Binocular Vision - Lecture 9 - Pupils

Binocular Vision - Lecture 9 - Pupils

• The lecture covers pupil size control pathways, testing methods, and abnormalities.
• Students will be able to identify afferent and efferent pathways controlling pupil size.
• Students will be able to describe parasympathetic and sympathetic nerve innervations.
• Students will be able to identify methods for testing pupil function.
• Students will be able to recognize common pupil function abnormalities.

Terms

• Afferent: Nerve carrying information towards the central nervous system.
• Efferent: Nerve carrying information away from the central nervous system.
• Sympathetic: "Fight and flight" response.
• Parasympathetic: "Rest and digest" response.
• Cholinergic receptors (muscarinic): Stimulated by acetylcholine.
• Adrenergic receptors (alpha 1): Stimulated by noradrenaline.

The Afferent Pathway

• Nerve carrying information towards the central nervous system.
• The pathway travels from the retina to the pretectal nuclei in the brain.
• Understanding the afferent pathway is essential for correlating damage effects to the pupils.
• Includes retinal ganglion cells, optic nerve, optic chiasm, optic tract, pretectal nucleus, Edinger-Westphal nucleus.

The Efferent Pathway

• Nerve carrying impulses away from the central nervous system toward the muscles controlling the pupil size.
• Includes:
  • Sphincter muscle (parasympathetic): responsible for pupil constriction.
  • Dilator muscle (sympathetic): responsible for pupil dilation.

Muscles Controlling Pupil Size

• The dilator and sphincter muscles work antagonistically to control pupil size.
  • Dilator muscle: The main muscle responsible for pupil dilation in response to signals in the long ciliary nerve. It is innervated by adrenergic sympathetic fibers.
  • Sphincter muscle: The main muscle responsible for pupil constriction in response to signals in the short ciliary nerve. It is innervated by cholinergic parasympathetic fibers.

Pupil Size in Different Light Conditions

• Dark lighting: Sphincter muscle relaxes, dilator muscle contracts, resulting in a larger pupil.
• Bright lighting: Sphincter muscle contracts, dilator muscle relaxes, leading to a smaller pupil.

Effects of Pharmaceutical Innovation

• Pharmaceuticals can either constrict or dilate pupils.
  • Mydriasis (dilated pupil): Tropicamide, Cyclopentolate, Atropine, Phenylephrine.
  • Miosis (constricted pupil): Pilocarpine.

Pupil Terminology

• Anisocoria: Inequality in pupil size.
• Miosis: Pupil constriction.
• Mydriasis: Pupil dilation.

Normal Pupils

• Typically round and equal in size.
• Diameter ranges from 3mm to 5mm in ambient light.
• Miotic pupils: Less than 3mm in diameter.
• Mydriatic pupils: Greater than 7mm in diameter.

Parasympathetic Pathway

• Runs from the Edinger-Westphal nucleus, through the oculomotor nerve (cranial nerve III), ciliary ganglion, short ciliary nerve, to the sphincter muscle in the iris.

Sympathetic Pathway

• Originates from the hypothalamus and travels to the superior cervical ganglion, subsequently to the dilator muscle in the iris via the long ciliary nerve.

Pupil Testing - Purpose

• To assess afferent and efferent neurological pathways responsible for pupil functions.
• Abnormalities can be life-threatening so careful evaluation is crucial.

Pupil Testing - Procedure

• Observation: Assess for anisocoria (unequal pupil sizes) in bright and dim light.
• Direct Response: Shine a light into one eye; the pupil should constrict.
• Consensual Response: Shine a light into one eye; the pupil in the other eye should also constrict.
• Swinging Flashlight Test: Rapidly move light between eyes to assess for pupillary escape/Marcus Gunn response.
• Near Reflex Test: Have the patient focus on a near object to assess accommodation and constriction.

Pupil Abnormalities

• Recording abnormalities using a standardized scale (e.g., 0 to 4+ for constriction).

Damage to the Afferent Pathway

• Key factors include light response in one eye versus the other, and differences in the direct and consensual responses.
• A lesion in the optic nerve or pathway can result in a reduced or absent direct and consensual response.

Damage to the Parasympathetic Pathway

• Results in a pupil that is consistently larger than its counterpart (common abnormalities include Adie's tonic pupil or damage to the 3rd nerve pathway with a fixed dilated pupil.)
• The inability of the pupil to respond to a light stimulus, particularly the consensual response, can be key in making the diagnosis.

Damage to the Sympathetic Pathway

• Pupil becomes smaller, known as Horner's syndrome.
• Common symptoms include ptosis, miosis, and anhydrosis, often with a possible etiology such as Pancoast tumor.

Argyll Robertson Pupil

• Small pupils that react poorly or not at all to light but respond normally to near (light-near dissociation)
• Associated with neurosyphilis and sometimes chronic diabetes or alcoholism.

Recording Abnormal Findings

• Use standardized terminology (e.g., PERRLA, RAPD) to objectively record pupil responses.
• Describe any abnormalities in pupil size or response to light and near situations.