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With much interest we read the article of Cariga et al1 who studied the capacity of the isolated spinal cord to generate a sympathetic skin response (SSR). We appreciate this comprehensive and well designed study, which encouraged us to suggest some of our own ideas. The authors recorded the palmar and plantar SSR to peripheral nerve electrical stimulation (median or supraorbital nerve above the lesion, and peroneal nerve below the lesion) in 29 patients with spinal cord injury (SCI) at various neurological levels and in 10 healthy control subjects. In complete SCI no SSR could be evoked below the lesion. It was concluded that the spinal cord isolated from the brain stem could not generate an SSR. Furthermore, the authors assume that supraspinal connections are necessary for the SSR.
The sudomotor response below a complete SCI has been widely studied in the past. However, the question whether the isolated spinal cord can generate SSRs is still under discussion. Wallin et al obtained sudomotor responses in complete SCI after manual pressure applied to the anterior abdominal wall. In this study the authors have shown, using microneurographic recordings from postganglionic axons in skin nerves, that several stimuli applied caudal to the lesion site induce bursts of neural impulses that contain sympathetic impulses of spinal origin.2 Previnaire et al recorded the palmar and plantar sudomotor responses in complete SCI during cystometry and found a sudomotor activation during bladder contraction.3
In a recent study we focused on sudomotor pathways in patients with complete SCI. Below the level of lesion the tibial and the pudendal nerve were stimulated electrically whereas palmar and plantar SSR were recorded. Tibial nerve stimulation was not found to elicit SSRs below a SCI lesion. This is in accordance with the results of Cariga et al and we agree that this type of electrical stimulation probably cannot activate the spinal sudomotor reflex circuit. However, pudendal nerve stimulation evoked plantar SSRs in patients with complete cervical and thoracic SCI. No SSRs after pudendal nerve stimulation were obtained in patients with lesions at L1 and more caudal. SSRs following pudendal nerve stimulation in complete SCI above the level L1 are probably mediated by sacral somatic afferents and a sympathetic pathway originating at the upper lumbar level. The underlying sacro-lumbar reflex circuit is organised on spinal level and requires intact lumbar segments. In conclusion, the complete isolated human spinal cord seems to be able to generate sympathetic sudomotor impulses.4
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