Abstract
Until a short time ago, the view prevailed worldwide that children were less sensitive to pain than adults, and such operations as circumcision were performed in babies without adequate anesthesia or analgesia. This view is now considered a misconception, as psychophysiological and behavioral studies show that even neonates have a well-functioning nociceptive system. Nociception generally refers to the neural and sensory aspects of pain, which do not necessarily include conscious experience. There is no discontinuity in the development of the nervous system during birth, and therefore it can be concluded that the fetus is also responsive to noxious stimuli. The question arises as to the stage of ontogeny of the human at which nociceptive behavior begins. Literature on the fetal nervous system reveals that the first signs of somatosensory system function occur at week 7 of gestation and at week 22 the synaptic connection from the nervous periphery to the somatosensory cortex is becoming established. During this period, motor behavior matures, from stereotyped reflexes to spontaneously generated complex motor patterns reminiscent of the repertory of voluntary movement. From week 22 onward the electroencephalogram (EEG) shows increasingly more varied patterns, and sleep-wake states can be discerned after week 30 of gestation. Somatosensory evoked cortial potentials have been recorded from gestational week 28 onward. Substance P, a neuropeptide associated with pain in the adult nervous system, is present in the fetal spinal cord as early as week 12 of gestation, while the antinociceptive opioid peptide enkephalin does not appear until week 24. From week 15 onward, opioid peptides such as β-endorphin appear in the pituitary; their release becomes sensitive to environmental stimuli from about week 20 onward, which can be considered the onset of pituitary stress responses. In particular, parturition and abortion induced the release of opioid peptides. Studies of conditioned behavior show that the fetus has the ability to learn. It has been hypothesized that the fetus and neonate possess a procedural memory, which is not transferred to the language-based memory of later phases of life. Learning and memory are the most essential elements for the construct of “consciousness.” Therefore, a primitive type or level of consciousness might exist in the fetus. Thus, a considerable range of sensorimotor function, including memory, develops during fetal life. Anatomical, physiological and behavioral data suggest that the nociceptive system is included in this development. Although we cannot be sure at present whether the fetus consciously experiences pain, beyond the protective nociceptive behavioral responses, anesthesia should be used for invasive procedures to protect the fetus and its nervous systems.
Zusammenfassung
Nach einer bis vor kurzem weltweit verbreiteten Einstellung sind Kinder weniger schmerz-empfindlich als Erwachsene, und (kleinere) chirurgische. Eingriffe wie Zirkumzision wurden bei Neugeborenen vielfach ohne ausreichende Anästhesie durchgeführt. Diese Sichtweise wird heute zunehmend angezweifelt, da psychophysiologische und verhaltensphysiologische Beobachtungen zeigen, daß bereits Neugeborene ein gut funktionierendes nozizeptives System besitzen. Nozizeption bezieht sich auf neuronale und sensorische Aspekte des Schmerzes, die nicht notwendigerweise eine bewußte Wahrnehmung einschließen. In der Entwicklung des Nervensystems bedeutet die Geburt keine Diskontinuität, und deshalb kann geschlossen werden, daß auch der Fet bereits Reaktionen auf noxische Reize zeigt. In diesem Beitrag soll der Frage nachgegangen werden, wann in der Ontogenese des Menschen das nozizeptive Verhalten beginnt. Aus der Literatur über die fetale Entwicklung des Nervensystems ist ersichtlich, daß die erstein Funktionszeichen des somatosensorischen System in der 7. Schwangerschaftswoche auftreten, und daß ab der 22. SSW die nervöse Peripherie über Synapsen zum somatosensorischen Kortex durchgeschaltet wird. Während dieser Zeitspanne reift auch die Motorik von stereotypen Reflexen zu spontan entstehenden komplexen motorischen Mustern, die einem Repertoire von Willkürbewegungen späterer Lebensphasen ähneln. Das Elektroenzephalogramm (EEG) zeigt ab der 22. SSW zunehmend differenzierte Muster, und ab der 30. SSW können Schlaf-Wach-Zustände unterschieden werden. Bereits in der 28. SSW wurden somatosensorische evozierte Kortexpotentiale abgeleitet. Das Neuropeptid Substanz P, das im erwachsenen Nervensystem als Neurotransmitter für Schmerzinformationen gilt, kann im Rückenmark des Feten ab der 12. SSW nachgewiesen werden; erst in der 24. SSW erscheint der antinozizeptive Transmitter Enkephalin im Rückenmark. Etwa ab der 15. SSW lassen sich Opioide wie β-Endorphin in der Hypophyse nachweisen; ihre Freisetzung reagiert ab der 20. SSW auf externe Reize, was als Beginn der hypophysären Streßantwort gelten kann. Insbesondere führen Geburt und Abort zu starken Ausschüttungen von Opioiden. Untersuchungen zur Konditionierbarkeit des Verhaltens bei Frühgeborenen zeigt, daß bereits der Fet lernfähig ist. Fet und Neonatus sollen ein “prozedurales Gedächtnis” haben, das nicht in das sprachorientierte Gedächtnis der späteren Lebensphasen übertragen wird. Lernen und Gedächtnis sind wichtige Bestandteile des Konstrukts “Bewußtsein”. Eine primitive Form (oder Vorstufe) von Bewußtsein beim Fet erscheint möglich. Während der Fetalzeit entwickelt sich somit ein umfangreiches Repertoire sensorisch-motorischer Funktionen, enschließlich Gedächtnis. Anatomische, physiologische und verhaltensmäßige Beobachtungen machen deutlich, daß das nozizeptive System bei dieser Entwicklung eingeschlossen ist. Obwohl wir nicht sicher sein können, ob der Fet bereits Schmerzen “erlebt”—über die protektiven Verhaltensreaktionen hinaus—, sollten bei invasiven Eingriffen anästhesiologische Maßnahmen zum Schutz des Feten und seines Nervensystems ergriffen werden.
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Herrn Prof. Dr. med. Dr. med. h.c. Hans Schaefer, em. Professor für Physiologie an der Universität Heidelberg, in Verehrung und Bewunderung zum 85. Geburtstag gewidmet.
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Zimmermann, M. Zur Frage der Schmerzempfindlichkeit des Feten: Neuro-, psycho- und verhaltensphysiologische Aspekte. Schmerz 5, 122–130 (1991). https://doi.org/10.1007/BF02528097
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DOI: https://doi.org/10.1007/BF02528097