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John R Heckenlively, Naheed W Khan; Occult optic neuropathy: A newly recognized disorder which superficially mimics RP. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):3796.
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To investigate and report on patients with night blindness, sudden or progressive visual field loss, normal electroretinograms, who are often sent on referral to rule out retinitis pigmentosa.
Nine patients with visual field loss, night blindness, normal fundus examinations and ERGs were studied. All patients had standardized electroretinograms, Goldmann visual fields, fundus photography, and optical coherence tomography (OCT) scans of their maculae and optic nerve heads ONHs) with in depth imaging. Best corrected visual acuities and photopic b-wave amplitudes and implicit times were measured. Using Heidelberg software designed to measure linear distances (that maintain standardized calibrations), nerve fiber thickness (NFL) was measured horizontally on the ONH midpoint looking at the maximal diameter of the optic nerve at the cribiform plate in three locations compared to the Bruch’s membrane opening diameter and to normal controls.
Patients were found to showed NFL thickness three to five times greater than normal between the macula and optic nerve head. OON patients have severe swelling at the optic nerve head and in the cribiform plate region. The diameter at the cribiform plate region was significantly wider (P<0.001) than normal controls while Bruch’s Membrane opening was not significantly different between controls and patients.
The OON patients have abnormally small visual fields, yet good photopic ERGs, which strongly, in itself, suggests a diagnosis of optic neuropathy; this was confirmed by the abnormal optic nerve fiber changes of the optic nerve head and by swelling at the cribiform plate region. It’s known in forms of glaucoma that distention and compression of the cribiform plate on optic nerve fibers is associated with pathologic visual field loss. In the OON patients, this putative pathologic mechanism would appear to be an important issue. Therapeutic measures for this disorder are not apparent and the pathophysiologic basis is not clear. These patients are often thought to be functional cases, but with the OCT imaging showing dysmorphic optic nerve head changes not seen clinically, these patients with their specific constellation of findings, should be easier to diagnose.
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