The Marcus Gunn pupil.

cannot be excluded. Allele association studies are hampered by several possible sources of error." Our study emphasises the need for additional, carefully designed studies when applying this potentially useful method in the search for genetic factors contributing to the aetiology of Parkinson's disease.tion between the oxidative polymorphism and early onset of Parkinson's disease. Genotype relative risks: methods for design and analysis of candidate-gene association The Marcus Gunn pupil Marcus Gunn (1850-1909) is most often remembered for his description of jaw-winking. The sign consists of widening of a ptosis when the patient chews or opens the jaw. Seen in congenital ptosis, there is an aberrant connection between the innervation of the pterygoids and levator palpebrae. However, Gunn also described an abnormal pupillary response: "dynamic anisocoria" that bears his name. "It is not sufficient" he says,' "to find that it [the pupil] contracts well or fairly well on exposure ; the eye must also be kept under direct stimulation of light and the pupil watched as to whether or not it shows that secondary dilatation under continued exposure that is found associated with the amblyopia of retro-ocular neuritis. If the vision of one eye only is affected, it is important to compare the behaviour of the two pupils when stimulated directly or consensually. Thus, in partial affection of the right optic nerve the right pupil will show this secondary dilatation during continued exposure to direct stimulation, while the left pupil will show the same behaviour on consensual stimulation. On the other hand, on stimulation of the left eye both the right and left pupil will behave normally. I need not remind you of the importance of this observation, inas-much as it not infrequently enables us to diagnose a retro-ocular neuritis in the absence of all ophthalmoscopic evidence." Levatin's "swinging flashlight test for pupillary escape"2 is a modification of Gunn's technique. However, the appropriate speed of swinging the light from one eye to the other has to be found by trial and error; and, the procedure is unreliable in the presence of bilateral afferent defects of light conduction.3 The Marcus Gunn pupil in effect is an adaptation of the light reflex during persistent stimulation. It corresponds to the decrement of evoked axonal potentials in the optic nerve of the rabbit that follows after a bipha-sic response to light (figure).4 In afferent lesions the input of the residual stimulus that normally triggers pupillary constriction …