European Journal of Cardiovascular Medicine

Airway management remains a cornerstone of anaesthetic practice, with the maintenance of a patent airway being the anaesthesiologist's foremost responsibility. One of the most critical challenges encountered during general anaesthesia is unanticipated difficult or failed tracheal intubation, which is a well-recognised cause of perioperative morbidity and mortality.¹ Failure to secure the airway effectively can result in catastrophic consequences, including hypoxic brain injury or death within minutes. The American Society of Anesthesiologists (ASA) reports that the development of an airway emergency increases the risk of hypoxic brain damage by 15-fold.² In fact, airway-related complications continue to be the second most common cause of anaesthesia-related malpractice claims, with poor preoperative airway assessment identified as a key contributor.²

Various bedside clinical predictors are employed to evaluate the risk of difficult laryngoscopy and intubation. Among these, the Modified Mallampati Classification (MPC) and Thyromental Distance (TMD) are widely accepted due to their simplicity and reliability.³ MPC is a visual assessment of oropharyngeal structures that estimates tongue size relative to pharyngeal space, whereas TMD provides an estimate of the mandibular space and the potential for alignment of airway axes during intubation. Both predictors, when used independently, offer reasonable sensitivity and specificity, but each has limitations that can lead to false-positive or false-negative predictions.⁴

Multiple studies have shown that using a combination of preoperative assessment tools enhances the ability to predict difficult airways compared to using a single test.⁵ A combined approach using both MPC and TMD has been proposed to improve predictive accuracy by compensating for the limitations of each method. Furthermore, multivariate tools such as the Modified Mallampati Score–Thyromental Distance–Anatomical Abnormality–Cervical Mobility (M-TAC) and the Simplified Airway Risk Index (SARI) are designed based on this rationale.⁵ However, their complexity limits their widespread use in routine pre-anaesthesia assessment.

In light of the need for a more accurate yet feasible method to predict difficult airways, this study was undertaken to evaluate the diagnostic accuracy of MPC and TMD—both independently and in combination—against the Modified Cormack-Lehane grading system during direct laryngoscopy as the gold standard. The objective is to establish whether combining these two simple, cost-effective assessments can offer improved sensitivity, specificity, and overall diagnostic performance for anticipating difficult laryngoscopy and intubation in patients undergoing elective surgery under general anaesthesia.

OBJECTIVE

To evaluate the predictive accuracy of the Modified Mallampati Classification and Thyromental Distance—individually and in combination—for anticipating difficult laryngoscopy and intubation in patients undergoing elective surgery under general anaesthesia.

Study Design and Setting

This study was designed as a diagnostic test evaluation conducted in a tertiary care setting. The research was carried out in the Pre-Anaesthesia Clinic and Major Operation Theatres of the Department of Anaesthesiology at Government Medical College, Thiruvananthapuram.

Study Population

The study population included patients aged between 18 and 65 years who were scheduled for elective surgery under general anaesthesia with tracheal intubation. Only patients belonging to ASA (American Society of Anesthesiologists) physical status I and II were included.

Inclusion and Exclusion Criteria

Inclusion criteria comprised patients aged 18–65 years undergoing elective surgeries under general anaesthesia with endotracheal intubation. Patients were excluded if they required rapid sequence intubation, were uncooperative or pregnant, had a body mass index (BMI) greater than 30, a history of previous difficult intubation, limited cervical spine mobility, or any maxillofacial anomalies.

Study Period

The study was conducted over a period of one year, from June 2021 to June 2022, following ethical approval.

Sample Size Calculation

The sample size was calculated based on a previously published study which reported a 96% sensitivity for the combined use of MPC and TMD in predicting difficult laryngoscopy. Assuming a 32% incidence of difficult intubation, the required number of true positives (n) was calculated using the formula n= 4pq/d², Hence, a total of 300 patients were enrolled using consecutive sampling.

Ethical Considerations

Institutional Ethics Committee approval was obtained prior to initiating the study. Written informed consent was obtained from all participants. Patient confidentiality was strictly maintained, and all study-related costs were borne by the investigator.

Study Variables

The primary measured variables were the Modified Mallampati Classification (MPC) and Thyromental Distance (TMD). The MPC was assessed with the patient in the sitting position, mouth fully open, and tongue protruded maximally without phonation. Scores ranged from 0 to 4, with grades 3 and 4 considered indicative of potential difficulty in laryngoscopy. The TMD was measured from the upper border of the thyroid cartilage to the mentum with the head fully extended. A TMD of less than 6.5 cm was considered a predictor of difficult intubation.

Outcome Variable

The outcome was the degree of difficulty in laryngoscopy, assessed intraoperatively using the Modified Cormack-Lehane grading system. Grades 1 and 2a were considered as indicating easy laryngoscopy, while grades 2b, 3, and 4 were classified as difficult laryngoscopy.

Study Procedure

Preoperative airway assessment using MPC and TMD was conducted in the Pre-Anaesthesia Clinic. Intraoperatively, laryngoscopy was performed after achieving optimal positioning and full neuromuscular blockade. External laryngeal manipulation was allowed if needed. The best laryngoscopic view achieved during the first or second attempt was recorded. A drop in oxygen saturation (SpO₂) or inability to intubate within three attempts was considered a failure, and the procedure was abandoned.

All patients were intubated with appropriately sized endotracheal tubes and the planned surgery was conducted. At the end of surgery, neuromuscular blockade was reversed and patients were extubated following standard protocols.

