Five main causes of light-near dissociation covered

Philadelphia-The five most important causes of light-near dissociation are deafferented pupils, midbrain lesions, Argyll-Robertson pupils, aberrant regeneration of the third nerve, and tonic pupils.

Laura J. Balcer, MD, reviewed these causes, with emphasis on how light-near dissociation is relevant to all pupillary abnormalities, and showed clinicians how to differentiate among them.

"Light-near dissociation is a situation in which the response of a pupil to a near target is greater than the light response. This may occur in patients who have pregeniculate afferent visual loss and a preserved near response," said Dr. Balcer, associate professor of neurology and ophthalmology at the University of Pennsylvania School of Medicine in Philadelphia. "Alternatively, lesions may develop along the efferent reflex arc of the light reflex, including within the midbrain pretectal nuclei and interneurons, the third nerve, and at the ciliary ganglion."

Dorsal midbrain syndromeIn patients with dorsal midbrain syndrome, light-near dissociation is a prominent feature, according to Dr. Balcer, as well as upgaze paresis, convergence retraction nystag- mus, and eyelid retraction.

"The lesion in these patients is located in the pretectal nuclei and ganglion cell axons as they enter the midbrain. A lesion in this region spares the supranuclear fibers that descend directly toward the ocular motor nerve complex. The result is pupillary light-near dissociation," she said.

"The most common lesions in this case are those caused by hydrocephalus, stroke, or tumor. The pupils in the dorsal midbrain syndrome are midposition or large," she said. Midbrain abnormalities that cause light-near dissociation are often bilateral, but they can be unilateral or asymmetric.

Argyll-Robertson pupilsArgyll-Robertson pupils are another prominent cause of bilateral pupillary light-near dissociation. Dr. Balcer noted that Argyll-Robertson pupils, which are miotic, are classically associated with neurosyphilis. Unlike the pupils of dorsal midbrain syndrome, Argyll-Robertson pupils also may be irregular and dilate poorly in the dark.

"The lesion is thought to occur within the interneurons that connect the pretectal nuclei and the Edinger-Westphal nuclei in the midbrain. This also spares the supranuclear fibers that subserve the pupillary near response," she said. She described a patient with Argyll-Robertson pupils who was shown to have pupils that did not respond well to light but contracted to near.

Aberrant regenerationWhen patients present with unilateral light-near dissociation, eye movement abnormalities, or signs of a third nerve palsy, there may be aberrant regeneration. Many different types of this pathology have been identified, but in the case of light-near dissociation, the fibers that used to innervate the medial or inferior rectus muscle of the third nerve are damaged. As they regrow, they reach the ciliary ganglion and result in miosis when the patient adducts the eye, according to Dr. Balcer.

She described a patient with pituitary apoplexy who had complete ophthalmoplegia of the right eye. This recovered into a partial pupil involving third nerve palsy. The pupil did not react to consensual light stimulation; however, upon adduction the pupil became more miotic than that of the left eye, Dr. Balcer demonstrated.

Compressive lesions and trauma are the most common causes of aberrant regeneration of the third nerve.

"When a patient with aberrant regeneration of the third nerve presents, the ophthalmologist should think of compressive lesions, most often aneurysms or tumors," she emphasized.

Tonic pupilsPatients with tonic pupils often notice one large or irregular pupil, which tends to become miotic when the condition is chronic. However, they exhibit light-near dissociation, Dr. Balcer explained, and in the early course patients may complain of accommodative insufficiency or glare. However, there are no eyelid or movement abnormalities.