Study Notes on the Pupil and Related Physiology

Introduction to the Pupil

  • The pupil is the black circle in the center of the colored iris of the eye.

  • Function: Allows light to enter the eye, which then passes through the lens to the retina at the back of the eye.

  • The retina is responsible for picking up light signals and transmitting them to the brain.

Regulation of Pupil Size

  • The size of the pupil is adjustable, allowing the body to control light entry.

  • Meiosis: The contraction or shrinking of the pupil.

  • Mydriasis: The dilation or increase in size of the pupil.

  • Regulation: Changes in pupil size are controlled autonomously and cannot be voluntarily manipulated.

Anatomy of the Iris

  • Iris: The colored portion surrounding the pupil, composed primarily of smooth muscle.

  • Muscle Groups:

    • Sphincter Pupillae:

    • Nature: Circular muscle.

    • Function: Contracts to decrease pupil size (meiosis).

    • Dilator Pupillae:

    • Nature: Radially oriented muscle with fibers extending from the center.

    • Function: Contracts to increase pupil size (mydriasis).

Neural Pathways of Meiosis (Pupillary Contraction)

  • Controlled by the Parasympathetic Nervous System.

  • Involves a two-neuron pathway:

    1. First Neuron: Begins at the Edinger Westphal nucleus in the midbrain (near cranial nerve III or oculomotor nucleus).

    2. Second Neuron: Travels to the ciliary ganglion, located behind the eye, where it synapses.

  • Transmission: Ciliary ganglion neurons signal the sphincter pupillae muscle via short ciliary nerves.

  • Neurotransmitter: Acetylcholine acts on muscarinic receptors.

Clinical Application: Rule of the Pupil

  • Important for evaluating cranial nerve III lesions.

  • Symptoms in lesions of cranial nerve III:

    • Affected eye appears down and out.

    • Assess pupil for dilation:

    • Dilated Pupil: Indicates impacted parasympathetic nerves, often compressed by a mass (e.g., posterior communicating artery aneurysm).

    • No Dilation: Suggests ischemic damage—often seen in diabetes.

Neural Pathways of Mydriasis (Pupillary Dilation)

  • Controlled by the Sympathetic Nervous System.

  • Activation involves the dilator pupillae muscle and inhibition of the sphincter pupillae.

  • Neurotransmitter: Norepinephrine acts on alpha-1 receptors.

  • Involves a three-neuron chain:

    1. First Neuron: Originates in the posterior hypothalamus, travels through the brainstem to the spinal cord (ends in ciliospinal center of Budge at C8 to T2).

    2. Second Neuron: Travels from the spinal cord to the superior cervical ganglion, exits at T1, and crosses lung's apical pleura.

    3. Third Neuron: Travels from the superior cervical ganglion to the dilator pupillae muscle alongside the internal carotid artery, passing through the cavernous sinus.

Clinical Condition: Horner Syndrome

  • Characterized by:

    1. Meiosis (small pupil) due to loss of sympathetic innervation.

    2. Ptosis: Eyelid droop, due to loss of sympathetic input to superior tarsal muscle.

    3. Anhidrosis: Absence of sweating on one side of the face.

  • Causes: Apical lung tumor, aortic dissection, carotid dissection, and certain strokes (e.g., PICA stroke).

  • Diagnosis: Disruption of sympathetic pathways can lead to emerging symptoms seen in Horner syndrome.

Diagnostic Tests for Horner Syndrome

  • Cocaine Test: Blocks norepinephrine reuptake; causes dilation in normal eyes but no effect in Horner syndrome.

  • Aproclonidine: Alpha agonist that can dilate the meiotic pupil in Horner syndrome, indicating functional dilator muscle despite lack of sympathetic innervation.

Anisocoria (Pupil Size Difference)

  • Definition: A condition where pupils are unequal in size.

  • Relation to conditions:

    • Horner Syndrome: Small pupil is abnormal.

    • Cranial Nerve III Palsy: Large pupil is abnormal.

    • Addy's Pupil: Affected pupil typically larger.

Pupillary Reflexes

  • Pupillary Light Reflex: Tests both eyes’ response to light:

    • Shine light in one eye; both should constrict (direct response in illuminated eye, consensual response in the opposite eye).

    • Signals travel to the pretectal nucleus in midbrain which sends signals to Edinger Westphal nuclei bilaterally.

    • Activated parasympathetic output results in sphincter pupillae muscle contraction.

Marcus Gunn Pupil

  • Describes a relative afferent pupillary defect where light in one eye produces less constriction than the other eye.

  • Diagnosis: Use the swinging flashlight test—reduced constriction on the affected side indicates afferent pathway issues.

  • Classic cause: Optic neuritis, often associated with multiple sclerosis.

Accommodation Reflex

  • Definition: Process allowing the eye to focus on near objects, involving changes in lens optical power.

  • Accompanied by miosis (pupil constriction).

  • Reflex actions during accommodation:

    1. Convergence of both eyes (medial movement).

    2. Accommodation (lens shape change).

    3. Miosis (pupil constriction).

Clinical Correlation: Argyll Robertson Pupil

  • Finding associated with neurosyphilis.

  • Characteristics: Bilateral small pupils that constrict to accommodation but not to light (light-near dissociation).

Clinical Documentation of Pupils

  • Normal finding is documented as PERRLA:

    • Pupils Equal, Round, and Reactive to Light and Accommodation.

    • Indicates a complete examination of pupillary reflexes showing normal function.

Conclusion

  • Summary of key points regarding pupil physiology, anatomy, neural pathways, clinical implications, and reflex testing.