Post by neurology admin on Jun 27, 2012 2:37:50 GMT -5
Lower cranial nerve palsies in various combinations involving the hypoglossal, glossopharyngeal, vagal, and the accessory nerves occur in internal carotid artery (ICA) dissection.1¨C8 17 The hypoglossal nerve (cn-XII) is most often affected. It is suggested that the close proximity of these nerves to the cervical ICA may subject them to mechanical injury either by the expansion of the dissected artery or by aneurysm formation.1 3 9 10 Dissection of the ICA may cause transient or permanent disruption of the blood supply to these cranial nerves.6 11 Therefore, cranial nerve injuries may result from ¡°mechanical, embolic, or haemodynamic¡± processes caused by ICA dissection.6 This hypothesis is supported by anatomical and clinical findings. 2 6 12 Each half of the genioglossus muscle that protrudes the tongue is innervated by contralateral corticobulbar (crossed) fibres whereas all other intrinsic and extrinsic tongue muscles that move the tongue within the mouth in all directions have bilateral cortical innervation.13 14 Unilateral cortical lesion in the topography representing the tongue may therefore cause contralateral genioglossus muscle weakness, resulting in deviation of the protruded tongue away from the side of the cortical lesion. By contrast, cn-XII injury causes ipsilateral genioglossus muscle weakness; hence the protruded tongue deviates to the side of the injured nerve.