Combined Ultrasound and Upper Lip Bite Test Assessment of Difficult Laryngoscopy in Obese Patients

Ultrasound-Measured Hyomental Distance Ratio Combined With the Upper Lip Bite Test as a Predictor of Difficult Laryngoscopy in Obese Patients: A Prospective Observational Study

The aim of this observational study is to evaluate the accuracy of the HMDR combined with the ULBT as a predictor of difficult laryngoscopy in obese patients undergoing elective surgery under general anesthesia, regarding:

• Correlation of HMDR-ULBT values with Cormack-Lehane grades during laryngoscopy.
• Comparison of HMDR-ULBT with standard HMDR in predicting difficult laryngoscopy.

Airway management is a cornerstone of safe anesthesia practice. Difficult laryngoscopy and intubation remain a critical challenge for anesthesiologists. The ability to predict difficult airway situations preoperatively remains crucial, particularly in high-risk populations such as obese patients. These patients have significantly higher rates of difficult mask ventilation and tracheal intubation.

Numerous bedside screening tests have been established to predict difficult laryngoscopy, including the Mallampati classification, thyromental distance, the upper lip bite test (ULBT), and hyomental distance (HMD) measurements. However, none of these tests alone possesses sufficient sensitivity or specificity, and their predictive performance in obese individuals remains variable .

The Hyomental Distance Ratio (HMDR), defined as the ratio of the hyomental distance in maximal head extension to that in the neutral position , is a dynamic parameter reflecting upper airway compliance and anatomical displacement. An HMDR value <1.2 has been associated with increased risk of difficult laryngoscopy in several studies . Traditionally, the HMDR is measured either manually with a ruler or more objectively using ultrasound.

The Upper Lip Bite Test (ULBT) is a simple, validated bedside test assessing mandibular protrusion and temporomandibular joint mobility, two key factors influencing the success of glottic visualization . The ULBT is performed in a conscious state, making it a practical tool in airway assessment.

Given that the ULBT involves active mandibular protrusion, it may mimic the anterior displacement of airway structures achieved during head extension or jaw thrust. Therefore, measuring the hyomental distance while the patient performs the ULBT could serve as a novel dynamic modification of the traditional HMDR-potentially offering a more functional and patient-cooperative measure.

Recent studies have validated the HMDR (extension/neutral) as a useful predictor of difficult laryngoscopy by ultrasound, due to its capacity to evaluate the anterior neck space and upper airway alignment . However, these studies primarily assess neck extension without incorporating mandibular movement, despite its key role in airway visualization. To date, no published studies have evaluated the hyomental distance combined with the ULBT using ultrasound or compared it with the traditional HMDR, leaving an interesting gap in the literature.

Combining the anatomical advantages of anterior neck space assessment with functional jaw mobility via the HMDR combined with the ULBT may yield a more comprehensive and accurate predictor of difficult airway, especially in obese or anatomically challenging patients. This novel approach may detect subtle impairments in upper airway dynamics not captured by the static HMDR alone. To our knowledge, this application has not yet been explored in existing literature.

This study, therefore, aims to evaluate the accuracy of the HMDR combined with the ULBT as a predictor of difficult laryngoscopy in obese patients undergoing elective surgery under general anesthesia, using difficult laryngoscopy-defined as Cormack-Lehane grade 3 or 4-as the primary outcome .

Inclusion Criteria:

• Adult obese patients aged 18 to 65 years.
• Both sexes.
• American Society of Anesthesiologists (ASA) physical status II- III.
• Body Mass Index (BMI) ≥ 30 kg/m² (classified as obese).
• Scheduled for elective surgical procedures requiring general anesthesia with endotracheal intubation.
• Ability to understand and provide informed written consent.
• Cooperative and able to perform the ULBT on command.
• Normal dentition (presence of upper and lower incisors to perform ULBT accurately).

Exclusion Criteria:

• History of maxillofacial trauma, surgery, or congenital facial anomalies affecting mandibular movement or neck anatomy.
• Known or suspected difficult airway requiring awake intubation.
• Patients with limited neck mobility (e.g., cervical spine disease or immobilization).
• Temporomandibular joint disorders or restricted mandibular mobility.
• Pregnancy.