Is there variable neuronal susceptibility to compression?
A case report of differential recovery from dorsal midbrain syndrome and compressive optic neuropathy following treatment for germinoma
A 12-year-old boy with suprasellar germinoma complicated by hydrocephalus was found to have dorsal midbrain syndrome and bilateral compressive optic neuropathy. Following a third ventricular neuroendoscopic biopsy and the initiation of chemotherapy, a follow-up magnetic resonance imaging scan of the brain showed significant regression of the tumor with resolution of hydrocephalus. On smooth pursuit, there was significant improvement in upgaze. However, light-near dissociation of his pupillary defect was persistent. Fundal examination showed persistent temporal pallor of the optic discs bilaterally, associated with a suboptimal best-corrected visual acuity of 20/40 and bilateral centrocecal scotoma.The differential recovery of neuronal function following relief of compression is poorly understood. In this case, both the optic nerve and pretectal axonal fibers responsible for pupillary reflex, with their neuronal cell bodies residing outside the direct compressive site at the midbrain, showed poor functional recovery. However, relief of pressure at the rostral interstitial medial longitudinal fasciculus and posterior commissure nuclear complexes resulted in significant clinical improvement in upgaze.
This case suggests that, for neurons, axons are apparently more susceptible to pressure than cell bodies. The predominant transmission of nerve impulses takes place through the length of axons, and thus is more sensitive to pressure changes, with the associated microvascular compromise and demyelination.