Rates of Mortality in Lumbar Spine Surgery and Factors Associated With Its Occurrence Over a 10-Year Period: A Study of 803,949 Patients in the Nationwide Inpatient Sample

INTRODUCTION

Orthopaedic procedures have a small but important risk for mortality as a consequence of surgical intervention. Quantification of mortality rates is vital when providing preoperative counseling and weighing risks and benefits with patients. Unfortunately, capturing true mortality rates may be difficult given their uncommon occurrence and lack of sufficiently large sample populations. Further, with the increasing age of the population and number of procedures being performed each year,¹ more broadly established mortality incidence is needed to identify patient populations at risk for this devastating complication.

Complications after lumbar spine surgery have received more attention, as postoperative morbidity is trending upward with the number of procedures being performed¹ and are thus better reviewed.^2–4 Mortalities after lumbar spine surgeries have been reported in the context of many specific complications: lumbar fusions for elderly patients with renal disease (mortality 1.1%), elderly patients after lumbar trauma (mortality 6.8%), and adult spinal deformity patients (mortality 0.18%).^5–7 Risk factors most frequently associated with mortality include but are not limited to patient age, sex, gender, and comorbidity status.^5,8–11 However, literature that evaluates mortality in lumbar spine surgery has traditionally evaluated specific comorbidity populations or spine surgeries. Data from a large database with the ability to consolidate risk factors that should be identified preoperatively in patients are limited or dated.¹²

This study aims to capture nationwide trends in mortality with further stratification based on different procedure types, patient demographics and comorbidities, and in-hospital complications. By examining data on multiple procedures, the current study aims to obtain a broader perspective on mortality in this patient population. Data from a national database will importantly allow for generalizable mortality incidence given the capture of wide geographic regions, hospital types, surgeon characteristics, and patient populations.

METHODS

RESULTS

A total of 803,949 patients (51.1% female; average age 53.1 years) undergoing lumbar spine procedures between 2003 and 2012 were identified in the NIS database. There were a total of 1068 mortalities reported. Overall mortality in lumbar spine procedures across the years 2003–2012 was found to be 0.13%. In temporal analysis, there was a significant decrease in the overall mortality rate from 2003 to 2012 (2003: 0.24%; 2012: 0.18%; P = .004) (Figure 1).

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Figure 1

Results of trend analysis by analysis of variance, describing a significant change in mortality rate over 2003–2012.

DISCUSSION

While mortality incidence in lumbar surgery is low, a fundamental aspect of preoperative counseling is educating the patient on risks associated with surgery based on evidence-based literature. For reference, the Centers for Disease Control and Prevention (CDC) reported a 0.81% overall mortality rate in 2011 for the general population of the United States.¹⁴ Patient comprehension of risk-benefit discussions can be enhanced with the use of quantitative values over statements, but despite the importance of mortality risk education, these values can be difficult to obtain for rare events such as death. Literature on informed consent found that patients not counseled using a numeric approach had a tendency to overestimate benefits and underestimate risk.^15,16 Therefore, the mortality incidence established in lumbar surgeries stratified by patient comorbidities and procedures undergone may be particularly useful for patient counsel.

Mortality incidences established by the current studies' data describe expected risk factors: increasing age, male gender, black race, more complex surgeries performed, concurrent complications, and certain comorbidities. These risk factors were isolated as independent risk factors by controlling for comorbidities, race, sex, and procedure performed (except when testing the respective factors). Increasing age as an independent risk factor is consistent with findings of an important earlier study by Deyo et al¹² that also looked at mortality in lumbar surgery but was limited to a single institution. A study by Schoenfeld et al⁸ used pooled data to examine the influence of patient sex on morbidity and mortality in spine surgery and found that males were at increased risk compared to females. This was not well characterized in lumbar spine procedures but our analysis suggests that this is applicable specifically to the lumbar spine as well. Further research is necessary to identify the etiologies responsible for the differences in mortality risk that are being observed by demographics. Black race is an established risk factor across many surgical specialties including spine.⁹ The reasons for this are multifactorial and may include health care access and utilization in addition to anatomic differences. Skolasky et al⁹ reported an OR of 1.59 (CI 1.30–2.96) in analyzing additional risk of mortality in cervical spine surgery in blacks relative to whites, comparable to our finding of 1.40 (CI 1.10–1.79).

