Title: Light-Near Dissociation
Author: Robert Henseler, 4th Year Medical Student, Rutgers University – New Jersey Medical School
PERRLA or Pupils Equal, Round, Reactive to Light and Accommodation might be the most common acronym to be seen in medical records on the physical exam. Unfortunately, many physicians do not properly asses all aspects of the acronym, so it is very possible to miss subtle changes in the pupils. To discuss light-near dissociation it is important to first have a discussion on the pupil itself and why it constricts. There are two main reasons for constriction, the first is due to stimulation by light (pupillary light response) and the second is due to accommodation or when a patient focuses on a near object.
Anatomy and Pathways:
For the pupillary light response, increased light on the retina leads to miosis or constriction of the pupil and less light leads to mydriasis or dilation of the pupil. The sensory input travels through the optic nerve (CN2) to the pretectal nucleus where it then travels to both Edinger-Westphal nuclei. It then travels to the oculomotor nerves (CN3) and to the ciliary sphincters where it constricts both pupils. This is what leads to direct (stimulated) and consensual (contralateral) pupil constriction.
Accommodation is slightly more complicated and actually consists of three actions. The pupil constricts (due to a similar pathway as above including the Edinger-Westphal nuclei), the lens shortens allowing the eyes to focus (due to relaxation of the zonular fibers due to action of muscles within the ciliary bodies), and the eyes converge on the object (meaning turn in via contraction of the medial rectus muscles).
During the physical exam it is important to do a thorough pupillary exam as abnormal responses can be diagnostic clues to potentially serious pathology. In a dark room observe the size of the pupils. Holding a pen light at a slight angle below the face allows for easy visualization of the pupils without the light hitting the retina and causing constriction of the pupils. The first step is to check the light reflex. Shine light individually into each eye twice. First watching for the direct response (constriction of the ipsilateral pupil) and next looking for the consensual response (constriction of the contralateral pupil). Reflexes should be brisk. A swinging light test, shining light from one pupil to the next can then be done to check for a relative afferent pupillary defect (RAPD). RAPD and its causes is described in more detail in another section. Following the light response, the physician should check that the pupils constrict on accommodation. In a dimly lit room have the patient look at your finger from 3-4 feet away and then move your finger towards their nose as the patient follows your finger observing the convergence of the eyes and the constriction of the pupils.
In light-near dissociation there is slow or incomplete pupillary light reflex, but the pupil will still constrict during accommodation.
So what does light-near dissociation mean clinically and what should be done diagnostically? There are a few entities to consider. Historically speaking this was very commonly seen with neurosyphilis. Hence the origin of the politically incorrect term, “prostitute’s pupil”. This specific diagnostic term is Argyll Robertson Pupil. In the developed world this has become extremely rare due to increased access to care, advancements in blood testing, and the use of antibiotics to treat syphilis in its early stages before it progresses to neurosyphilis. Regardless, a patient with a light-near dissociation should be worked up for syphilis, with appropriate history, physical exam, and bloodwork.
Adie’s or Tonic pupil is probably the most common cause of light-near dissociation. It is a disorder in which one, (or possibly both) pupils are abnormally dilated. The light response is slowed. It is due to injury to the parasympathetic oculomotor nerve. In the acute stages the reaction to accommodation can be delayed as well, but chronically light near dissociation is common. Also, chronically the pupil tends to get smaller in size due to regeneration of nerve fibers to the iris sphincter. Adie’s pupil is generally considered a benign condition and might not require further workup. Interestingly, Adie’s pupil is associated with hyporeflexia, especially of the knee and ankle reflexes (Adie’s Syndrome). Adies pupil can also be seen in diabetes, Sjogrens syndrome and other autonomic disorders.
Lastly, a more rare cause of light near dissociation is Parinaud Syndrome. This syndrome is characterized by paralysis of upward gaze, light-near dissociation, eyelid retraction, and convergence retraction nystagmus. If these signs are seen it is important to work up the patient for Parinaud Syndrome and its multiple causes, which are all connected in that they involve direct or indirect injury to the structures involved in the pathway including the Edinger-Westphal nucleus in the midbrain of the brainstem. Pineal tumors leading to compression of nearby structures are the most common cause of Parinaud Syndrome so appropriate brain imaging should be done. Multiple sclerosis (MS) is another cause, so anyone suspected of MS should have a full workup. Lastly, stroke can cause direct injury to the midbrain.
Light-near dissociation detection requires a thorough pupillary exam by a physician. While rare, it can be a sign of serious pathologic conditions and more work-up and testing should be performed for anyone found to have this pupillary finding.