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.
1. Rosenberg H, Davis M, James D, Pollock N, Stowell K. Malignant hyperthermia. Orphanet J Rare Dis. 2007;2:21
2. Torpy JM, Lynm C, Glass RM. JAMA patient page. Malignant hyperthermia. JAMA. 2005;293:2958
3. Brady JE, Sun LS, Rosenberg H, Li G. Prevalence of malignant hyperthermia due to anesthesia in New York State, 2001-2005. Anesth Analg. 2009;109:1162–6
4. Cullen KA, Hall MJ, Golosinskiy A Ambulatory Surgery in the United States, 2006. 2009 Hyattsville, MD National Center for Health Statistics DHHS Publication No. (PHS) 2009–1250
5. National Center for Health Statistics.Health, United States, 2012: With Special Feature on Emergency Care. 2013 Hyattsville, MD National Center for Health Statistics DHHS Publication No. 2013–1232
6. Preti L, Senathirajah M, Sun C Evaluation of the State Ambulatory Surgery Databases, Available Through the HCUP Central Distributor, 2008. 2011 Rockville, MD US Agency for Healthcare Research and Quality Series Report 2011–02
7. Larach MG, Gronert GA, Allen GC, Brandom BW, Lehman EB. Clinical presentation, treatment, and complications of malignant hyperthermia in North America from 1987 to 2006. Anesth Analg. 2010;110:498–507
8. Elixhauser A, Steiner C, Palmer L Clinical Classifications Software (CCS). 2004 Rockville, MD U.S. Agency for Healthcare Research and Quality Report 2004–02
9. Deyo RA, Cherkin DC, Ciol MA. Adapting a clinical comorbidity index for use with ICD-9-CM administrative databases. J Clin Epidemiol. 1992;45:613–9
10. Quan H, Sundararajan V, Halfon P, Fong A, Burnand B, Luthi JC, Saunders LD, Beck CA, Feasby TE, Ghali WA. Coding algorithms for defining comorbidities in ICD-9-CM and ICD-10 administrative data. Med Care. 2005;43:1130–9
11. Rosner B Fundamentals of Biostatistics. 20066th ed Belmont, CA Duxbury:145–50
12. Rosero EB, Adesanya AO, Timaran CH, Joshi GP. Trends and outcomes of malignant hyperthermia in the United States, 2000 to 2005. Anesthesiology. 2009;110:89–94
13. Litman RS, Joshi GP. Malignant hyperthermia in the ambulatory surgery center: how should we prepare? Anesthesiology. 2014;120:1306–8
14. Rosenfeld RM, Schwartz SR, Pynnonen MA, Tunkel DE, Hussey HM, Fichera JS, Grimes AM, Hackell JM, Harrison MF, Haskell H, Haynes DS, Kim TW, Lafreniere DC, LeBlanc K, Mackey WL, Netterville JL, Pipan ME, Raol NP, Schellhase KG. Clinical practice guideline: tympanostomy tubes in children. Otolaryngol Head Neck Surg. 2013;149:S1–35
15. O’Malley KJ, Cook KF, Price MD, Wildes KR, Hurdle JF, Ashton CM. Measuring diagnoses: ICD code accuracy. Health Serv Res. 2005;40:1620–39
16. Pinyavat T, Rosenberg H, Lang BH, Wong CA, Riazi S, Brady JE, Sun LS, Li G. Accuracy of malignant hyperthermia diagnoses in hospital discharge records. Anesthesiology. 2015;122:55–63
17. Mezei G, Chung F. Return hospital visits and hospital readmissions after ambulatory surgery. Ann Surg. 1999;230:721–7
© 2016 International Anesthesia Research Society
18. Twersky R, Fishman D, Homel P. What happens after discharge? Return hospital visits after ambulatory surgery. Anesth Analg. 1997;84:319–24