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Evidence for the Use of Preoperative Risk Assessment Scores in Elective Cranial Neurosurgery: A Systematic Review of the Literature

Reponen, Elina MD*; Tuominen, Hanna MD, PhD*; Korja, Miikka MD, PhD†‡

doi: 10.1213/ANE.0000000000000234
Neuroscience in Anesthesiology and Perioperative Medicine: Review Article

BACKGROUND: Preoperative risk scores are designed to guide patient management by providing a means of predicting operative outcome. Several risk scores are used in neurosurgery, but studies on their clinical relevance are scarce. Therefore, it is not clear whether these risk scores are beneficial or helpful in predicting outcome after elective cranial neurosurgery. In this review, we summarize the current scientific evidence for using preoperative risk scores in elective cranial neurosurgery.

METHODS: A systematic review of the MEDLINE, Embase, and PubMed databases in November 2013 yielded 25 relevant studies with a minimum of 30 patients per study. The studies evaluated the value of the preoperative ASA physical status classification, the Karnofsky performance score (KPS), the Charlson comorbidity score, the modified Rankin Scale and the sex, KPS, ASA physical status classification, location, and edema (SKALE) score in assessing postoperative outcome in cranial neurosurgery. Surgery-related and nonsurgical complications were assessed separately whenever reported in the original article. For this purpose, the studies were placed into 4 categories based on the reported outcome: surgery-related outcome, nonsurgical outcome, morbidity, and mortality. The Preferred Reporting Items for Systematic reviews and Meta-analyses guidelines for systematic reviews were followed.

RESULTS: KPS has the strongest support in the literature for predicting surgery-related outcomes. There is no strong support in the literature for the use of any preoperative scores in predicting nonsurgical outcomes after elective craniotomies. KPS and ASA physical status classification seem to predict early (≤ 30-day) morbidity of intracranial tumor patients. The Charlson comorbidity score may be applicable in predicting mortality of elective intracranial aneurysm patients. Only 4 studies were prospective in design.

CONCLUSIONS: Large prospective studies are needed to validate the use of the reviewed risk scores in elective cranial neurosurgery. It appears, however, that the patient’s preoperative physical and functional status can be used to predict the short- and long-term outcome in elective cranial neurosurgery.

From the Departments of *Anesthesiology and Intensive Care Medicine, and Neurosurgery, Helsinki University Central Hospital, Helsinki, Finland; and Australian School of Advanced Medicine, Sydney, Australia.

Accepted for publication March 5, 2014.

Funding: No funding was received.

The authors declare no conflicts of interest.

Reprints will not be available from the authors.

Address correspondence to Elina Reponen, MD, Department of Anesthesiology and Intensive Care Medicine, Helsinki University Central Hospital, P.O.Box 266, 00029 HUS, Helsinki, Finland. Address e-mail to

Intraoperative and postoperative morbidity play the most important roles in determining the quality of life and functional outcome of surgical patients and have a major impact on the overall cost-effectiveness of surgical treatments.1–3 Preoperative risk assessment scores are designed to help patients and clinicians in anticipating operative risks before surgery. Several preoperative scores and models have been developed and used to predict operative outcome, but their widespread use has been limited by their poor specificity and sensitivity.4 A few subspecialties in surgery have succeeded in designing predictive preoperative models; the European System for Cardiac Operative Risk Evaluation (EuroSCORE) risk stratification system, for instance, is widely used in assessing the risk of patients scheduled for cardiac surgery.5 Similarly, ASA physical status classification system has been a useful assessment score in predicting outcome in major abdominal surgery as well as in renal cell carcinoma surgery.6,7 Moreover, the ASA physical status classification system predicts the outcome of general and vascular surgery as well as spinal surgery patients, because patients with higher ASA physical status grades have a higher risk of morbidity.8,9 Recently, a systematic review of the risk stratification tools for predicting morbidity and mortality in a heterogenous cohort of surgical patients (noncardiac/nonneurological) suggested that the most promising risk predictors were the Portsmouth-Physiology and Operative Severity Score for the enUmeration of Mortality (P-POSSUM) and the Surgical Risk Scale.10

