He Consensual reflex Is any reflex observed on one side of the body when the other side has been stimulated. This reflex is evidenced mainly in the process of contracting the pupil of both eyes by illuminating only one of them (Dictionary, 2017).
The pupillary response to light is the reduction in pupil size when the eye is directly illuminated. This is the most common stimulus applied to the contraction of the hole in the center of the iris. The process of contracting both pupils uniformly when the stimulus is generated in a single eye is known as a consensual reflex (Backhaus, 2011).
Consensual reflex is important in determining whether there is neurological or central nervous system damage. If the contraction of the pupils occurs unevenly it can be concluded that there is damage to the cranial nerves of the patient. Similarly, the consensual reflex can help determine if there are damage to the retina or oculomotor nerves.
There are several tests and light stimuli that can be used to evidence the normal reaction of the consensual reflex in both pupils. These tests include the gradual lighting of a room, the direct application of light on one of the two eyes or the test of oscillating light.
The consensual reflex is different from the photomotor reflex, the latter being the one that takes place in the eye in which the light stimulus is applied directly and whose effect is also the contraction of the pupil.
How it is produced
The size of the pupil is determined by the interaction of the sympathetic and parasympathetic nervous systems, which are connected to the iris.
These systems are controlled by the central nervous system, which sends signals to the brain Influenced by numerous factors, such as illumination, observation distance, surveillance status and cognitive status (Dragoi, 1997).
The reduction in pupil size occurs when the circular muscle of the eye, controlled by the sympathetic nervous system, contracts in response to an external stimulus of light. The pupillary constriction of each eye occurs when the retina, or optic nerve and the pretectal nucleus of each eye take sensory information from the outside.
When one of the eyes of one individual is covered and the other eye is illuminated, the contraction of the pupil of both eyes must occur simultaneously and uniformly.
This happens by sending an afferent signal through the optic nerve that connects with the Edinger-Westphal nucleus, whose axons traverse the oculomotor nerves of both eyes.
Manifestation of the consensual reflex
The size and shape of the pupil under normal light conditions is in the range of 1 to 8 millimeters. When the pupils function correctly, they are said to be isochronous, meaning that they react in the same way to the stimulus of light. When this stimulus is modified, the pupils must grow symmetrically and simultaneously.
To evaluate that the pupils have normal functioning, a consensual reflex test is usually applied.
This test consists in illuminating both eyes independently so that a direct pupil response occurs in the eye being illuminated and an indirect response in the eye that does not receive is stimulation.
If the optic nerve of the illuminated eye is damaged, the pupil reflex does not take place, therefore the consensual reflex does not take place, since the eye that is not being stimulated receives no message.
However, if the optic nerve of the eye being illuminated and the oculomotor nerve of the eye that is not being stimulated are in perfect condition, the consensual reflex will take place, as the signal may be sent by one eye and received by the other (Bell, Wagoner, & Boyd, 1993).
Abnormalities of the pupil
There are some disorders that can occur in the nervous system of the eye that can affect the process of contraction of the pupil.
These disorders can affect the parasympathetic system and lead to a consensual response to light occurring irregularly (Levatin, 1959). Some of these disorders may include the following:
- Inflammation of the optic nerve (Optic neuritis).
- High intraocular pressure (severe glaucoma).
- Direct or indirect ocular trauma (Traumatic optic neuropathy).
- Tumor of the optic nerve.
- Disease in the orbit of the eye.
- Optic atrophy.
- Infections or inflammation of the optic nerve.
- Diseases of the retina
- Intracranial pathologies
- Brain Injury
- Pharmacological blockages (Lowth, 2017)
Swing light test
The oscillating light test is used to detect the presence of reactive pupillary afferent defects. This means that the test is used to determine if there is any difference in the way both eyes respond to the application of light over one of the two eyes.
The test is useful for detecting diseases of the retina or optic nerve that cause the pupils to contract asymmetrically (Broadway, 2012).
The steps to carry out this test are as follows:
- Use a flashlight that can be focused close to the eye in a dimly lit room.
- Ask the patient to look from a distance while the eye is illuminated. This will prevent the pupil from contracting due to the reaction to the proximity of the flashlight during the test.
- Move the flashlight deliberately from one eye to another, illuminating each eye independently. Be careful not to move the flashlight near the nose, as this may stimulate the pupil's response to a nearby object.
- Continue to move the flashlight at the same distance from each eye to make sure that each eye is receiving the same stimulus.
- Hold the flashlight for three seconds in each eye, allowing the movement of the pupil to stabilize. Notice what happens to the other pupil during this process.
- Repeat the test several times to identify what happens to the pupil of each eye when illuminated.
- Backhaus, S. (2011). Pupillary Light Response, Pupillary Response. In J. S. Kreutzer, J. DeLuca, & B. Caplan, Encyclopedia of Clinical Neuropsychology (P.2086). New York: Springer ScienceþBusiness Media.
- Bell, R., Wagoner, P., & Boyd, W. (1993). Clinical grading of relative afferent pupillary defects. Arch Ophthalmol , 938-942.
- Broadway, D.C. (2012). How to test for a relative afferent pupillary defect (RAPD). Community Eye Health Journal Pp. 79-80; 58-59.
- Dictionary, T. F. (2017). The Free Dictionary . Retrieved from Consensual light reflex: medical-dictionary.thefreedictionary.com.
- Dragoi, V. (1997). Neuroscience Online . Retrieved from Chapter 7: Ocular Motor System: http://neuroscience.uth.tmc.edu.
- Levatin, P. (1959). Pupillary escape in disease of the retina and optic nerve. Arch Ophthalmol. , 768-779.
- Lowth, M. (2017, 14). Patient . Retrieved from Pupillary Abnormalities: patient.info.