As a medical student I rotated through an elite hospital where it was believed that every patient admitted to the medicine service needed a rectal exam. The rationale was to avoid ever missing a case of rectal or prostate cancer. Eventually, the utility of digital rectal examination as a cancer-screening tool was debunked. Thankfully, this practice has fallen out of favor.
Checking gag reflexes seems like a similar practice: an uncomfortable test which is engrained in our practice during training. Despite evidence that it's unhelpful, it continues to be commonly performed. This post pushes back against the practice of gagging patients by proposing an extreme viewpoint: this test should be abandoned in living patients.
Neuroanatomy of the pharyngeal reflex
The pharyngeal reflex involves stimulating the posterior pharynx, which will usually elicit a reflexive constriction of the pharynx with elevation of the uvula. Sensation is predominantly due to CN IX (glossopharyngeal nerve), whereas the pharyngeal musculature is mostly controlled by CN X (vagus nerve). However, this division isn't absolute; in particular, the pharyngeal muscles are enervated by both nerves. More sensitive patients also gag in response to stimulation of the soft palate, which is enervated by cranial nerve V (trigeminal).
The pharyngeal reflex should be dissected into two components:
- Palatal reflex: Upward movement of the soft palate. This component is subtle, requiring continuous attention to the soft palate.
- Gag reflex: Retching (which may rarely progress to vomiting). This component is unpleasant and obvious from across the room.
The gag reflex appears to always be accompanied by the palatal reflex. However, normal patients often lack a gag reflex, despite retaining a palatal reflex (Lim 2009). The palatal reflex seems to be more hard-wired, whereas the gag reflex may be more susceptible to influence from higher brain centers (e.g. emotional inputs, habituation). The palatal reflex is probably a better indicator of true pathology. However, the gag reflex is more dramatic and thus has received far more attention.
Reproducibility of the gag reflex
There is no standardization of how the gag reflex is tested:
- Most texts recommend touching the posterior pharynx, but some recommend touching the back of the tongue. This makes a big difference. Among 104 medical students, touching the posterior pharynx was far more likely to stimulate a gag response than the posterior tongue (91% vs. 18%; Lim 2009).
- The instrument used and amount of pressure applied to the pharynx isn't standardized. Since the gag reflex may be stimulated by both touch and pressure, more aggressive probing of the pharynx is probably more likely to elicit a gag.
- The technique for testing a gag reflex in an intubated patient is even murkier. Some sources recommend shaking the endotracheal tube, whereas others recommend inserting a tongue depressor or suction catheter into the posterior pharynx.
Davies 1995 found that inter-observer agreement was poor even within a single research study using a standardized protocol (agreement in only 10/15 patients, which isn't much better than pure chance). In clinical practice reproducibility is probably even worse, because clinicians use a variety of different techniques.
Specificity of the gag reflex
If it were perfectly specific, the gag reflex would be present in 100% of patients without neurologic disease. However, healthy people frequently lack a gag reflex. The table below shows the frequency of absent gag reflex among neurologically normal individuals (1):
There is enormous variability in the frequency of absent gag reflex. This may partially reflect different patient populations, with gag reflex less common among the elderly or patients with sleep apnea (Valbuza 2011). It may also partially reflect different techniques used to elicit the reflex. Regardless, the specificity is poor and non-reproducible.
Performance of the gag reflex for predicting aspiration
The greatest amount of evidence about the gag reflex regards its ability to predict aspiration. Several studies have found that the gag reflex has little relationship to aspiration, for example:
- Leder 1997 prospectively evaluated the gag reflex in a mixed population of patients undergoing a videofluoroscopic examination with modified barium swallow procedure. Among patients without a gag reflex, most patients had no aspiration (6/7). Alternatively, among patients with a normal gag reflex, half had evidence of aspiration (11/21). The authors concluded that there was no relationship between the gag reflex and aspiration.
- McCullough 2001 prospectively evaluated the gag reflex in sixty patients with stroke who were undergoing videofluoroscopic examination. No statistically significant relationship was found between gag reflex and aspiration.
A poor relationship between gagging and aspiration may seem counterintuitive, but this is easily explained. Gagging seems evolutionarily designed to prevent choking on large pieces of food, not to prevent liquid aspiration. Avoiding aspiration of liquids requires reflexive swallowing, a far more complex maneuver involving five cranial nerves and 26 muscles. Thus, it is entirely possible for a patient to have an intact gag reflex, yet freely aspirate liquids.
