A dilated and fixed pupil raises a red flag suggesting the possibility of serious oculomotor 3rd nerve disease. The most serious is aneurysm. A less serious alternative diagnosis with this clinical picture is Adie's pupil. This is a condition predominately of young females who are usually a decade younger than the average age for a symptomatic aneurysm.
Adie's pupil starts out with dilitation or paralysis of the pupil called iridoplegia. This can be differentiated from the iridoplegia caused by a third nerve lesion. In time the Adie's pupil iridoplegia is converted to an Argyll Robertson (AR) type with light near dissociation reaction. But Adie's differs in certain aspects from a true AR pupil. The Adie's pupil is unilateral and is the larger of the two pupils whereas the true AR pupil is a bilateral phenomenon with relatively small pupils. An Adie's pupil can be smaller when you examine it if the patient has been reading for a period of time before your examination. The tonic nature of the Adie's pupil applies to dilation as well as contraction. The Adie's pupil can be bilateral in 10% of cases. The pupils are not really rigid to light but react tonically, moving if the light is held in front of the eye for a minute. If a light such as the light from a slit lamp is continuously shone on the pupil, it reacts slowly and tonically rather than briskly. It is this tonic phenomenon that differs from a true AR pupil reaction. The tonic feature also affects the near reaction. This reaction occurs more rapidly than the light reaction thus giving it an AR like reaction. The different degree of reaction occurs because the ratio of fibers innervating the light reflex versus the near reflex is 40 to 1 in favor of the ciliary body near reflex.
The location of the lesion in Adie's is in the ciliary ganglion in the orbit. A long established physiologic principle is the more peripheral the nerve injury, the more responsive it is to its effector substance. This is why the Adie's pupils react to dilute Mechoyl or Pilocarpine. In the past we used 2.5% Mecholyl and the affected pupil reacted with miosis and the normal one did not. The percent of Mecholyl that would affect both pupils was 14%, which gives us a wide margin for a proper response. Mecholyl is no longer available. We now use .125% Pilocarpine. We make this by diluting commercially available 0.5% Pilocarpine. There may be some reaction in the normal pupil but a much greater reaction in the Adie's pupil. How soon this sensitivity occurs after the onset in humans is not known. Sensitivity in cats occurs between 5 and 7 days. Even if the pharmacologic test for Aide's pupil is inconclusive, the physical features particularly the tonic reaction is diagnostic. This is best observed with the magnification of the slit lamp. The contraction is not symmetrical but segmental due to sector iridoplegia. Its contraction has been likened to the movement of a bag of worms.
The important fact in identifying this phenomenon is to realize that it is not vision or life threatening and requires no further work up.
There is also an Adie's syndrome. These patients in addition to the pupil findings have absent knee and ankle reflexes. This condition also has no health threatening consequences. A few cases have come to autopsy for other reasons. They only show some drop out of a few anterior horn cells, but no other degeneration.
The only problem caused by the pupiloplegia may be a large pupil letting in too much light which can be uncomfortable and also some paralysis of the near reflex in only one eye which could cause some blur or "confusion" when reading. Sunglasses or a weak percent of pilocarpine may be helpful if needed.