FUNCTION -- Facial Sensation and Biting

TEST I -- General Sensation
During this crude test of sensation, the clinician asks the patient to close the eyes, following which the face will be touched with the fingers corresponding to the three trigeminal dermatomes (i.e., ophthalmic, maxillary, mandibular). In particular, the intention is to determine whether the stimulus is detected and whether it is perceived as the same on both sides of the face.

INTERPRETATION
The test may be sensitive to general losses of sensation related to unilateral or bilateral deficits affecting the trigeminal dermatomes.

PITFALLS AND SAFEGUARDS
Attempts should be made to tap the skin with the finger rather than to brush the skin, particularly when coarse facial hair is present. The brushing of course hair can create an audible noise, which permits the patient to detect the stimulus by means other than tactile receptors and their afferent pathways. Also, the stimuli should be applied at some distance laterally from the midline of the face, as areas around the nose and mouth transmit tactile information through both the left and the right trigeminal nerves, possibly leading to failures to detect unilateral sensory deficits.

TEST II -- Sharp and Dull
This test assesses the ability of the patient to detect a noxious tactile stimulus applied to the face, again according to the three-dermatomal framework of the trigeminal nerve. The clinician is to stimulate the skin (with some measure of randomness) with either a pointed or blunt object. Comparing one side of the face to the other, each dermatome should be stimulated with either the sharp or dull object. After each stimulus, the patient should report whether it was experienced as sharp or dull.

INTERPRETATION
Failures to report the sharp stimuli as feeling sharp should be interpreted as failures of the nociceptive system. Exact interpretation will depend on whether this finding occurs in isolation or along with other sensory deficits involving the trigeminal sensory system. Pathology leading to deficits in this system can arise anywhere within the pathway between the face, pons, spinal tract and nucleus of the trigeminal nerve, trigeminal lemniscus, thalamus, internal capsule, corona radiata, postcentral gyrus, and the parietal lobe. Note that the patient in the video clip misidentifies the dull stimulus applied to the left side of the jaw as being sharp. In a case such as this, the clinician would be well-advised to assess the site again to confirm or refute the suggestion that the error by the patient reflects sensory pathology.

PITFALLS AND SAFEGUARDS
As in the previous test, the stimuli should be applied at some distance laterally from the midline of the face, as areas around the nose and mouth transmit tactile information through both the left and the right trigeminal nerves, possibly leading to failures to detect unilateral sensory deficits. Also, one must compromise between the sharpness of the sharp object and the probability that stimulation may be accompanied by small puncture wounds and bleeding. As further protection for the patient, sharp objects should not be used for multiple patients but should be subject to disposal immediately following use. At the end of the right/left comparisons, the dermatomes should be assessed again in a pseudorandom pattern to determine the tendency of the patient to give false positive responses.

TEST III -- Hot/cold
In this instance, the stimuli used during assessments of cranial nerve V are two vials filled with water, one hot water and the other with cold. With the eyes of the patient closed, the clinician should bilaterally assess each of the trigeminal dermatomes at some distance from the midline. The patient must correctly report the stimulus as being either hot or cold.

INTERPRETATION
The neural pathway being assessed is the trigeminal nerve, spinal tract and nucleus of the trigeminal nerve, trigeminal lemniscus, ventral posteromedial nucleus of the trigeminal nerve, and primary and secondary somatosensory cortices.

PITFALLS AND SAFEGUARDS
To avoid scalding of the patient the temperature of the water should not exceed 40 degrees Celsius.

TEST IV -- Light Touch
The stimulus for this test is brief light stroking of the face (using a wisp of clean cotton, with assessments made of each of the trigeminal dermatomes with comparisons between left and right sides of the face featuring prominently. The objective is to determine whether the patient can detect the stimulus, as indicated by a verbal report.

INTERPRETATION
Pathology leading to deficits in this system can arise anywhere within the pathway between the face, pons, main sensory nucleus of the trigeminal nerve, trigeminal lemniscus, thalamus, internal capsule, corona radiata, postcentral gyrus, and the parietal lobe.

PITFALLS AND SAFEGUARDS
Once again, it is critical that, for each of the trigeminal dermatomes, the left and right sides are compared. Also, when the patient is uncertain as to whether stimulation occurred or simply fails to report awareness of the stimulus applied to a particular part of the face, it is not helpful to leave the cotton in contact with the skin: The receptors responsible for detecting the cotton adapt rapidly and are thus responsive to initial rather than sustained contact. In addition, note that facial hair may compromise the validity of the test. As an alternative to this test, the clinical may opt to assess tactile (two-point) discrimination.

TEST V -- Mastication
To test masticatory function, the clinician should place his/her thumb over the masseteric muscles bilaterally, with the remaining fingers distributed across the temporalis muscles. At the same time, the patient should be instructed to clench the teeth repeatedly. The clinician should feel the muscles contract strongly.

INTERPRETATION
Weakness affecting the biting muscles may arise following injuries of the frontal lobe or connections between the frontal lobe and the pons, particularly when normal or increased muscle tone is evident. Weakness with diminished muscle tone may reflect defects involving the pontine motor nucleus of the trigeminal nerve, the mandibular division of the trigeminal nerve, or the muscles themselves.