Data Analysis

Data were recorded in Microsoft Excel and analysed using SPSS software version 20. Categorical variables were summarised as frequencies and percentages. The diagnostic accuracy of MPC and TMD—individually and in combination—was evaluated against the Modified Cormack-Lehane grade as the gold standard. Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and overall accuracy were calculated. A p-value of less than 0.05 was considered statistically significant.

This diagnostic test evaluation study included 300 patients undergoing elective surgery under general anaesthesia with tracheal intubation. The goal was to assess the predictive accuracy of two commonly used bedside airway assessment tools — Modified Mallampati Classification (MPC) and Thyromental Distance (TMD) — for predicting difficult laryngoscopy and intubation, using the Modified Cormack-Lehane (CL) grade during direct laryngoscopy as the gold standard.

1Predictive Accuracy of Modified Mallampati Classification (MPC)

Table 1.1: Distribution of Study Population by MPC Grade

Most patients were classified as MPC Grade 1 or 2, indicating a likely easy intubation. Only 6.7% had MPC Grade 3, which is a predictor of difficult laryngoscopy.

Table 1.2: MPC-based Airway Difficulty Prediction

Based on the MPC grading, 20 patients (6.7%) were predicted to have a difficult airway (i.e., MPC GFigure: Percentage distribution of the sample according to Modified Cormack Lehane Grade

Note: Grades 2B, 3, and 4 were considered "difficult."

Table 1.4: Diagnostic Accuracy of MPC Against CL Grading

• Sensitivity: 95%
• Specificity: 99.6%
• PPV: 95%
• NPV: 99.6%
• Accuracy: 99.3%

The MPC demonstrated excellent diagnostic performance. The high specificity and negative predictive value make it a dependable screening tool to rule out difficult airways when the grade is 1 or 2. Its sensitivity was also strong at 95%, correctly identifying 19 of 20 true difficult intubations.

2. Predictive Accuracy of Thyromental Distance (TMD)

Table 2.1: TMD-Based Prediction of Difficult Intubation

The TMD test predicted 18 patients (6.0%) as difficult and 282 (94.0%) as easy, using the threshold of <6.5 cm for difficult airway. This is consistent with the known prevalence of difficult intubation in elective surgery settings.

Table 2.2: Diagnostic Accuracy of TMD Against CL Grading

• Sensitivity: 90%
• Specificity: 100%
• PPV: 100%
• NPV: 99.3%
• Accuracy: 99.3%

TMD showed perfect specificity and PPV — meaning every patient identified as difficult truly had a difficult airway. Its slightly lower sensitivity (90%) means it missed 2 out of 20 difficult cases. However, its overall accuracy remains excellent.

This diagnostic test evaluation study was undertaken to assess the individual predictive accuracy of two commonly used bedside airway assessment tools—Modified Mallampati Classification (MPC) and Thyromental Distance (TMD)—for predicting difficult laryngoscopy and intubation in patients undergoing elective surgery under general anaesthesia.

Airway management remains a fundamental responsibility of the anaesthesiologist, and difficult or failed intubation continues to be a significant cause of anaesthesia-related morbidity and mortality. It is estimated that difficult intubation occurs in approximately 1 out of every 200 general surgical cases ⁶. The ability to anticipate difficulty in airway access allows for adequate preparation, including the arrangement of alternative equipment and techniques such as video laryngoscopy or fibreoptic intubation, thereby improving patient safety⁷.

In our study, among 300 patients evaluated preoperatively, the Modified Mallampati Classification demonstrated excellent diagnostic properties. Specifically, it showed a sensitivity of 95%, specificity of 99.6%, positive predictive value (PPV) of 95%, and negative predictive value (NPV) of 99.6%, with an overall accuracy of 99.3%. This is consistent with findings from previous studies which have highlighted MPC’s utility in predicting difficult laryngoscopy^8,9. Although the test is subjective and may vary based on patient cooperation and examiner experience, its ease of use and reproducibility make it valuable in routine clinical practice.

Similarly, the Thyromental Distance assessment also proved to be a reliable screening test. Our results showed that TMD had a sensitivity of 90%, specificity of 100%, PPV of 100%, NPV of 99.3%, and accuracy of 99.3%. These findings suggest that while TMD is particularly good at confirming the presence of a difficult airway (owing to its high PPV and specificity), it may slightly underperform in terms of sensitivity compared to MPC. Nonetheless, it remains a vital component of the preoperative airway assessment, as supported by several other studies^10,11.

Both MPC and TMD individually exhibited high predictive accuracy and can be considered effective diagnostic tools for anticipating difficult laryngoscopy and intubation. However, they should ideally be interpreted within the broader context of the patient’s anatomy, comorbidities, and other clinical parameters to minimize risk.

This diagnostic evaluation study aimed to assess the individual predictive value of Modified Mallampati Classification and Thyromental Distance in predicting difficult laryngoscopy and intubation in patients undergoing elective surgery under general anaesthesia. Based on our findings:

• Modified Mallampati Classification shows high sensitivity, specificity, and overall accuracy, making it a reliable bedside tool.
• Thyromental Distance also offers high specificity and PPV, indicating its strength in confirming a difficult airway when identified.
• Both tests are simple, cost-effective, and applicable in routine clinical practice.
• The individual predictive accuracies of MPC and TMD were comparable, with both tests demonstrating over 99% diagnostic accuracy.

These findings support their continued use in preoperative airway evaluation and highlight the importance of clinical judgment in interpreting their results.

Conflict of interest: Nil

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