Pathology of lumbar nerve root compression Part 2: Morphological and immunohistochemical changes of dorsal root ganglion

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Abstract
This study is to investigate the changes of dorsal root ganglion (DRG) induced by mechanical compression using in vivo model. The effect of axonal flow disturbance induced by nerve root compression was determined in DRG. The dorsal root ganglion should not be overlooked when considering the mechanism of low back pain and sciatica, so it is important to understand the morphologic and functional changes that occur in primary sensory neurons of the dorsal root ganglion as a result of nerve root compression. However, few studies have looked at changes of neurons within the dorsal root ganglion caused by disturbance of axonal flow and the axon reaction as a result of mechanical compression of the dorsal root through which the central branches of the primary sensory nerves pass. In mongrel dogs, the seventh lumbar nerve root was compressed for 24 h, one week, or three weeks using a clip with a pressure of 7.5 gf. Morphologic changes of the primary sensory neurons in the dorsal root ganglion secondary to the axon reaction were examined by light and electron microscopy. Changes of immunostaining for substance P (SP), calcitonin gene-related peptide (CGRP), and somatostatin (SOM) in the primary sensory neurons affected by central chromatolysis after nerve root compression were also examined. Light microscopy showed central chromatolysis of neurons in the dorsal root ganglion from one week after the start of compression. Electron microscopy of the affected neurons revealed movement of the nucleus to the cell periphery and the loss of rough endo-plasmic reticulum and mitochondria from the central region. Immunohistochemical studies showed a marked decrease of SP, CGRP, and SOM staining in small ganglion cells with central chromatolysis when compared with cells from control ganglia. It is important to be aware that in patients with nerve root compression due to lumbar disc herniation or lumbar canal stenosis, dysfunction is not confined to degeneration at the site of compression, but also extends to the primary sensory neurons within the dorsal root ganglion as a result of the axon reaction. Patients with sensory disturbance should therefore be fully informed of the fact that these symptoms will not resolve immediately after surgery.