conditions. Comorbidities were identified by applying the Charlson Comorbidity Score to ICD-9-CM codes in the data sets.9,10 We calculated the prevalence of MH for 5-year intervals and examined trends in MH prevalence from 2002 to 2011 and also calculated the 95% confidence intervals (CIs) using the normal approximation method.11 Comparisons of prevalence among different patient groups were performed with SAS version 9.4 (SAS Institute, Cary, NC); and χ2 or Fisher exact test was used to compare categorical variables. Statistical significance was defined as P < 0.05.
The SASD database for 2002 to 2011 contained 17,092,765 ASC discharge records; of them, 31 had an MH diagnosis. The overall prevalence of MH was 0.18 (95% CI, 0.12–0.25) per 100,000 discharges (Table 1). The difference in MH prevalence across all groups of exposure to anesthesia, including exposure to no anesthesia, was not statistically significant (P = 0.065; Table 1). However, the prevalence of MH for patients who received general anesthesia was 2.5 times (95% CI, 1.12–5.56, P = 0.025) the prevalence for those receiving other anesthesia including local and regional anesthesia (Table 1). The estimated prevalence of MH diagnosis per 100,000 discharges increased from 0.11 (95% CI, 0.03–0.18) in 2002 to 2006 to 0.24 (95% CI, 0.14–0.34) in 2007 to 2011 (P = 0.001; Table 1). The difference in MH prevalence between sexes was not statistically significant (P = 0.76; Table 1). Through cross-referencing revisit files (except for 2002 SASD that was unavailable) and other discharge variables such as race, birth year, and birth month, all 31 patients with a diagnosis of MH were deemed unique individual patients. Based on revisit files of 2009 to 2011, 28.5% of ASC patients had repeat visits within 3 years.
Characteristics of MH Cases
Of the 31 patients with a diagnosis of MH, 53.6% were younger than 45 years, 88.0% were Caucasian, and 55.0% were male (Table 1). All of the patients with a diagnosis of MH were discharged to home or self-care with the exception of 1 patient being transferred to a short-term hospital. Based on the Charlson-Deyo Comorbidity Score ≥1, 26.0% of the MH patients had at least 1 significant preexisting medical condition. Of 31 MH patients, 50.0% received general anesthesia, whereas 33.0% received administration of other anesthesia, such as local and regional anesthesia (Table 1). More than half of those with an MH diagnosis had private insurance as the primary payer for receiving medical services from ASCs, whereas approximately 39.0% of those diagnosed as MH susceptible were covered by Medicare and Medicaid (Table 1). All 31 patients with a diagnosis of MH were alive at the time of discharge from the ASCs. All patients diagnosed with MH were recorded in discharges from hospital-based ASCs. Based on the CCS codes, the difference in MH prevalence across all diagnosis categories was not statistically significant (P = 0.68; Table 2). The MH prevalence per discharge differed significantly across all CCS surgery categories (P = 0.01), with the highest prevalence (1.66 per 100,000 discharges; 95% CI, 0.00–3.39) being found in surgeries of the ear, such as tympanoplasty, myringotomy, and mastoidectomy (Fig. 1).
The number of freestanding ASCs in New York State increased almost threefold within 10 years, whereas the number of hospital-based ASCs remained unchanged (Fig. 2). Furthermore, the number of surgical visits to freestanding ASCs increased over the decade. Based on the SASD of 2002 to 2007, 92% of the ASC visits were surgical visits, and the remaining were likely preoperative assessment visits and postoperative follow-up visits because there were no procedure codes recorded for these visits. The top 3 surgery categories performed in ASCs from 2002 to 2007 were digestive system (29.7%), musculoskeletal system (12.5%), and eye (12.2%).
This study provides valuable information for understanding the epidemiologic patterns of MH in ASC settings. The results indicate that the prevalence of MH among ASCs in New York State between 2002 and 2011 is 0.18 per 100,000 discharges (95% CI, 0.12–0.25). In previous studies, the prevalence of MH in hospital surgical patients was found to be approximately 1 per 100,000 in New York State3 and 1.3 patients per 100,000 in the United States.12 To our knowledge, this study is among the first to estimate the prevalence of MH in ambulatory settings. The results indicate that the overall prevalence of MH in ASCs in New York State is much less than reported in hospital surgical patients, probably due to enhanced MH awareness and preferential treatment of MH-susceptible patients in the inpatient setting. It is important to note that the prevalence of MH discussed in this study encompasses both MH susceptibility and MH incidents.
Our results also indicate that 16.7% of patients with an MH diagnosis did not receive anesthesia. Because patients in ASCs usually undergo low-risk surgical procedures such as extraction of lens, which are less complex and less invasive than those performed in hospital settings, the anesthesia care often involves only regional and local anesthetics, decreasing the risk of triggering MH among MH-susceptible patients. Compared with hospitals, freestanding ASCs have fewer resources and capabilities to provide advanced treatment for urgent and life-threatening events. Many ASCs do not have immediate laboratory access to make a diagnosis and monitor its treatment.13 Therefore, MH-susceptible patients or patients who have been diagnosed with MH might choose to undergo surgery without general anesthesia or in hospital settings that are better equipped to manage a MH crisis, providing another plausible explanation for the lower prevalence of MH in ASCs.
