8 September 2003
RELIABILITY OF PLANTAR RESPONSE REQUIRES CONSISTENCY OF THE APPLIED STIMULUS
We read with interest the paper by Gupta and Gupta1. We agree with their conclusions about the reliability of the plantar response in neonates. It appears, however, that they are somewhat uncertain about the nature of the plantar response in general and they did not mention some of the recent published work concerning these issues.
The plantar response (flexor and extensor) has been shown by Landau and Clare2 to represent a continuum in which the flexor plantar response is achieved through a polysynaptic reflex arc initiated by a nociceptive stimulus to the S1 dermatome and subsequent contraction of the short hallux flexors, the extensor hallucis brevis, the tibialis anterior, and the extensor digitorum longus. Contraction of the extensor hallucis is not normally seen in the mature nervous system. However, if there is a lack of modulation of the reflex arc, as occurs with pathology of the corticospinal tract or with immaturity of the nervous system, the high threshold extensor hallucis longus is recruited to the response. An initial flexor response may therefore be seen before the threshold of the extensor hallucis is reached. Once recruited, the extensor hallucis longus overpowers the flexor response, resulting in the up-going toe of the Babinski sign. If one observes physiological principles in the application of a supramaximal stimulus to the organism, the response is a reliable sign, if however, a submaximal or inadequate stimulus is applied, the response may be variable and is not reliable3.
Jaynes et al. studied 349 infants in the first day of life and found that 90% had extensor responses4. Furthermore, there were only 3% equivocal responses (compared to 7.7% in the article by Gupta and Gupta) and there were essentially no asymmetrical responses. The difference lies in the nature of the stimulus applied. It is important that the test subject be in the correct state of alertness but position is also important and the head must be in the midline and the leg must be extended at the knee in order to achieve the most reliable response3. It is also difficult to apply a noxious stimulus with the thumbnail (unless the nail has been sharpened) without at the same time applying pressure to the foot, which in the neonate will elicit competing reflexes such as the plantar grasp and withdrawal which make interpretation of the plantar response more difficult. A more appropriate instrument might be the plain end of a cotton-tipped applicator4. Application of an inadequate or insufficient stimulus may result in unreliable and unrepeatable responses, a fact which probably explains the discrepancies between the various studies of the plantar response in infants3. If one considers the physiology involved in the neonate with unmyelinated corticospinal tracts, it should be apparent that in the healthy infant, asymmetry of the reflex is unlikely and when observed is mostly due to improper application of the stimulus used to elicit the reflex. Even in infants with major hemispheric injury due to in-utero strokes, the reflexes are not asymmetrical until the cortical-spinal tracts approach full myelination. In a serial study of 169 infants, Gingold et al. found that the mature plantar response (down) becomes reliable between 6 and 8 months of age. This is about the time that the neurological signs of a hemiparesis begin to be identifiable and correlates with the maturation of several other infantile reflexes5.
The conclusion of Gupta and Gupta, that the plantar response is of limited usefulness in the neonate is true but it is not because of the high incidence of asymmetrical responses, rather it is because of the normal condition of immaturity of the central nervous system in these children6.