The clinical relevance of preoperative risk assessment scores in predicting the neurological or overall outcome of cranial neurosurgery patients has been poorly studied. Given that the average age of neurosurgical patients and the prevalence of age-related comorbidities are constantly increasing, preoperative risk assessment scores could potentially help to identify high-risk patient groups in cranial neurosurgery.11,12

Addressing this issue, the search strategy in this systematic review was based on a specific question “What is the evidence for the use of risk assessment scores in elective cranial neurosurgery?” In addition to summarizing the available evidence, we discuss the strengths and weaknesses of these risk assessment scores, with particular focus on their end point-prediction quality and applicability in elective cranial neurosurgery.

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We used the published standards for reporting systematic reviews.13 The Preferred Reporting Items for Systematic reviews and Meta-analyses (PRISMA) checklist was also used (Appendix 1).14

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Definitions for the Purposes of This Study

For the purposes of this review, surgery-related outcomes included hematomas, cerebral infarctions, neurological deficits, and altered functional status (category 1). Systemic and infectious complications, such as surgical site infection or meningitis, pneumonia, thromboembolic, and cardiac complications were considered nonsurgical outcomes (category 2). If associations with the preoperative risk assessment scores and outcomes were not reported separately for surgery-related and nonsurgical end points, the outcome was defined as morbidity (category 3). The fourth category consisted of studies in which mortality was the only reported outcome. The terms “early” and “short-term” are used to refer to a follow-up period of up to 1 month postoperatively, whereas “long-term” refers to follow-up periods lasting from several months to several years. The length of follow-up for each study is reported in detail in Table 1. The association of the preoperative risk score and outcome end point was considered significant when yielding a P value of <0.05.

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Search Strategy

We performed a literature search for English-language articles published between January 1, 1980, and November 14, 2013, using the MEDLINE, Embase, and PubMed databases as described in Appendix 2.

After the broad initial search, the titles and abstracts of the search results were screened, and all studies reporting original data for at least 30 patients were selected for a full-text review (Fig. 1, Appendix 2). The reference lists of the selected full-text articles were further checked for additional studies. Reviews, editorial notes, and case reports were excluded, as were studies without online abstracts. The selected cutoff number of 30 patients was based on the assumption that very small study cohorts probably lack statistical power in their outcome analyses. Studies reporting only survival were excluded, since the vast majority evaluated tumor-related long-term survival. In addition, if only 1 eligible study reported the use of a specific preoperative assessment method, the study was excluded due to the possibility of a publication bias. Studies including only emergency operations and/or <24-hour follow-up periods were also excluded.

The initial MEDLINE, Embase, and PubMed searches resulted in 265, 425, and 1539 articles, respectively. Of the 2229 articles, 2101 were excluded on the basis of the reviewing criteria (Fig. 1, Appendix 2). One hundred twenty-eight full-text articles were carefully reviewed (Fig. 1). Of these, only 25 articles (which evaluated 5 different risk assessment scores) met the inclusion criteria and were included in the final review (Fig. 1).

The PRISMA guidelines were followed for the description of this review. All 25 studies were graded as II+ (well-conducted case-control or cohort studies with a low risk of confounding or bias and a high probability that the relationship is causal). Our systematic review was graded as II++ (high-quality systematic reviews of case-control or cohort studies) for strength of evidence. Quantitative analyses (i.e., meta-analysis) were not performed since the included studies were highly heterogenous.

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Models and scores that were used in the reviewed articles included the ASA physical status classification,15,16 the sex, Karnofsky performance score (KPS), ASA physical status classification, location, and edema (SKALE) score,17 the KPS,18 the modified Rankin Scale (mRS),19,20 and the Charlson comorbidity score.21 The P-POSSUM score22,23 was also used in 2 articles. However, since P-POSSUM includes intraoperative variables, it cannot be used in a preoperative risk assessment setting, and therefore, the scale was not included in the review. A summary of the key elements of the reviewed studies is presented in Table 1.

