Pediatric Pain Assessment

At What Age are Children Capable of Experiencing Pain?

To experience pain, one must possess the ability to perceive a peripheral noxious stimulus via a functioning nociceptive system and to develop a motor, autonomic, metabolic, psychological, behavioral, or emotional response. Neonates sense noxious stimuli and routinely demonstrate all these responses. In the developing fetus, cutaneous sensation begins in the 7th week of gestation in the perioral region and soon spreads to the face, hands, feet, and trunk. By the 15th week of gestation, cutaneous sensation has spread to the extremities, and by the 20th week of gestation, sensory perception is present in all cutaneous and mucosal regions. Substance P appears in fetal nerve tissue by the 10th week and endogenous opioids are detected at the 22nd week of gestation. Synapses begin to form between peripheral sensory neurons and dorsal horn neurons by this time. Myelination of nerve tracts in the spinal cord and brainstem begins during the 22nd week of gestation and is complete by the third trimester. Peripheral nerve myelination is not fully completed until after birth. However, one of the major nociceptive neurons is unmyelinated (C-fibers) and the other is thinly myelinated (A-δ fibers). This does not mean that noxious signals are not transmitted, but that they are transmitted more slowly.

Centrally, the cerebral cortex begins to develop at 8 weeks and will contain 10 billion neurons by the 20th week of gestation. Fetal electroencephalographic patterns, though intermittent and unsynchronized, appear by the 22nd week; by the 27th week, signals are synchronized in both hemispheres. By the 30th gestational week, cortical evoked potentials can be detected. At the beginning of the third trimester, all elements of the nociceptive system required to process noxious stimulation are present. The only component of the nociceptive system that is not present at birth is the descending inhibitory pathway, which develops during the first 6 months of antenatal life. Thus the neonate is not capable of attenuating nociceptive signals. The dorsal horn cells of the neonate that are responsible for transmitting nociceptive signals centrally have wider receptive fields and lower excitatory thresholds than in older subjects. These properties mature quickly in the postnatal period. Excitatory thresholds of dorsal horn neurons are further lowered by repetitive minor injuries such as daily heel lancing. Neonates may experience more pain in response to a given noxious stimulus than older children or adults.

Some practitioners erroneously assume that, because neonates cannot remember a painful event, it is of no consequence. But the metabolic and behavioral stress response that accompanies neonatal pain is associated with increased morbidity and mortality. This response can be reduced by using regional anesthesia, opioids, or general anesthesia before painful procedures.

Though little is known about neonatal consciousness and the perception of pain, there is evidence of complex, integrated cortical responses to nociceptive stimulation. Neonates that are subjected to noxious events (circumcision, repeated heel lancing, phlebotomy, etc.) demonstrate abnormalities in short-term behavior such as periods of increased crying, and feeding and sleeping abnormalities. Furthermore, painful experiences during early infancy affect future responses to painful events. In some instances, physiologic responses are enhanced, while behavioral responses are blunted. Opposite changes may occur under different circumstances. For example, infants who were circumcised at birth without anesthesia will demonstrate an exaggerated pain response to immunizations in the first year of life compared with infants who received some form of pain control during newborn circumcision.

To summarize, neonates may not be able to interpret or remember painful events but even premature newborns are capable of perceiving noxious events and mounting a variety of physiologic and behavioral responses. These responses may translate into short- and long-term behavioral changes during subsequent painful stimuli. Because these stress responses and adverse outcomes can be reduced by the judicious use of analgesics, pain should be anticipated whenever possible, recognized when present, and treated appropriately.