Pupil Pathologies 1 Flashcards
What is a pupil?
The pupil is the dark circular opening in the centre of the iris, where light enters the eye. It is equivalent to the aperture in a camera, while the iris is equivalent to the diaphragm
What is isocoria?
When pupils are the same size.
(iso = equal, korē = pupil, ia = condition)
Why does pupil size vary in diameter?
- Modulate retinal illumination (i.e. regulate retinal illuminance). (though only by ~1.2 log units, vs. the 12 log units in which we operate)
- Optimise optics, by balancing the competing demands of diffraction (high for small apertures) and optical aberrations (high for large apertures).
- Increase depth of field (re: the “near triad”)
Mechanically what is pupil size/control regulated by?
In mechanical terms, pupil size is regulated by two sets of smooth muscles in the iris:
- A circular group called the sphincter pupillae, which when contracted causes the pupil to constrict (miosis)
- A radial group called the dilator pupillae, which when contracted causes the pupil to dilatate (mydriasis)
Which part of the Autonomic Nervous system is the Sphincter muscle innervated by, describe its pathway.
The sphincter (pupil constriction) is innervated by the parasympathetic branch of the ANS (autonomic nervous system), with signals originating at the Edinger-Westphal nucleus( in the midbrain). The fibres enter the orbit with CNIII (3rd cranial nerve) nerve fibres and ultimately synapse at the ciliary ganglion.
Which part of the Autonomic Nervous system is the Dilator muscle innervated by, describe its pathway.
The dilator (pupil dilatation) is innervated by the sympathetic branch of the ANS, with signals traveling down from ganglia (superior cervical ganglion) near the spinal cord, and ultimately entering the orbit through the trigeminal nerve.
What two reflexex does the pupil show?
- Responses to changes in viewing distance (the Pupillary Near Reflex; part of the ‘near triad’ of accommodation, convergence and pupil size, all of which vary together in concert)
- Responses to changes in ambient light (the Pupillary Light Reflex). Notably they constrict to direct illumination (direct response) and to illumination of the opposite eye (consensual response)
What is the left consensual pupillary reflex?
•Left consensual pupillary reflex is the left pupil’s indirect response to light entering the right eye.
What is the right consensual pupillary reflex?
•Right consensual pupillary reflex is the right pupil’s indirect response to light entering the left eye.
Describe the pathway of the pupillary light reflex (PLR)
For each eye, the PLR is subserved by 1 afferent limb (carrying sensory inputs up through fibres in the optic nerve [CN II]), and 2 efferent limbs (carrying pupillary motor outputs down through fibres in the oculomotor nerve [CN III]).
How do the three limbs of the pupillary light reflex make up 8 different neural pathways?
Essentially depending on the response (i.e. swinging light test result) you can predict where the lesion or defect is
Gnerally speaking if a defect is seen in only one eye in the swinging light’s test (e.g no right direct or right consenual repsonse) what can we assume about the pathology?
The pathology lies in the efferent pathway.
When pathology lies in the efferent pathway only one eye is affected
[The picture shows an example]
What can we assume about the pathology if an abnormal response to the swinging light’s test is seen between two eyes?
There is pathology of the afferent nerve.
[The picture shows an example]
What is the swinging light’s test also known as?
The Marcus Gunn Test
What are pupil pathologies generally indications of?
Neural Pathologies - autonomic nervous system