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Outcomes Reporting

There was considerable heterogeneity in the chosen outcome end points as well as in the length of follow-up periods between the reviewed studies (ranging from the in-hospital period to 8 years). Short-term mortality rates varied between 0% and 23% and reported short-term morbidity rates varied between 3% and 56%.

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The most widely known and applied model of preoperative risk assessments is the ASA physical status classification, introduced in the 1940s and revised in 1963.15,16 The ASA physical status classification is a 5-tier classification system describing the preoperative physical status of a patient. ASA physical status I refers to a healthy patient, whereas ASA physical status V refers to a patient who is likely to die within 24 hours without surgery (Table 2). It is a concise scale and easy to use in a variety of clinical settings.

Ten studies evaluated the use of the ASA physical status classification in assessing morbidity and/or mortality of craniotomy patients.17,24–32 Two studies reported that both elective and emergency patients were included in the study protocol,28,32 but the remaining 8 studies did not report whether the participants were emergency or elective patients.17,24–27,29–31 Considering the indications given for surgery, these 8 studies included probably more elective than emergency craniotomy patients. However, the ASA physical status classification system is also applicable to emergency craniotomy patients, and therefore, this estimate is not reliable. Eight17,25,26,28–32 of the 1017,24–32 studies, one32 of which was prospective and 717,25,26,28–31 retrospective in design, found a positive correlation between the ASA physical status classification and outcome.

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Surgery-Related Outcomes

Two studies evaluated the role of the ASA physical status classification in predicting surgery-related outcome measures.24,25 Both were retrospective in design. One of the studies evaluated a group of 208 patients with unruptured intracranial aneurysms, 157 of whom underwent craniotomy.24 In this study, the only variable correlating with poor outcome (Glasgow Outcome Scale <4) at 6 months after the surgery was calcification of the aneurysm.24 The other retrospective study on 96 elderly intracranial meningioma patients concluded that a preoperative ASA physical status classification score of III or IV predicts poor outcome, defined as a KPS score of 70 or less at 4-month follow-up.25

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Nonsurgical Outcomes

In a prospective study of 390 patients undergoing craniotomy, craniectomy, cranioplasty, or burrhole operations for various indications, 36.9% of the operations were elective.32 The authors found that a preoperative ASA physical status classification score of II or higher predicted surgical site infections.32 One retrospective study evaluated the incidence of postoperative meningitis in a cohort of 453 patients with both elective and emergency craniotomies.28 Higher preoperative ASA physical status classification scores were associated with an increased incidence of postoperative meningitis.28

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Morbidity and Mortality

Two retrospective studies on 36 and 203 patients with meningiomas and malignant tumors, respectively, reported the association of a preoperative ASA physical status classification score of III or higher with an increased postoperative morbidity.,29,30 Four retrospective studies on elderly patients with intracranial meningiomas (305 patients) reported postoperative mortality as the primary outcome measure.17,26,27,31 In 3 of these studies, the association of the higher ASA physical status classification scores and mortality was significant;17,26,31 however, 1 study of 30 patients found no such association.27

In summary, 2 of the 10 studies using ASA physical status classification concluded that ASA physical status classification was not associated with outcome.24,27 Both of these studies were retrospective in design and included 157 and 30 craniotomy patients who underwent surgery for unruptured intracranial aneurysms24 and meningiomas,27 respectively. Most studies found evidence suggesting that ASA physical status classification may be useful in the preoperative risk assessment of patients in cranial neurosurgery. Surgery-related and nonsurgical outcome measures were poorly defined in the articles. There is some evidence that the preoperative ASA physical status classification may predict surgical site infections, postoperative functional status, and postoperative meningitis.25,28,32 It should be noted, however, that most of these studies were relatively small (<300 patients in 8 of 10 studies), retrospective (9 of 10 studies) in design, and did not evaluate short-term (<30 days) morbidity and mortality (5 of 10 studies). Thus, to conclude whether ASA physical status classification can play a role in predicting early or late postoperative outcome in cranial neurosurgery is somewhat unclear. In 1 of the 2 studies,24,27 that did not associate ASA physical status classification with outcome, another scaling system (KPS) was found to be associated with outcome.27

