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Somatic pain is a subjective sensory experience resulting from the intermixing of three main components[5] [6] : motivational-directive, sensory-discriminatory, and cognitive-evaluative (see Chapter 73 ). The motivational-directive component is conveyed by unmyelinated C fibers (i.e., slow pain or true pain). It leads to protective reflexes such as autonomic reactions, muscle contraction, and rigidity. C fibers are fully functional from early fetal life onward. Sensory-discriminatory influxes are propagated by myelinated Aδ fibers (i.e., fast pain). This allows accurate identification and location of the nociceptive stimuli and elicits withdrawal reactions. Because these fibers are thinly
| Peculiarities (Mainly Infants) | Consequences and Specific Risks | Implications for Regional Anesthesia |
|---|---|---|
| Enzyme immaturity | Slower metabolism of aminoamides and aminoesters but with no clinical implications | Increased accumulation with reinjections or continuous infusions of local anesthetics |
| Incomplete myelinization of nerve fibers | Easier crossing of nerve envelopes by local anesthetics | Shortened onset time and increased efficacy of diluted local anesthetics |
| Incomplete ossification of vertebrae | Possible damage to ossification nuclei and danger of crossing cartilaginous structures | Use short, short-bevel, and not-too-thin caudal, epidural, and spinal block needles |
| Lower termination of the spinal cord and dural sac | Increased risk of trauma to the spinal cord | Avoid epidural approaches above L3 if possible |
| No fusion of sacral vertebrae | Existence of sacral intervertebral spaces | Sacral approaches to epidural spaces are possible |
| Delayed curvatures of the spine | Cervical lordosis (3–6 mo old) | Adjust needle orientation accordingly when approaching the spinal canal |
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Lumbar lordosis (8–9 mo old) |
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| Axis change of coccyx | Location of sacral hiatus more difficult in children >6–8 yr old | Caudal anesthesia more difficult in children >6–8 yr old |
| Loose attachment of sheaths and aponeuroses to underlying structures | Increased diffusion along nerve paths that may reach distant spaces and nerves | Required volume of local anesthetics decreased for peripheral blocks but increased for neuraxial (leakage along nerve roots) |
| Increased cardiac output and local vascularity | Increased systemic absorption of local anesthetics | Decreased duration of blockade, increased effectiveness of epinephrine |
| Sympathetic immaturity, diminished autonomic adaptability of the heart, smaller vascular bed in lower extremities | Excellent hemodynamic stability during neuraxial blocks | Fluid preloading and use of vasoactive agents unnecessary |
| Delayed acquisition of body scheme and conceptualization, anxiety | Inability of patients to locate precise body areas, concept of paresthesia not understandable, difficult cooperation | Physical means required to locate nerve and spaces; do not perform a "dangerous" technique while the child is awake (risk of unanticipated panic attack at the wrong time, even in apparently cooperative children) |
| Increased fluidity of epidural fat | Increased diffusion of local anesthetics up to 6–7 yr old | Excellent blockade after caudal anesthesia up to 6–7 yr old |
Connections between C fibers and dorsal horn neurons are not mature before the second week of postnatal life. However, nociceptive stimulations transmitted to the dorsal horn by C fibers elicit long-lasting responses, probably because of extensive depolarization of surrounding neurons in response to the production of large amounts of substance P. As the number of dorsal horn receptors to substance P decreases during the first two weeks of life, this exaggerated response of neonates to nociceptive stimulation progressively disappears. The inhibitory control pathways, which are immature at birth, develop concomitantly.
Pain thresholds are low in infants, and there is a negative correlation with age. Premature infants submitted to repeat nociceptive cutaneous stimulations develop sensitization at the level of peripheral nociceptors and at that of dorsal horn neurons. N-methyl-D-aspartate (NMDA) receptors seem to play an important role. In the case of repeat nociceptive stimulations, there is an overstimulation of NMDA receptors that results, at least experimentally, in excitotoxic lesions of neurons in development.[7]
A major difficulty is the assessment and sometimes the identification of pain in children. The younger the patient, the greater is the difficulty to communicate because the ability to express distress and discomfort is limited. During the past 2 decades, pediatric pain has received considerable attention, and reliable age-related pain scales have been developed to evaluate the severity of pain and the efficacy of its treatment.
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