Pupillary Abnormalities & Its Significance

• The pupil is an aperture – (a round hole) in the center of the iris.
• It is black in color.
• The pupil measures 3-4mm.
• It is surrounded by the iris, a contractile structure, which controls its size.

• During mydriasis, the iris is pulled into numerous ridges and folds; during miosis, its anterior surface appears relatively smooth.
• The pupil’s function is to allow light to pass through and enter the eye.
• This light then interacts with the cells of the retina, working as part of the visual pathway to provide the ability of sight.
• The pupillary margin is formed by a ring of circular smooth muscle fibers within the iris measuring 1mm wide called the Sphincter Pupillae Muscle.
• The iris with its smooth muscles (intrinsic eye muscles) controls the amount of light entering the eye by controlling the size of the pupil.

Anterior Relations

The aqueous humor in the anterior chamber

Posterior Relations

The anterior surface of the lens and posterior chamber.

• Sphincter Pupillae: Supplied by Parasympathetic N. through the postganglionic fibers in short ciliary Ns from 3rd Nerve. Its stimulation causes pupillary constriction - Miosis.
• Dilator Pupillae: Supplied by Sympathetic N. through postganglionic fibers in long ciliary Nerves from the posterior hypothalamus. Its stimulation causes pupillary dilatation - Mydriasis.

When the Sphincter pupillae muscles are stimulated, they contract and constrict the pupil, while the Dilator pupillae when stimulated, contracts and dilates the pupil.

THE PARASYMPATHETIC PATHWAY

This is responsible for

1. The Pupillary light reflex
2. The Near Reflex

The Pupillary Light Reflex

Is a four Neuron arc

1. 1st Order Neuron
• Connects the retina to the pretectal nucleus in the midbrain – at the level of Sup. Colliculus.
• Reflex is mediated by the retinal photoreceptors, then the ganglion cells by small caliber fiber types.
• Retinal nasal fibers decussate at the optic chiasma, pass up optic tract + terminate in the contralateral pretectal nucleus.
• Impulses from temporal retinal, conducted by uncrossed fibers in the optic tract terminate in the ipsilateral pretectal nucleus.
• The pupillary afferent fibers leave the optic tract just before the LGB to synapse in the pretectal nucleus in the midbrain.
2. 2nd Order Neuron
• Connects each pretectal nucleus to both Edinger-Westphal nuclei in the 3rd Nerve nucleus complex, equally distributed.
• Called internuncial neurons, Also called intercalated fibers
• Together with the chiasmal crossing - explains the consensual light response which is symmetrical
• Thus a uniocular light stimulus evokes bilateral and symmetrical pupillary constriction.
3. 3rd Order Neuron
• Connects (pre-ganglionic) Edinger-Westphal nucleus (3rd N) to the ciliary ganglion.
• The parasympathetic fibers pass through the oculomotor nerve, enter its inferior division and reach the ciliary ganglion via the nerve to the inferior oblique muscle
• These are called pupillomotor fibers & they mediate the efferent pathway
4. 4th Order Neuron
• Connects (post-ganglionic fibers) the ciliary ganglion and the sphincter pupillae ms. through the short ciliary Nerves
• The ciliary ganglion is located within the muscle cone, just behind the globe.
• It should be noted that, although the ciliary ganglion serves as a conduit for other nerve fibers, only the parasympathetic fibers synapse there
• Approximately 3% - 5% of short ciliary nerves supply the pupillary sphincter.
• The rest (95-97%) are designated for the ciliary muscle and are concerned with the near reflex and accommodation

The Near Reflex

• A triad of:
• Convergence of the visual axes
• Constriction of the pupils
• Increased accommodation
• When the eyes are directed from a distant object to a near one, 3 things happen:
• Contraction of the medial recti: Brings about convergence of the ocular axes
• The lens thickens: Increases the refractive power by contraction of the ciliary muscles.
• The pupils constrict: Limiting the light waves to the thickest central part of the lens hence increasing the depth of focus
• The afferent impulses travel through the optic N, optic chiasma, optic tract, LGB, and optic radiations to the visual cortex.
• The visual cortex is connected to the frontal eye field from where cortical fibers:
• Descend via the internal capsule to the medial rectus subnuclei of the oculomotor nuclei in the midbrain, the 3rd N travels to the medial recti muscles.
• Some of the descending cortical fibers synapse with the parasympathetic nuclei (Edinger-westphal nuclei) of the 3rd nerve. These fibers travel through the 3rd N to the ciliary ganglion to the ciliary muscle + the sphincter pupillae muscles.
• The midbrain center for the near reflex is located in a more ventral position than that for the light reflex (pretectal Nucleus).
• Hence compressive lesions like pinealomas preferentially involve the dorsal pupillomotor fibers and spare the ventral near reflex fibers until late in the development of the lesion - Light near dissociation

THE SYMPATHETIC NERVE SUPPLY

• 3 NEURONE ARC
• 1ST-central neurone
• 2nd- preganglionic neurone
• 3rd – postganglionic neurone
• First order neurone
• Connects the posterior hypothalamus and the ciliospinal centre of Budge (between C8 –T2) having descended uncrossed down the brainstem.
• Involves brainstem + cervical cord
• 2nd – Preganglionic Neurone
• Connects the ciliospinal centre of Budge with the superior cervical ganglion in the neck.
• Involves the chest + Neck.
• Has a long course where it is closely related to the apical pleura + the subclavian artery.
• 3rd – Postganglionic Neurone
• Connects the superior cervical ganglion to the iris dilator muscles through the nasociliary nerve and the long ciliary nerve.
• Passes thru the ciliary ganglion but does not synapse
• It ascends along the internal carotid artery to enter the base of skull and passes through the cavernous sinus to the orbit where it joins the ophthalmic division of the 5th N. (Nasociliary N → long ciliary N).
• Any total or partial interruption of the sympathetic chain anywhere along its course from the hypothalamus to the eye causes the syndrome called Horner’s syndrome.