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The KPS score is used in determining the functional status of cancer patients. It has also been used in predicting the risk of an unfavorable outcome or morbidity after surgical operations. The functional status is evaluated on a scale of 0 to 100, 0 being deceased and 100 representing a normal healthy person with no signs of illness (Table 3).18

Sixteen studies used KPS as a preoperative predictor of outcome in cranial neurosurgery, and 14 of these studies reported a positive correlation between the preoperative KPS and outcome.17,25–27,30,31,33–40 Three of these 14 studies were prospective in design,33,39,40 whereas the remaining 11 were retrospective.17,25–27,30,31,34–38 Outcome was defined as early neurological deficit,33,35,36 short-term mortality, and/or short-term morbidity (defined as reoperation or complications prolonging the hospital stay;27,38,40,41 short-term neurological, medical [e.g., thromboembolic complications, sepsis, pneumonia, or heart failure], or operative [e.g., wound infection or cerebrospinal fluid leakage] complications;30,34,37 and long-term functional status,39 mortality,17,26,31 or surgical [e.g., hematoma or cerebral infarction] or medical complications such as meningitis, pulmonary embolism, or cardiorespiratory failure).30 The studies included 610 craniotomy patients who underwent surgery for meningiomas,17,25–27,31,36 876 who underwent surgery for malignant intracranial tumors/metastases,30,33,35,39 and 4569 who underwent surgery for unspecified intracranial tumors.34,37,38

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Surgery-Related Outcomes

Six studies used KPS for the preoperative assessment of surgery-related risks.25,33,35,36,39,42 Five studies (2 of which were prospective33,39 and 3 retrospective25,35,36) found a positive correlation between a preoperative KPS and surgery-related outcome. In a prospective study of >400 craniotomy patients with malignant gliomas, KPS was deemed an important neurological outcome predictor at the 21-day follow-up.33 Another prospective study of 80 patients with high-grade gliomas found no significant change in the patients’ preoperative versus postoperative KPS at 6-month follow-up, indicating a positive prognostic association.39 In a retrospective study of 94 patients with insular gliomas, a high preoperative KPS was associated with a favorable outcome, defined as a postoperative KPS of 80 or more within a 3-month follow-up period.35 In a series of 96 elderly patients with intracranial meningioma, a preoperative KPS of 70 or less was predictive of a poor postoperative outcome, defined as a KPS 70 or less.25 In a retrospective study of 75 patients with meningiomas, KPS was associated with early (at 1 week) but not late (at 3 months) neurological deficits.36 One study of 196 intraaxial brain tumor patients (175 of which underwent an elective operation) failed to find a significant correlation between the preoperative KPS and the postoperative neurological status or operative complications.42

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Nonsurgical Outcomes

Nonsurgical outcome measures were reported in 3 retrospective studies.34,38,42 In a study of 84 elderly (70 years or older) patients with intracranial tumors, low preoperative KPS was found to be associated with postoperative systemic complications such as pneumonia, acute heart failure, acute renal failure, and sepsis during the 4-week follow-up period.34 In a large retrospective series of 4293 patients with intracranial tumors, a KPS of 70 or less was statistically significantly associated with the development of deep venous thromboembolism or pulmonary embolism within 30 days after craniotomy.38 One study of 196 intraaxial brain tumor patients, 175 of which underwent elective operation, failed to find a significant correlation between the preoperative KPS and systemic complications.42