Consistent with previous studies of inpatient populations, the age group for patients with the highest prevalence of MH in ASCs is <45 years.12 Although previous studies in hospital surgery patients reported a higher prevalence of MH in males than in females,3,12 this study did not find a significant difference between sexes in the prevalence of MH in ASCs, due in part to the modest number of MH cases included in the study. One possible explanation for the divergent finding from hospital surgery patients might be differences in utilization patterns of ASCs between sexes. Another finding of the study is that the category of ear surgery has the highest prevalence of MH. Insertion of tympanostomy tubes is the most common ambulatory surgery performed on children in the United States.14 One possible reason for the increased prevalence of MH in patients undergoing ear surgery might be that these patients are more likely to develop postoperative fever, and thus, the fever may be misdiagnosed as MH. It is also possible that the heightened prevalence of MH in ear surgery resulted from confounding factors, such as type of anesthesia. Further research using larger datasets and more sophisticated statistical techniques may help understand whether there is a causal relationship between ear surgery and excess risk of MH.
Our study has several limitations. First, our data sets did not contain variables that would allow us to distinguish incident MH events from visits where patients have a family history of MH or documented MH susceptibility. ICD-9 codes are likely to be more accurate for calculating disease prevalence than for calculating disease incidence because incidence requires identification of new cases or cases without previous documentation.15 The prevalence of MH as found in this study is the combined cases of incidence and MH-susceptible individuals during their stay in the ASCs. As suggested by an earlier study, to improve specificity of searches for incident MH events using administrative databases such as SASD, it might be meaningful to include information on surgical procedure and dantrolene administration.16 Another approach is to review medical records for patients with a recorded diagnosis of MH in combination with ICD codes because clinical evidence will be useful in differentiating incident MH cases from MH susceptibility in ASCs.15 The absence of ICD-9-CM surgical procedures since 2008 in the data set made it impossible for us to examine the associations between MH prevalence and types of surgical procedures among ambulatory surgery patients in recent years.
Second, SASDs are proprietary databases that focus on discharge diagnoses. The accuracy and completeness of MH diagnosis and coding may vary across facilities. Based on a recent study, the most common reason for inaccurate MH coding for hospital discharge records is because of high fever unrelated to anesthesia.16 The study also finds that prevalence of MH susceptibility is more likely to be captured accurately by ICD-9-CM code than MH incidence,16 implying that by using administrative databases such as SASD, the MH prevalence found in our study could be reflecting a larger proportion of MH-susceptible patients rather than MH incidence. Since the diagnosis code was first introduced in 1997,16 familiarity of MH coding among physicians and medical coders might vary significantly across ASCs, leading to potential underreporting, misdiagnosis, or miscoding of MH cases in our data sets. As demonstrated by our study, the number of freestanding ASCs has increased threefold over the decade, but no MH case was reported from any freestanding ASCs. This observation suggests that compared with hospital-based ASCs in which physician experts or well-trained medical coders could identify and report MH cases, freestanding ASCs might lack experienced medical staff to recognize the disease and record MH cases accurately. Further evaluation of the sensitivity and specificity of MH coding in outpatient administrative databases might help improve the accuracy of epidemiologic studies in ambulatory care settings.
Third, the prevalence reported in our study was based on the number of ambulatory surgery discharges, rather than unique individual patients. MH patients who were transferred from other health care facilities and died in the emergency room before admission to the hospital are not included in the SASD. Unplanned admissions to hospitals after ambulatory surgery were estimated to occur in 0.5% to 1.5% of cases.17,18 Finally, because of the modest number of MH cases in the study sample, our analysis was limited to descriptive statistics. In the absence of multivariate analysis, it was not possible for us to identify the risk factors for MH in ASCs with adjustment for confounding factors.
Despite these limitations, the large sample size of the SASD database enable epidemiologic analyses of a rare medical condition such as MH. Among ASC discharges in New York State, the overall prevalence of MH was less than among hospital discharges. Even though our study shows that all 31 patients with a recorded diagnosis of MH were discharged alive from ASCs, we cannot conclude that it is safe for patients with MH susceptibility to have surgery in an ambulatory setting. Conversely, MH susceptibility is not a contraindication for anesthesia and surgery in an ambulatory setting. All facilities where anesthesia is provided should be prepared to recognize, treat, and manage an MH crisis according to the guidelines established by accrediting agencies and the Malignant Hyperthermia Association of the United States. The findings of our study should be valuable for informing health care policy and developing clinical guidelines to ensure patient safety in ASCs.
Name: Zhen Lu, MPH.
Contribution: This author reviewed the literature, analyzed the data, and wrote the draft manuscript.
Attestation: Zhen Lu has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Conflicts of Interest: Zhen Lu declares no conflicts of interest.
Name: Henry Rosenberg, MD.
Contribution: This author helped design the study, interpret the findings, and revise the manuscript.
Attestation: Henry Rosenberg has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Conflicts of Interest: Henry Rosenberg reported a conflict of interest with Eagle Pharmaceuticals (Woodcliff Lakes, NJ) and received a 1-time speaking fee from Eagle Pharmaceuticals, a company that manufactures Ryanodex, a concentrated formulation of dantrolene approved for the treatment of malignant hyperthermia.
Name: Joanne E. Brady, PhD.
Contribution: This author helped supervise the statistical analysis, interpret the results, and write the manuscript.
Attestation: Joanne E. Brady has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Conflicts of Interest: Joanne E. Brady declares no conflicts of interest.
Name: Guohua Li, MD, DrPH.
Contribution: This author designed the study, acquired the data, supervised the data analysis, and oversaw the development of the manuscript.
Attestation: Guohua Li has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.
Conflicts of Interest: Guohua Li declares no conflicts of interest.
This manuscript was handled by: Sorin J. Brull, MD.
a Details of the record selection criteria are provided in the New York State Ambulatory Surgery Databases file composition notes. Available at http://www.hcup-us.ahrq.gov/db/state/sasddist/sasddist_filecompny.jsp. Accessed July 20, 2014.
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