What is the term for the reflex that does not disappear and is demonstrated when an infant tries to clear its throat?

Introduction

The gag reflex, also known as the pharyngeal reflex, is an involuntary reflex involving bilateral pharyngeal muscle contraction and elevation of the soft palate. This reflex may be evoked by stimulation of the posterior pharyngeal wall, tonsillar area, or base of the tongue. The gag reflex is believed to be an evolutionary reflex that developed as a method to prevent swallowing foreign objects and prevent choking. It is an essential component of evaluating the medullary brainstem and plays a role in the declaration of brain death. [1]

Issues of Concern

In certain instances, a lack of a gag reflex may be a symptom of a more severe medical condition, such as cranial nerve damage or brain death. Contrast this with a hypersensitive gag reflex (HGR), which may be caused by anxiety, postnatal drip, acid reflux, or oral stimulation such as during dental treatments. 

Development

The gag reflex is mediated by CN IX and CN X. Embryologically, the glossopharyngeal nerve (CN IX) is associated with the derivatives of the third pharyngeal arch. The vagus nerve (CN X) is associated with the derivatives of the fourth and sixth pharyngeal arches. [2]

Function

The gag reflex is a natural somatic response in which the body attempts to eliminate unwanted agents or foreign objects from the oral cavity through muscle contraction at the base of the tongue and the pharyngeal wall.[3] 

In the first few months of life, the gag reflex is triggered by any food that the nucleus tractus solitarius (a region of the brain stem) deems to be too large or solid for a baby to digest. Starting around six or seven months of age, the gag reflex diminishes, allowing babies to swallow more solid foods. 

Mechanism

There are two types of gagging; somatogenic (from a physical stimulus) and psychogenic (from a mental trigger). The gag reflex is controlled by both the glossopharyngeal (IX) and vagus (X) nerves, which serve as the afferent and the efferent limbs for the reflex arc, respectively. The nerve roots of cranial nerves IX and X exit the medulla through the jugular foramen and descend on either side of the pharynx to innervate the posterior pharynx, posterior one-third of the tongue, soft palate, and the stylopharyngeus muscle. [4]

The stimulus is provided by sensation to the posterior pharyngeal wall, the tonsillar pillars, or the base of the tongue. These sensations are carried by CN IX, which acts as the afferent limb of the reflex to the ipsilateral nucleus solitarius (also referred to as the gustatory nucleus) after synapsing at the superior ganglion located in the jugular foramen. In turn, these nuclei send fibers to the nucleus ambiguus, a motor nucleus present in the rostral medulla. Efferent nerve fibers to the pharyngeal musculature traverse from the nucleus ambiguus through CN X. This ultimately results in the bilateral contraction of the posterior pharyngeal muscles.

Contraction of the pharyngeal musculature ipsilateral to the side of the stimulus is known as the direct gag reflex, and contraction of the musculature on the contralateral side is known as the consensual gag reflex.

Stimulation of the soft palate can also elicit the gag reflex; however, the sensory limb, in this case, is the trigeminal nerve (CN V). Here, sensory stimulation of the soft palate travels through the nucleus of the spinal tract of the trigeminal nerve.  

Related Testing

Equipment 

The gag reflex can be elicited using a tongue blade or soft cotton applicator. In an intubated patient, a suction device may be most convenient to use for testing.

Technique

The examiner stimulates the posterior pharynx using a tongue blade or cotton applicator. After doing so, the patient will produce a gagging reaction, which may lead to vomiting in some patients. Additionally, the elevation of the bilateral posterior pharyngeal muscles requires examination. In a study among 104 medical students assessing the gag reflex, researchers noticed that stimulation of the posterior pharynx was more likely to elicit a gag reflex when compared to stimulation of the posterior tongue.[5]

An asymmetric response or absence of response when stimulating one side indicates the presence of pathology and warrants further assessment. 

• Alternatives: The soft palatal reflex can help to assess the function of CN IX and X, as this reflex may be intact in the absence of gag reflex. The voice is evaluated by looking for hoarseness and dysphonia to determine CN X pathology. Research has also found that the cough reflex was better reproduced in intubated patients than the gag reflex to test for brainstem function.[6]

• Embracing health care team options: As there are a variety of techniques used to assess the gag reflex, there is poor inter-observer agreement. Hence, a standard method of examining patients for specific determined clinical scenarios is warranted. However, the gag reflex remains imperative in assessing brainstem function, especially in the setting of brain death. 

Contraindications

During airway assessment for intubation in an obtunded patient, the gag reflex should not be performed due to the risk of vomiting and subsequent aspiration.[7] It may be difficult to assess the oral cavity in patients with a hypersensitive gag reflex. These patients may benefit from intravenous sedation during prosthodontic treatment.[8]

Pathophysiology

As previously stated, individuals may suffer from a lack of a gag reflex or hypersensitive gag reflex (HGR). 

It is not uncommon for an individual to lack a gag reflex. According to one study involving 140 people, 37% were found to have an absent gag reflex.[9] This percentage may be higher in patients with a history of smoking or tobacco use. However, clinical judgment is indicated, as in certain instances a lack of a gag reflex may be a symptom of a more severe medical condition, such as cranial nerve damage or brain death. 

Testing the gag reflex can help assess damage to CN IX and CN X. To test the gag reflex, you gently touch first one and then the other palatal arch with a cotton swab or tongue blade, waiting each time for gagging. If the glossopharyngeal nerve (IX) is damaged on one side, there will be no response when touched. If the vagus (X) nerve is damaged, and either side is touched, the soft palate will elevate and move toward the affected side. If both CN IX and X are damaged on one side, touching the intact side will result in a unilateral response with deviation of the palate to that side. There will be no response when touching the damaged side.