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Morbidity and Mortality

Overall morbidity was reported as an end point in 5 studies,30,37,40–42 only one of which was prospective.40 In a retrospective study of 207 patients who underwent surgery for supratentorial tumors, a KPS of 50 or less correlated with a complicated outcome (neurological, medical, or operative complications) or increased length of stay within a 30-day follow-up period.37 In a retrospective study of 203 patients who underwent surgery for malignant intracranial tumors, KPS was associated with postoperative complications, including postoperative hematomas, local infections, fistulas, thromboembolic complications, sepsis, and cardiac failure, within a 30-day follow-up period.30 Three studies reported both overall morbidity and mortality. Two retrospective studies of 20041 and 19642 patients with gliomas/metastases and intraaxial brain tumors, respectively, found that neither postoperative morbidity nor 30-day mortality associated with the preoperative KPS. Furthermore, KPS was not associated with a prolonged hospital stay, life-threatening situation, reintervention, or readmission.41 In a prospective study of 327 patients with gliomas or metastases, preoperative KPS was associated with 30-day mortality but not with morbidity.40 Finally, all 4 studies with mortality as the primary end point reported a positive correlation with a preoperatively high KPS.17,26,27,31 Collectively, these studies included 305 elderly patients with intracranial meningiomas.

In summary, most studies suggest that the preoperative KPS is a valuable tool for assessment of not only long-term17,25,27,31,35,36,39 but also short-term27,30,33–37,40 postoperative outcome in patients with intracranial tumors, regardless of the nature of the tumor. Fourteen of 16 previous studies especially evaluated the usefulness of KPS in the assessment of short-term (<30 days) operative outcome. KPS seems to be well suited for the preoperative assessment for surgery-related risks, since 4 of 5 studies addressing this end point found a correlation. However, 5 of 7 studies found no significant correlation between preoperative KPS and mortality. In addition to most reviewed retrospective studies,17,25–27,31,34–37 3 prospective studies reported that KPS correlated with short-term mortality,40 morbidity,33 and long-term functional status at 6 months.39 The results of these studies showed that patients with a lower KPS tended to have a poor outcome. Unfortunately, only one of the studies stated whether the patients were elective or emergency cases or a mixed population of both.39

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The SKALE score is a proposed risk assessment system for meningioma patients. The score consists of 5 independent factors: sex, KPS, ASA physical status classification, meningioma location, and peritumoral edema. A patient is scored 0, 2, or 4 points for each factor, and a low total score is supposed to predict an unfavourable neurological outcome (Table 4).17

Only 2 studies used the SKALE score to assess outcome in cranial neurosurgery, and both were retrospective in design.17,31 In these studies, outcome was defined as long-term (12 months) mortality.17,31 A study of 164 meningioma patients did not find the SKALE score to be more useful than ASA physical status classification alone in predicting the risk of mortality within a 12-month postoperative period.31 Another study of 74 elderly meningioma patients17 concluded that a SKALE score of 8 or higher is associated with a significantly lower mortality rate at 12 months; however, the authors stated that prospective studies were needed to validate this grading system.17 In summary, there are no studies linking the preoperative SKALE score and short-term neurological outcome in neurosurgery patients, but the preoperative SKALE score may correlate with the long-term mortality of surgical elderly meningioma patients.17

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The mRS was originally developed in the late 1950s to classify the functional status of stroke patients due to neurological deficits.19 It was modified to its current form in the late 1980s20 and describes the patient’s functional status on a scale from 0 to 6; 0 being a symptomless patient and 6 being dead (Table 5).

Two retrospective studies evaluated the preoperative mRS in outcome prediction.43,44 In a series of 59 patients with brainstem cavernous malformations (54 of whom underwent craniotomy), a low preoperative mRS (0–2) was found to be an independent predictor of surgical outcome, defined as an mRS <2 at 1-year follow-up.44 In the other study, 120 patients with 136 unruptured posterior circulation aneurysms underwent 121 craniotomies. In this series, the preoperative mRS was not found to be a significant predictor of surgical morbidity or mortality.43 Therefore, the evidence on the applicability of mRS in the preoperative risk assessment of elective craniotomy patients is very limited.

