LEMON

Difficult Airway Assessment Acronym

Author: Anna Hernández, MD

Editor: Alyssa Haag, MD

Editor: Ian Mannarino, MD, MBA

Editor: Kelsey LaFayette, DNP, ARNP, FNP-C

Illustrator: Jannat Day

Modified: Jan 06, 2025

What is LEMON?

LEMON is a commonly used acronym to predict the presence of a difficult airway before attempting intubation. Endotracheal intubation is a life-saving procedure where a flexible plastic tube called an endotracheal tube is inserted through a person’s mouth and through their vocal cords to keep their airway open and facilitate mechanical ventilation. This procedure is often performed in emergency situations, during general anesthesia, and in intensive care units.

The ability to predict a difficult airway is essential, as it allows healthcare providers to develop a safe airway management strategy in case intubation becomes challenging. The acronym LEMON goes over the main factors that constitute a difficult airway, which can help reduce the chances of unexpectedly encountering airway management difficulties.

An infographic detailing the difficult airway acronym, LEMON.

What does the “L” in LEMON mean?

The “L” in LEMON stands for look externally, meaning observing any external conditions that can make intubation difficult. These can include signs of facial trauma (e.g., wounds, burns, fractures), a short neck or a small mouth, prominent teeth, a large tongue, and a beard or moustache.

What does the “E” in LEMON mean?

The “E” in LEMON stands for evaluate the 3-3-2 rule. This rule asks three different questions:

Is the person able to fit 3 fingers between their teeth?
Is the length from the bottom of the chin to the hyoid bone at least 3 fingers?
Is the distance between the hyoid bone and the thyroid cartilage (i.e., Adam’s apple) at least 2 fingers?

The first question assesses the person’s ability to open their mouth so that the endotracheal tube and laryngoscope can reach the airway comfortably. The second question evaluates if there is enough space for intubation within the pharynx, and the third question assesses the position of the pharynx relative to the larynx (i.e., voice box). If the distance between the hyoid bone and the thyroid cartilage measures less than two fingers, it means the larynx is positioned higher in the neck, which may make it difficult to see the vocal cords.

What does the “M” in LEMON mean?

The “M” in LEMON means Mallampati score, which is used to assess the anatomy of the oral cavity in relation to the airway. Class I means the soft palate, uvula, and tonsillar pillars are completely visible. Class II means the soft palate and uvula are visible, but the tonsillar pillars are not. Class III means only the soft palate and base of the uvula are visible. Finally, Class IV means only the hard palate is visible and the soft palate is not visible at all.

A higher Mallampati score (Class III or IV) is associated with a more difficult airway because of limited visibility and a higher risk that the tongue or other soft tissues might obstruct the airway.

What does the “O” in LEMON mean?

The “O” in LEMON stands for obstruction, referring to conditions that may cause an airway obstruction, such as foreign bodies, a tumor, or soft tissue swelling. This part of the airway assessment is particularly important in individuals with burns or traumatic injuries where the risk of airway compromise is increased.

What does the “N” in LEMON mean?

The “N” in LEMON stands for neck mobility. In awake and cooperative individuals, it is important to check neck mobility by observing if they can extend their neck backward. Neck extension (i.e., sniffing position) is an essential maneuver for placing the individual in the optimal position for direct laryngoscopy and intubation. This part of the assessment may not be possible in individuals who are wearing a cervical collar in the setting of trauma.

What are the most important facts to know about the LEMON acronym?

LEMON is a difficult airway assessment acronym that can help reduce the chance of unexpectedly encountering a difficult airway when attempting intubation. It includes the following steps: Look externally, Evaluate the 3-3-2 rule, Mallampati score, Obstructed airway, and Neck mobility. Endotracheal intubation is a life-saving procedure where a flexible tube is inserted through a person’s mouth and through the vocal cords to secure the airway and assist with mechanical ventilation.

References

Apfelbaum JL, Hagberg CA, Caplan RA, et al. Practice guidelines for management of the difficult airway: An updated report by the American Society of Anesthesiologists Task Force on management of the difficult airway. Anesthesiology. 2013;118(2):251-270. doi:10.1097/ALN.0b013e31827773b2

Derakhshan P, Nikoubakht N, Alimian M, Mohammadi S. Relationship between airway examination with LEMON criteria and difficulty of tracheal intubation with IDS criteria. Anesth Pain Med. 2024;13(6). doi:10.5812/aapm-142921

Crawley SM, Dalton AJ. Predicting the difficult airway. BJA Educ. 2015;15(5):253-258. doi:10.1093/bjaceaccp/mku047

Sun F, on behalf of Chinese Collaboration Group for Emergency Airway Management, Wang Y, et al. Clinical consensus of emergency airway management. J Thorac Dis. 2017;9(11):4599-4606. doi:10.21037/jtd.2017.10.79

Williamson D, Nolan J. Airway assessment. In: Emergency Airway Management. Cambridge University Press; 2015:41-50.