Monday, November 03, 2008
The issue of abortion is obviously a contentious one, from a moral, medical and emotional point of view. Of central importance to this debate is the question of whether a fetus can feel pain during surgery or during other operative procedures. Necessary for this is an objective assessment of when a fetus can feel pain, or rather, when during development do the mechanisms underlying pain reception and processing emerge to form a functioning system.
In this paper by Vanhatalo and Niewenhuizen (2000), a review of the development of the pain systems of the fetus is given, followed by the discussion of a particularly pertinent and scientific (i.e. not emotionally laden) question: what is the effect of noxious (read: painful) stimuli on the subsequent development of the fetus?
Given that a formal definition of pain (as stated by the aptly named International Association for the Study of Pain) is given as "a unpleasant sensory and emotional experience associated with actual or potential tissue damage", it may be seen that pain is an inherently subjective phenomenon. Indeed, all of the standard measures of pain rely on subjective scales and the verbal reports of the patients - which are clearly not suitable for neonate use. Indirect measures such as behavioural reactions and autonomic responses to noxious stimuli must therefore be used.
In addition to these indirect indicators of pain, an understanding of the development of the development of the pain mechanisms themselves. Three levels are described in the following paragraphs: the somatosensory functions, the physiological reflexes and the pain behaviours.
The somatosensory pain system involves the pain receptors and associated neural pathways. The development of these takes place relatively early on in development: by the 7th week receptors appear around the mouth, and by the20th week they are present all over the body. This is followed by synapse connection to the spinal cord. These lower levels have been relatively well characterised. At higher levels (sub-cortical and cortical regions), the picture is less clear. Thalamo-cortical connections occur at approximately week 25, but it is not until week 29 that evoked potentials can be measured from the cortex, indicating the formation of meaningful functional pathways at this time. Despite this, there are likely to be other connections formed between the sensory periphery and "deeper brain areas" when the spine and thalamic connections are formed at week 20, allowing the potential for sub-cortical processing of pain at this time. There are naturally many other relevant developmental milestones, but it is important to note that the initial pain related pathways which form in the fetus may be very different from those in the mature individual - giving an indication of the importance of individual development both pre- and post-natally.
The second element of fetal pain characterisation is the behavioural reflexes displayed, including withdrawal reflexes, other body movements and even vocalisations. These reflexes are, however, known to be mediated entirely by the spinal cord, and therefore do not involve "higher perception of pain". Additionally, there is no clear correlation between intensity of the stimulus, and strength of the reflex - meaning that the reflex cannot be used as a reliable indicator of 'painfulness'. However, there is some evidence that facial expressions, for fetus's as young as 26 weeks, may more accurately give an indication of pain level.
The third level in characterising pain is the development of the autonomic and endocrine systems. In mature humans for example, changes in autonomic responses (e.g. increased heart rates, and production of various hormones such as cortisol) may be used as indicators for pain - similar mechanisms may also apply to the developing fetus. However, as yet (or at least at the time of the paper), there appears to be no reliable correlation upon which to base a measurement scale.
Finally, the question of how prenatal pain affects the subsequent development of the fetus is examined. It is noted that subcortical pain processing may occur a number of weeks prior to any noxious stimuli processing in the cortex. Also noted is the plasticity of the nervous system throughout this period, meaning that development is strongly shaped by external input - a number of studies have provided evidence for ...correlations of the early pain experiences to later behavioural variables or to later developmental outcomes..." - particularly the effects on stress responses. In summary: the presence of pain during development can have profound long-term effects on both neurological and psychological development.
In conclusion, the authors note that while the fist sensory experiences (read: conscious experience?) of pain are theoretically possible around the 26th week of gestation (corresponding to the growth of the thalamocortical connections), noxious stimuli could have a damaging effect on fetal development irrespective of processing in the brain. This is because pain also results in activation of stress responses which do not rely on cortical processing. Hence, the authors point to week 10 (development of spinal cord afferents) as being of significance in terms of effect of pain on development – and finish with the question: “which sensory experiences are potentially harmful for the development of a fetus?”.
Note: this paper is from 2000 - those mentions of mechanisms as not being well characterised may now be well understood. If anyone knows of any such advancements, I'd be happy to hear about them – I'm by no means an expert on this topic :-)
Sampsa Vanhatalo, Onno van Niewenhuizen (2000). Fetal Pain? Brain and Development, 22, 145-150