EQUINE VETERINARY EDUCATION / AE / OCTOBER 2014


527


Clinical Commentary


Traumatic brain injury manifested as optic neuropathy in the horse: A commentary and clinical case


D. E. Brooks*, C. E. Plummer, S. L. M. Craft† and J. D. Struthers† Department of Large Animal Clinical Sciences, University of Florida, Gainesville, USA; and †Department of Infectious Diseases and Pathology, University of Florida, Gainesville, USA. *Corresponding author email: brooksd@ufl.edu


Keywords: horse; optic nerve; chiasm; trauma; avulsion


Summary Blindness from head trauma is a serious debilitating disease in the horse. It appears that blunt trauma to the skull results in intracranial injury to the optic nerve at the optic chiasm from mechanical and vascular mechanisms. Systemically administered iron and calcium chelation therapy may enable some horses to recover vision.


Introduction


The optic nerve is not truly a cranial nerve but a white matter tract of the diencephalon composed principally of the axons of retinal ganglion cells (RGC). Retinal ganglion cells axons project without synapses from the retinal nerve fibre layer (NFL) through the optic chiasm and the optic tracts to either the lateral geniculate nucleus, the superior colliculus, the hypothalamus, or to the pretectal nucleus and other midbrain centres (Brooks et al. 1999a). The optic nerve provides the sole connecting link between photoreceptors of the retina and the more central components of the visual system and is thus an extremely vulnerable segment of the visual pathway (Brooks et al. 1999b). The optic nerve consists of 4 anatomic regions (Fig 1)


(Brooks et al. 1999a): the intraocular optic nerve includes the RGC layer, the NFL, the optic nerve head or optic disc, and the intralaminar region within the scleral lamina cribrosa. Posterior to the globe the optic nerve consists of the intraorbital optic nerve, the 3 cm long intracanalicular optic nerve of the horse (within the optic canal), and the short intracranial optic nerve that merges into the optic chiasm (Brooks et al. 1999a). Optic nerve axons consist of an outer sheath of myelin, a hollow thin, collapsible, bilayered lipid cell wall, and a gel-like viscoelastic axoplasm containing transmitter molecules, proteins, microtubules and organelles such as mitochondria (Brooks et al. 1999b). Meningeal sheaths of pia mater, arachnoid, and outer dura mater surround the intraorbital optic nerve (Brooks et al. 1999a). Arteriolar and capillary blood vessels pierce the dura mater, cross the trabeculae of the arachnoid and branch to enter the pia mater. The dural sheath of the intraorbital optic nerve merges with the sclera anteriorly, and is continuous posteriorly at the orbital apex with the periosteum of the optic canal (Brooks et al. 1999a). The optic nerve enters the bony optic canal of the sphenoid bone at the orbital apex surrounded by the origins of the rectus muscles where it is tightly fixed to the bone at the anterior foramen and less adherent to bone posteriorly (Joseph 1995). The internal ophthalmic artery is also present in the optic canal. The dura of the optic nerve and the periosteum of the sphenoid bone are fused in the optic canal, but the subarachnoid space of the


intraorbital optic nerve contains cerebrospinal fluid and communicates with the intracranial subarachnoid. The orbital optic nerve is longer than the direct distance between the globe and the optic canal of the horse such that it has an S-shaped curve in it to allow for globe movement (Brooks et al. 1999a).


Traumatic brain injury: force vectors resulting from blunt trauma to the head


The pathogenesis of traumatic brain injury (TBI), stroke, shaken baby syndrome and intracerebral haemorrhagic stroke in man may provide insight into how to understand and treat traumatic optic neuropathy in horses. Traumatic brain injury is an alteration in brain function caused by a redistribution of kinetic energy to intracranial tissues (Pieramici and Parver 1995; Feary et al. 2007). This can be manifested as isolated focal damage to the optic nerves or optic chiasm, and is then


io


or


ic ch


Fig 1: The 4 parts of the optic nerve: intraocular (io), intraorbital (or), intracanalicular (ic), and intracranial or chiasmal (ch). Modified from Polyak (1957).


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