On the other hand, another study showed that 10-15% have a HGR.[10] Most often, those with an HGR gag while eating thick or sticky foods that tend to get stuck in the mouth, such as bananas and mashed potatoes.  

As mentioned above, Following intraoral stimulation, afferent fibers from the trigeminal, glossopharyngeal, and vagus nerves pass to the medulla oblongata. From here, efferent impulses give rise to spasmodic and uncoordinated muscle movements characteristic of gagging. The center in the medulla oblongata is close to the vomiting, salivary, and cardiac centers, which may be stimulated during gagging[3]. 

This explains why gagging may be accompanied by excessive salivation, lacrimation, sweating, fainting, or even a panic attack in a minority of patients. Furthermore, neural pathways from the gagging center to the cerebral cortex allow the reflex to be modified by higher centers, thus making it possible to initiate gagging just by imagining a disagreeable experience or conversely by controlling the reflex to some extent by distractive action. [11]

Clinical Significance

The gag reflex once served as a method to detect dysphagia in the setting of acute stroke. In one study comparing gag reflex to bedside swallowing assessment in 242 patients, the researchers found that the absence of gag reflex was specific for and consistent with the inability to swallow as assessed at the bedside but not sensitive in stroke patients. This study showed that the specificity of the gag reflex in detecting dysphagia was 96%, with a sensitivity of 39%. However, an intact gag reflex does indicate the presence of protection against long-term swallowing issues and predicts a decreased requirement for enteral feeding in the future.[12]

Research has found that the posterior pharyngeal muscles, which control the gag reflex, are independent of the muscles responsible for swallowing. Therefore, clinicians should not rely upon an absent gag reflex as a predictor for aspiration in stroke patients. Indirect laryngoscopy has been demonstrated to be a better alternative to performing the gag reflex to assess airway safety. Researchers have also noted that one out of three people may lack a gag reflex through habituation or be influenced by emotions through higher centers. Pharyngeal sensation, in contrast, is rarely absent and is thus used as an alternative to gag reflex testing and could prove better at predicting future problems with swallowing.[12]

Finally, performing the gag reflex is a must when assessing brainstem function as part of determining brain death. Confirmation of brain death is done in part by absent brainstem reflexes, which include an absent gag reflex.[13]

Review Questions

Figure

Gag Reflex. Contributed by Shruthi Sivakumar, MBBS

References

Park MJ, Byun JS, Jung JK, Choi JK. The correlation of gagging threshold with intra-oral tactile and psychometric profiles in healthy subjects: A pilot study. J Oral Rehabil. 2020 May;47(5):591-598. [PubMed: 32003041]

Frisdal A, Trainor PA. Development and evolution of the pharyngeal apparatus. Wiley Interdiscip Rev Dev Biol. 2014 Nov-Dec;3(6):403-18. [PMC free article: PMC4199908] [PubMed: 25176500]

Bassi GS, Humphris GM, Longman LP. The etiology and management of gagging: a review of the literature. J Prosthet Dent. 2004 May;91(5):459-67. [PubMed: 15153854]

Klimaj Z, Klein JP, Szatmary G. Cranial Nerve Imaging and Pathology. Neurol Clin. 2020 Feb;38(1):115-147. [PubMed: 31761055]

Lim KS, Hew YC, Lau HK, Lim TS, Tan CT. Bulbar signs in normal population. Can J Neurol Sci. 2009 Jan;36(1):60-4. [PubMed: 19294890]

Polverino M, Polverino F, Fasolino M, Andò F, Alfieri A, De Blasio F. Anatomy and neuro-pathophysiology of the cough reflex arc. Multidiscip Respir Med. 2012 Jun 18;7(1):5. [PMC free article: PMC3415124] [PubMed: 22958367]

Mackway-Jones K, Moulton C. Towards evidence based emergency medicine: best BETs from the Manchester Royal Infirmary. Gag reflex and intubation. J Accid Emerg Med. 1999 Nov;16(6):444-5. [PMC free article: PMC1343414] [PubMed: 10572821]

Yoshida H, Ayuse T, Ishizaka S, Ishitobi S, Nogami T, Oi K. Management of exaggerated gag reflex using intravenous sedation in prosthodontic treatment. Tohoku J Exp Med. 2007 Aug;212(4):373-8. [PubMed: 17660702]

Davies AE, Kidd D, Stone SP, MacMahon J. Pharyngeal sensation and gag reflex in healthy subjects. Lancet. 1995 Feb 25;345(8948):487-8. [PubMed: 7861875]

Neumann JK, McCarty GA. Behavioral approaches to reduce hypersensitive gag response. J Prosthet Dent. 2001 Mar;85(3):305. [PubMed: 11264940]

Eachempati P, Kumbargere Nagraj S, Kiran Kumar Krishanappa S, George RP, Soe HHK, Karanth L. Management of gag reflex for patients undergoing dental treatment. Cochrane Database Syst Rev. 2019 Nov 13;2019(11) [PMC free article: PMC6953338] [PubMed: 31721146]

Ramsey D, Smithard D, Donaldson N, Kalra L. Is the gag reflex useful in the management of swallowing problems in acute stroke? Dysphagia. 2005 Spring;20(2):105-7. [PubMed: 16172818]

Goila AK, Pawar M. The diagnosis of brain death. Indian J Crit Care Med. 2009 Jan-Mar;13(1):7-11. [PMC free article: PMC2772257] [PubMed: 19881172]

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