Primitive & Frontal-Release Reflexes

The primitive reflexes are a small group of stereotyped motor responses that are present and normal in the infant, are progressively suppressed as the frontal lobes mature, and reappear, or are “released,” when frontal-lobe or diffuse cerebral disease removes that cortical inhibition. For this reason they are also termed frontal-release signs. They are encountered in a range of conditions characterized by frontal or widespread cerebral dysfunction, including the dementias, normal-pressure and obstructive hydrocephalus, diffuse small-vessel cerebrovascular disease, and focal lesions of the frontal lobes. Their interpretation requires restraint. These signs are nonspecific, and a single sign in isolation, particularly the glabellar or palmomental response, occurs in a substantial proportion of cognitively normal elderly individuals. It is therefore the number of signs present and their persistence, rather than the appearance of any one sign, that carries diagnostic weight; isolated findings should be interpreted with caution and in the context of the remainder of the examination.

The Individual Reflexes

Grasp Reflex

The grasp reflex is elicited by stroking the patient's palm, drawing the examiner's fingers across the skin from the radial border toward the fingers, ideally while the patient's attention is engaged elsewhere. The response is an involuntary grasping of the stimulating fingers, and characteristically the grip tightens when the examiner attempts to withdraw, so that the patient appears unable to release the hand on command. Of the frontal-release signs, the grasp reflex is among the more specific for structural disease, and it points to pathology in the contralateral frontal lobe, particularly the medial and premotor regions. When clearly present, and especially when unilateral, it is a finding of real localizing value rather than a nonspecific marker of diffuse disease.

Palmomental Reflex

The palmomental reflex is sought by briskly stroking or scratching the thenar eminence of the palm. The response is a brief contraction of the ipsilateral mentalis muscle, producing a visible twitch and wrinkling of the skin of the chin on the same side. The reflex is easy to elicit and to observe, but its specificity is low: it is among the most frequently encountered of the primitive reflexes in healthy older adults, and in isolation it should not be regarded as evidence of disease. Its significance rises only when it is reproducible, prominent, or accompanied by other frontal-release signs.

Glabellar (Myerson) Sign

The glabellar sign is tested by tapping repetitively over the glabella, the smooth area of the forehead between the eyebrows, with the examiner's finger positioned so that the patient cannot see the approaching hand, since visual anticipation confounds the response. In the normal individual the reflex blink habituates, ceasing after the first few taps. The abnormal response, the Myerson sign, is a failure of habituation: the patient continues to blink with each successive tap. This sign is characteristic of parkinsonism and is also seen in frontal-lobe disease. It reflects loss of the normal cortical suppression of the blink reflex rather than any abnormality of the brainstem arc itself.

Snout Reflex

The snout reflex is elicited by light tapping or gentle pressure on the philtrum or the lips. The response is puckering and protrusion of the lips, a pouting movement that recalls the rooting and feeding reflexes of infancy. Its presence indicates loss of frontal inhibition and is most meaningful as one element of a constellation of release signs in diffuse cerebral or frontal disease.

Rooting and Sucking Reflexes

The rooting reflex is elicited by stroking the cheek or the corner of the mouth; the response is turning of the head and mouth toward the stimulus, as in the infant seeking the nipple. The sucking reflex is elicited by contact with or light stroking of the lips, which provokes involuntary sucking movements. Both are normal and prominent in infancy and are suppressed in childhood. Their reappearance in the adult indicates advanced diffuse cerebral disease and is generally a late finding, encountered in advanced dementia and other states of widespread cortical dysfunction.

Significance and Interpretation

These responses belong to the broader category of frontal-release signs, and their clinical value lies less in any single reflex than in their aggregate pattern. When multiple signs are present together, they correlate with frontal and executive dysfunction and lend support to a diagnosis of frontal-lobe or diffuse cerebral disease. Conversely, a solitary glabellar or palmomental sign in an otherwise intact older patient is of limited consequence. The examiner should therefore record which signs are present, whether they are unilateral or bilateral, and how they relate to the cognitive, behavioral, and other neurologic findings, rather than treating any one reflex as diagnostic in itself.