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The Charlson comorbidity score is a weighted index that considers the severity of the patient’s comorbidities. It was originally developed to predict the 10-year survival of medical patients.21 Each of the 22 comorbidities used in this system is assigned a score of 1, 2, 3, or 6, and the total score represents the patient’s overall comorbidity; the higher the score, the higher the risk of an adverse outcome after surgery. In a later modification (the Charlson Comorbidity Index), age increases the total score by 1 point for each decade over the age of 40 years (Table 6).

Three retrospective studies have studied the association of the preoperative Charlson comorbidity score and postoperative outcome.45 In these studies, outcome was defined as postoperative inpatient mortality,45–47 and 2 studies included postoperative complications, increased length of stay, and total hospital costs.46,47 These register-based retrospective studies included 3738 operated patients with unruptured intracranial aneurysms,45 4907 patients with metastatic brain tumors,46 and 5717 patients with intracranial meningiomas.47 Two studies included both elective and emergency patients,45,47 while in the third study, this information was not disclosed. All 3 studies found that the Charlson comorbidity score correlates with outcome in craniotomy patients. In the aneurysm study, high scores were associated with in-hospital mortality.45 In the study of elderly patients (65 years or older) with metastatic brain tumors, each 1-point increase in the Charlson score was associated with a 12% increase in the odds for inpatient death, an increase of 0.52 days in length of stay, stroke, and pulmonary complications.46 In the study of elderly intracranial meningioma patients (mean age 73.6 years), the Charlson score was associated with higher odds for inpatient death and major complications after surgery.47

According to these large register-based retrospective studies, the Charlson comorbidity score may be associated with the surgical outcome of aneurysm patients and elderly patients with intracranial tumors.45–47 However, 2 of these 3 studies were reported by the same authors from Johns Hopkins, Baltimore, Maryland, and therefore, the external validity may be low, and the findings should perhaps be interpreted with caution.

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The neurosurgical literature on preoperative risk assessment methods is still scarce, but, given that most of the neurosurgical studies have been published after 1998, interest in the topic is likely to be increasing. In comparison to cardiac surgery where >300 articles on preoperative risk assessment methods were published in 2011 alone, we found only 25 studies relevant to neurosurgery. None of the included studies was based on an unselected consecutive series of elective craniotomy patients, and only 16% of the studies were prospective in design. Moreover, the number of patients in one-third of the studies was smaller than 100, and the assessment methods, study settings, and reporting principles varied considerably, making direct comparisons and risk calculations (e.g., predictive values of risk assessment scores) practically impossible.

Preoperative risk assessment scores can be subdivided into risk scores and risk prediction models.10 All the scales discussed in this review represent risk scores (Tables 2–6), which cannot provide an individualized risk prediction of an adverse outcome.48 Multivariate risk prediction models, such as P-POSSUM, are capable of such individualized risk prediction, but they are much more complex and time-consuming to use in the daily clinical setting. According to our review of the risk scores in elective cranial neurosurgery, a patient’s functional status (KPS) is associated with both short- and long-term outcomes in patients with intracranial tumors after elective craniotomy. ASA physical status and comorbidity (Charlson comorbidity score) assessments are associated with postoperative outcome after elective intracranial surgeries in selected patient populations. In comparison, in the EuroSCORE (which predicts outcome after cardiac surgery),5 functional status and comorbidities are also an important part of the risk calculator. Whether the functional and physical status along with comorbidities can be incorporated into a single preoperative risk evaluation scale for neurosurgery remains a subject for future studies.

