The parasympathetic nervous system controls pupillary constriction while the sympathetic nervous system controls pupillary dilation. The afferent part of the PLR shares some common pathways with the central visual pathways up to the level of the lateral geniculate nucleus after which the axons decussate again to synapse with the oculomotor nucleus ipsilateral to the simulated eye. The efferent part of the PLR is the parasympathetic portion of the oculomotor nerve (CN III). The presence of intact menace response on both eyes indicates that the central visual pathways are intact and in particular the portions of the visual pathways that are common with the pupillary light reflex (PLR) pathways (i.e., afferent part of the PLR). The lesion is therefore located to the efferent part of the PLR, i.e. ipsilateral parasympathetic portion of the oculomotor nerve.
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