The gag reflex should never be used to determine whether a patient requires intubation
There are several reasons that the gag reflex shouldn't be used to determine whether a patient is able to protect their airway from aspiration:
- Absent gag reflex has poor specificity and poor reproducibility.
- The gag reflex has been proven to be unable to predict aspiration.
- Gagging may rarely elicit vomiting, which is potentially disastrous in an obtunded patient without a secured airway.
Experts have cautioned against using the gag reflex to evaluate airway protection for decades (Mackway-Jones 1999). Unfortunately, knowledge translation has been sluggish:
Although it persists, inexplicably, in clinical practice, the gag reflex largely has disappeared from research evaluations -Walls Manual of Emergency Airway Management, 4th edition Airway Manual, 4th edition 2012.
Value of gag reflex in neurologic examination?
The gag reflex is a traditional component of the neurologic examination to evaluate the brainstem. I couldn't find any large studies regarding the performance of the gag reflex for detection of focal neurologic lesions. Most commonly a single gag maneuver is performed, which probably has poor performance:
- The sensitivity of an absent gag reflex is probably poor. If the pharynx is stimulated on one side, this would be expected to be at most 50% sensitive for detecting unilateral brainstem pathology (which would cause ablation of the reflex on only half of the pharynx).
- The specificity of the gag reflex is poor (perhaps ~70%), as explored above.
If we estimate that a single gag reflex has a sensitivity of ~50% and specificity of ~75%, this would give it a positive likelihood ratio of 1.7 and a negative likelihood ratio of 0.7. These performance characteristics cannot justify the time and discomfort involved in performing the test.
What about checking bilateral gag reflexes? A unilaterally absent gag reflex might be expected to have greater specificity for focal pathology. However, a unilaterally absent gag was found among 4/138 of normal people by Davies 1995. Although this rate is low, it would still be higher than the rate of occult brainstem pathology – so that a unilaterally absent gag would still probably represent a red herring.
Better ways to test CN IX/X?
CN IX and X are very close to each other, with diseases often involving both nerves together. For a general neurologic examination, the following tests may be more humane and more replicable than the gag reflex:
- Soft palate movement with phonation: Ask the patient to say “ahh” while observing the uvula. Symmetric upward movement of the uvula suggests intact function of IX/X. Unilateral damage to IX/X will cause the uvula to be pulled to the normal side (picture below). Lim 2009 found soft palate movement to be intact among 206 normal patients (including patients with no gag reflex), suggesting that this may have superior specificity compared to the gag reflex.
- Voice: CN X is involved in enervation of the vocal cords. Lesions involving this nerve may therefore be suggested by a breathy, nasal, or hoarse voice.
- Cough reflex: CN X is required for the afferent pathway of this reflex (Polverino 2012). Among intubated patients, this may be more reproducible than the gag reflex.
We are legally required to confirm the absence of a gag reflex in every patient who is declared brain dead. This is a sensible practice, because the presence of a gag reflex would indicate brainstem function and thereby disprove brain death. This is the only situation where the gag reflex is a highly specific test (2). Thus, the brain-dead patient is the last bastion of clinical utility for the gag reflex.
- The gag reflex is a traditional component of the neurologic examination, but isn't evidence-based.
- The reproducibility of the gag reflex is poor, due to variation in the techniques used to elicit it.
- The specificity of the gag reflex is poor, being absent in ~20% of younger patients and ~40% of elderly patients.
- The gag reflex is an unreliable predictor of aspiration, because it tests only a small fraction of the nerves and muscles required to control secretions via swallowing.
- The gag reflex should not be used to assess whether patients can protect their airways.
- The only rational use of the gag reflex appears to be certification of brain death.
- 95% confidence intervals calculated using http://vassarstats.net/prop1.html. Although this calculation is unnecessary – even without the 95% confidence intervals it is clear that this data is all over the place. References for this table are as follows: Lim 2009, Davies 1995, Beltrani 1994.
- The last post on sensitivity and specificity discussed how a useless test could occasionally be rendered useful by reversing its interpretation (see Footnote #3). That is essentially what has been done here. Typically, a positive test result is regarded as an “absent gag reflex” – but this has poor test characteristics. If we flip the test around and a consider a positive test to be a “present gag reflex” then this becomes 100% specific for the absence of brain death.