Considering surgery-related and nonsurgical outcomes in the reviewed studies, preoperative KPS appears to be associated with surgery-related outcome in elective cranial neurosurgery33,35,36,39,42 (Table 7). In contrast, however, nonsurgical outcomes such as systemic infections seem to be much more difficult to relate to the preoperative scores in cranial neurosurgery, even though there is some evidence supporting the use of both ASA physical status28,32 and KPS34,38 for this purpose. When outcome was defined as morbidity (i.e., adverse events that were not recored as surgery-related or nonsurgical end points), it seems that ASA29,30 physical status and Charlson comorbidity score46,47 can be used preoperatively to estimate any adverse outcome after cranial neurosurgery. Finally, 5 of the 25 reviewed studies used mortality as the only measure of outcome. Since death is a rare complication in modern elective neurosurgery, none of the reviewed preoperative scoring systems may be sensitive enough for postoperative mortality. Despite this though, preoperative ASA physical status,17,26,31KPS,17,26,27,31,40 SKALE,17,31 and Charlson comorbidity score45–47 seem to be associated with mortality in elective cranial neurosurgery.

Ease of use is an imperative factor for any preoperative risk assessment score to achieve wide clinical applicability. Even though the simplicity and familiarity of the ASA physical status classification are its major strengths in clinical use, it has been largely criticized for being subject to the subjective interpretations of assessing clinician.,49,50 In addition, many countries and institutions have developed various modifications of ASA physical status classification, which hinders comparisons of the study results. Our physical and social surroundings and the prevalence and incidence of many diseases have changed tremendously since the introduction of ASA physical status classification; however, the classification itself has remained virtually untouched. Most importantly, what was originally intended as a tool for scientific and statistical purposes is now often understood as a classification of the anesthesiological risk of a patient or even as a preoperative measurement of the total operative risk of a patient. Given this, it is perhaps unfair to review ASA physical status classification as a preoperative risk assessment method, especially since its limitations were carefully discussed in the original article by Saklad16 in 1941. However, since ASA physical status classification is in daily use in a number of neurosurgical units, presenting these controversial views may give rise to a critical appraisal of its use in elective neurosurgery. KPS is also a well-established, easy-to-use, and relatively simple scoring system. The assessment is usually performed by an oncologist or surgeon. Perhaps the most advantageous risk assessment aspect of KPS is that it considers information that is obtained directly from the patient. This information pertains to the activities of daily living and functional capacity of the patient and can be acquired either through verbal communication or by adding relevant questions to a preoperative questionnaire. Despite the strengths of KPS, it is unclear whether it can reliably predict morbidity and mortality in categories other than intracranial tumor patients. Since the SKALE score was developed only for meningioma patients, it is not applicable to other neurosurgical patients.17 Moreover, it is a relatively time-consuming and complex scoring system, and it is not easily implemented into everyday practice. Whether this score provides any benefits over the individual ASA physical status classification or KPS that are incorporated into the SKALE score is unclear. An mRS is a simple representation of the patient’s functional status and can easily be applied in the clinical setting. An mRS score of 3 is often considered a critical cutoff value, since it distinguishes between persons with functional independence and persons requiring assistance. However, large, unselected, and prospective studies assessing the usefulness of mRS are lacking, and the role of mRS in the preoperative risk assessment of elective cranial neurosurgery is unclear. Finally, the Charlson comorbidity score is a complex scoring system that requires a good knowledge of the patient’s underlying illnesses as well as of the grading system itself. However, it may be used in predicting inpatient mortality or complicated postoperative recovery in cranial neurosurgery, even without considering the nature and location of the lesion.

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This review has 2 main strengths. It is based on relatively broad and flexible literature search criteria to increase the likelihood of including most of the relevant data available. Moreover, since the outcome of patients undergoing emergency surgery is strongly associated with their underlying acute illness at the time of surgery,51 the selection of studies that only report elective craniotomy patients can be considered beneficial. However, we were unable to fully exclude emergency surgery patients from the review, because this information was not clearly stated in every article. It is also noted that preoperative scales (e.g., ASA physical status classification and the Charlson comorbidity score) are used for both elective and emergency patients, and therefore, exclusion is neither practical nor perhaps warranted on the basis of the intended use of the scales themselves.

