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Cogan's Rule

Named after David G. Cogan, who described a series of 31 anatomically verified cases in 1959,¹ this rule states that, in a patient complaining of homonymous hemianopia, if there is an asymmetric optokinetic response (lack or diminished response in the direction of the corrective saccade when the optokinetic nystagmus (OKN) stimulus moves towards the affected lobe²), the lesion is more likely to involve the deep portions of the parietal lobe that are outside the territory of the posterior cerebral artery and therefore the lesion is unlikely of vascular origin.³ In contrast, the asymmetric optokinetic response is usually associated with tumors, as tumors are more common in the parietal than the occipital lobe.⁴

References

1. Smith JL, Cogan DG. Optokinetic nystagmus: a test for parietal lobe lesions; a study of 31 anatomically verified cases. Am J Ophthalmol 1959;48:187–193.
2. Sundaram V, Barsam A, Barker L, Khaw PT, editors. Training in Ophthalmology, 2nd edition. London: Oxford University Press; 2016.
3. Smith JL. Homonymous hemianopia. A review of one hundred cases. Am J Ophtalmol 1962;54:616–623.
4. Cogan DG, Loeb DR. Optokinetic response and intracranial lesions. Arch Neurol Psychiatry 1949;61(2):183–187.

Submitted by: Roberto Rodríguez MD, Neurology Resident at the National Institute of Neurology and Neurosurgery (México City, México)

Dr. Rodríguez-Rivas reports no disclosures.