The results obtained on patient comorbidities suggest that liver disease is associated with a significantly increased mortality rate. This may be related to the liver's multiple physiologic roles, including coagulation and its effect on blood loss. A large study looking at perioperative morbidity and mortality in spinal fusion found that the highest risk of mortality was associated with a finding of coagulopathy.¹⁷ This may be compatible with our finding of liver disease and increased mortality. Congestive heart failure and history of myocardial infarction as predictors for mortality in noncardiac surgery is unsurprising, as they are well established.^18–20 A history of cerebrovascular disease was also identified as a significant patient comorbidity associated with mortality. This is consistent with the findings of Jorgensen et al^21,22 in a novel study that found a history of stroke to be associated with adverse outcomes after surgery. Their recommendation to wait 9 months in this patient population until elective surgery may be warranted. Interestingly, controlling for metastatic tumor diagnosis caused malignancy diagnoses to remain insignificant as a mortality risk factor. Malignancies included lymphoma and leukemia but excluded malignant neoplasms of the skin.

A few comorbidities stood out with large patient cohorts and high mortality: 5183 patients with mild liver disease sustained mortality of 1.91%, 16,683 congestive heart failure patients sustained mortality of 1.42%, and 9238 patients with cerebrovascular disease sustained mortality of 1.09%. These patient groups, with high risk of death, deserve further study to establish if the risk factors established by the current study, ranging from 3.92 to 10.04, could have been avoided via different or nonsurgical treatment.

In analyzing diagnostic risk factors for mortality in lumbar spine surgery, disc displacement, stenosis, disc degeneration, and spondylolisthesis, by far the highest-volume surgeries, demonstrated decreased risk relative to surgeries such as fracture, scoliosis, and cord injuries. After controlling for the type of surgery being performed (simple fusion, complex fusion, or just decompression), these high-volume surgeries continued to show decreased risk of death relative to less frequent, commonly more complex surgeries. Surprisingly, surgeries associated with a scoliosis diagnosis were not associated with an increased risk of death. This defies expectations, as scoliosis is frequently associated with deformity correction and a plethora of comorbid factors but is backed up by literature. For example, Smith et al²³ reported mortality incidence of only 1.8 per 1000 in over 100 000 deformity surgeries.

The limitations to our study arise from the design of the NIS database. Procedure type was divided to roughly stratify case complexity, but the available data prevent us from detailing the surgical procedure (ie, estimated blood loss, surgery length, preoperative American Society of Anaesthesiologists grade) and identifying confounders for mortality. In addition, patient comorbidities are not fully specified (eg, severity, length of disease, patient medication compliance). Another limitation to recognize is that the severity of complications is not quantified by the NIS, with the end point noted only if it involved mortality. Also, due to our data analysis procedure, the operative diagnosis was not examined for many groups. The operative diagnosis could explain, for example, the justification for operation on patients with liver disease or metastatic cancer. Finally, because the NIS database is an inpatient sample, it is unable to capture complications and death in the postoperative period following discharge. While this underestimates the overall mortality rate, the Scoliosis Research Society database, including approximately 100 000 cases, found that close to 80% of postoperative mortality occurred during the patient's initial hospital stay.²³ This study is applicable to elective procedures but cannot be generalized to all lumbar surgeries because it does not include emergency admissions or cases involving trauma, fracture, or malignancy. This underestimates the overall mortality associated with lumbar spine surgery, and our findings should be interpreted in the provided context. An earlier study of the NIS from 1998 to 2008 by Memtsoudis et al¹¹ included both emergent and urgent cases and found a higher mortality rate of 0.20% in patients undergoing lumbar spinal fusion.

CONCLUSIONS

Between 2003 and 2012, there was an overall mortality incidence rate of 0.13%. After controlling for comorbidities, race, sex, and procedure performed, increased risk of mortality was seen with complex fusion and older age, male sex, and/or black race. Significant comorbidities associated with increased risk included a history of liver disease, congestive heart failure, and cerebrovascular disease. These results may be helpful to surgeons in determining risk prognosis and identifying high-risk patients prior to lumbar surgical procedures with the hopes of improving outcomes and reducing patient mortality.