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An outcome prediction scale applicable to a consecutive series of elective craniotomy patients regardless of age and diagnosis would probably be beneficial. As stated by Schul et al.,31 better preoperative patient assessment methods are needed, and these should be based on a large patient series. According to a common belief, the nature and location of the neurosurgical lesion together with the experience of the surgeon determine surgical outcome to such an extent that preoperative risk scores cannot be used in outcome prediction unless these factors are somehow incorporated in the score. Inevitably, such factors do play an important role in outcome prediction, but it is likely that a patient’s comorbidities, physical status, and functional status predict morbidity and mortality in neurosurgery as well as other surgeries, such as cardiac and major abdominal surgery, that has been suggested in this and other reviews.5,7 For example, comorbidities may have effects on intraoperative (e.g., hemodynamics, urinary output, cerebral blood flow, coagulation cascades, brain metabolism, etc.) and postoperative (e.g., anti-inflammatory responses, mobilization, etc.) variables, which in turn may contribute to morbidity and mortality. In a large consecutive cohort of operative patients, the role of surgeon-related outcome factors could be minimized. Unfortunately, none of the reviewed studies fulfilled these criteria, but results from large register-based retrospective series of patients suggest that a patient’s physical status (in terms of comorbidities) may predict neurosurgical outcome, despite the location and nature of the lesion.46,47 Today, shared decision-making with patients and a patient-centered approach are emphasized across the field of health care. In keeping with this focus, future studies should be aimed at developing and validating reliable and easy-to-use preoperative risk assessment scores for elective neurosurgery. Ideally, the risk score should be simple and accessible for preoperative risk assessment even by the patients themselves.

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In light of current knowledge, the most applicable preoperative risk assessment scales for elective craniotomy patients may be the KPS, Charlson comorbidity score, and ASA physical status score. The most applicable risk assessment score for each individual case, however, depends on which kind of outcome is deemed significant, because according to this review, different scores predict different outcomes in specific patient populations. There is no evidence supporting the use of any preoperative risk assessment score for predicting mortality or neurological outcome in a consecutive series of elective craniotomy patients. In keeping with our findings and the current emphasis placed on patient safety aspects worldwide, patient-derived risk assessment methods should also be studied in outcome prediction in elective neurosurgery. Hopefully, this review encourages clinicians to conduct reasonable and well-designed studies on preoperative risk assessment methods in the field.

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Appendix 2. Search Strategy


The MEDLINE search was conducted on the November 5, 2013. A search term “ or Craniotomy/” was combined with “risk or Risk Assessment/,” “risk or Risk Adjustment/ or “Outcome Assessment (Health Care)”/” or “,” which provided 136, 82 and 47 unrelated articles, respectively.

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The Embase search was conducted on the November 5, 2013. A search term “ or craniotomy/” was combined with “risk or risk assessment/” (which included the search term “risk adjustment”) or “ or prediction/,” which provided 262 and 163 unrelated articles, respectively.

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The PubMed search was conducted on the November 14, 2013. Keyword terms “ASA,” “ASA physical status,” “American Society of Anesthesiologists,” “outcome,” “preoperative,” “preoperative assessment,” “Rankin Scale,” “modified Rankin Scale,” “Glasgow outcome scale,” “Charlson comorbidity score,” “Karnofsky performance score,” “ECOG,” “Eastern Cooperative Oncology Group Performance Status,” “POSSUM,” “Physiological and Operative Severity Score for the enUmeration of Mortality and Morbidity,” “neurosurgery,” and “craniotomy” were used for PubMed searches.

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Name: Elina Reponen, MD.

Contribution: This author helped in designing and conducting the study, writing the manuscript, and selecting relevant articles.

Attestation: Elina Reponen approved the final manuscript.

Name: Hanna Tuominen, MD, PhD.

Contribution: This author helped in designing the study, writing the manuscript, and selecting relevant articles.

Attestation: Hanna Tuominen approved the final manuscript

Name: Miikka Korja, MD, PhD, Associate Professor.

Contribution: This author helped in designing the study, writing the manuscript, and selecting relevant articles.

Attestation: Miikka Korja approved the final manuscript.

This manuscript was handled by: Gregory J. Crosby, MD.

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