Pupil reactions

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Light reflex

The light reflex is mediated by retinal photoreceptors and 4 neurons.

1. The first neuron (sensory) connects each retina with both pretectal nuclei of the midbrain at the level of the superior colliculus. Impulses arising in the temporal retina are conducted by uncrossed fibers (the ipsilateral optic tract), which terminate in the ipsilateral iretectal nucleus.
2. The second neuron (interneuron) connects each pretectal nucleus with both Edinger-Weslphal nuclei. A monocular light stimulus causes bilateral symmetrical pupillary constriction. Damage to interneurons causes dissociation of reactions to light and near distances in neurosyphilis and insalomas.
3. The third neuron (preganglionic motor) connects the Edinger-Westphal nucleus with the ciliary ganglion. Parasympathetic fibers are part of the oculomotor nerve and, entering its lower branch, reach the ciliary ganglion.
4. The fourth neuron (postganglionic motor) leaves the ciliary ganglion and, passing in the short ciliary nerves, innervates the sphincter of the pupil. The ciliary ganglion is located in the muscular cone, behind the eye. Different fibers pass through the ciliary ganglion, but only the parasympathetic ones form a synapse in it.

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Approach reflex

The approach reflex (a synkinesis, not a true reflex) is activated by shifting the gaze from a distant to a near object. It involves accommodation, convergence, and miosis. Vision is not necessary for the approach reflex, and there is no clinical condition in which the light reflex is present but the approach reflex is absent. Although the terminal pathways for the approach and light reflexes are identical (i.e., oculomotor nerve, ciliary ganglion, short ciliary nerves), the approach reflex center is poorly understood. Two supranuclear influences are likely: from the frontal and occipital lobes. The midbrain approach reflex center is probably more ventral than the pretectal nucleus, which is why compressive lesions such as pinealomas preferentially affect the dorsal interneurons of the light reflex, sparing the ventral fibers to the last.

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Sympathetic innervation of the pupils

Sympathetic innervation includes 3 neurons:

1. The first-order (central) neuron originates in the posterior hypothalamus and descends, uncrossed, along the brainstem to terminate in the ciliospinal center of Budge in the lateral interstitium of the spinal cord between C8 and T2.
2. The second-order neuron (preganglionic) runs from the ciliospinous center to the superior cervical ganglion. Along its route, it is closely associated with the apical pleura, where it can be affected by bronchogenic carcinoma (Pancoasl tumor) or by neck surgery.
3. The third-order neuron (postganglionic) ascends along the internal carotid artery to the cavernous synapse, where it joins the ophthalmic branch of the trigeminal nerve. Sympathetic fibers reach the ciliary body and dilator pupillae via the nasociliary nerve and long ciliary nerves.

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Afferent pupillary defects

Absolute afferent pupillary defect

Absolute afferent pupillary defect (amaurotic pupil) is caused by complete damage to the optic nerve and is characterized by the following:

• The eye on the affected side is blind. Both pupils are of equal size. Neither pupil responds to light stimulation of the affected eye, but both pupils respond normally to stimulation of the normal eye. The approach reflex is normal for both eyes.

Relative afferent pupillary defect

Relative afferent pupillary defect (Marcus Gunn pupil) is caused by incomplete lesion of the optic nerve or severe retinal damage, but is not caused by dense cataract. Clinical manifestations are similar to amaurotic pupil, but milder. Thus, the pupils react sluggishly to stimulation of the diseased eye, while those of the normal eye react briskly. Differences in pupillary response in both eyes are emphasized by the "flashlight swing" test, in which the light source is moved from one eye to the other and back, stimulating each eye in turn. The normal eye is stimulated first, causing both pupils to constrict. When the light is moved to the diseased eye, both pupils dilate instead of constricting. This paradoxical dilation of the pupils in response to illumination occurs because the dilation caused by the diversion of light from the normal eye outweighs the constriction caused by stimulation of the diseased eye.

In afferent (sensory) lesions, the pupils are of equal size. Anisocoria (unequal pupil size) is a consequence of lesions of the efferent (motor) nerve, iris, or pupillary muscles.

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Dissociation of pupillary reflexes to light and near distances

The reflex to light is absent or sluggish, but the reaction to approach is normal.

Causes of dissociation of pupillary reflexes to light and near distances

One-sided

• afferentation conduction defect
• Adie's pupil
• herpes zoster ophthalmicus
• aberrant regeneration of n. oculomotorius

Double sided

• neurosyphilis
• diabetes type 1
• myotonic dystrophy
• Parinaud's dorsal midbrain syndrome
• familial amyloidosis
• encephalitis
• chronic alcoholism

Symptoms

• Moderate ptosis (usually 1-2 mm) as a result of weakness of the Müller muscle.
• Slight elevation of the lower eyelid due to weakness of the inferior tarsal muscle.
• Miosis due to unimpeded action of the sphincter of the pupil, with the development of anisocoria, which intensifies in low light, since Horner's pupil does not dilate, like the paired one.
• Normal reaction to light and proximity,
• Decreased sweating is ipsilateral, but only if the lesion is below the superior cervical ganglion, since the fibers innervating the skin of the face run along the external cervical artery.
• Hypochromic heterochromia (irises of different colors - Horner's pupil is lighter) is visible if the lesion is congenital or has existed for a long time.
• The pupil dilates slowly.
• Less important symptoms: hyperactivity of accommodation, ocular hypotonia and conjunctival hyperemia.

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Pupil of Argyll Robertson

It is caused by neurosyphilis and is characterized by the following:

• Manifestations are usually bilateral but asymmetrical.
• The pupils are small and irregular in shape.
• Dissociation of reactions to light and proximity.
• The pupils are very difficult to dilate.

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Adie's pupil

Adie's pupil (tonic) is caused by postganglionic denervation of the sphincter pupillae and ciliary muscle, possibly due to a viral infection. Usually occurs in the young and is unilateral in 80% of cases.

Symptoms

• Uniformly dilated pupil.
• The light reflex is absent or sluggish and is combined with worm-like movements of the edge of the pupil, visible in a slit lamp.
• The pupil reacts slowly to the approach of an object, and subsequent dilation is also slow.
• Accommodation may exhibit similar tonicity. Thus, after fixation on a close object, the time of refocusing on a distant object (relaxation of the ciliary muscle) is increased.
• Over time, the pupil may become small ("little old Adie").

In some cases, this is accompanied by weakening of deep tendon reflexes (Holmes-Adie syndrome) and autonomic dysfunction.

Pharmacological tests. If mecholyl 2.5% or pilocarpine 0.125% is instilled into both eyes, the normal pupil will not constrict, but the affected one will constrict due to denervation hypersensitivity. Some patients with diabetes may also have this reaction, and in healthy people both pupils constrict very rarely.

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Oculosympathetic paralysis (Horner syndrome)

Causes of Horner's syndrome

Central (first order neuron)

• brainstem lesions (vascular, tumors, demyelination)
• syringomyelia
• alternating wallenberg syndrome
• spinal cord tumors

Preganglionic (second order neuron)

• Pancoast tumor
• carotid and aortic aneurysms and dissections
• neck diseases (glands, trauma, postoperative)

Postganglionic (third order neuron)

• cluster headaches (migraine neuralgia)
• internal carotid artery dissection
• nasopharyngeal tumors
• otitis media
• cavernous sinus neoplasm

Pharmacological tests

Diagnosis is confirmed with cocaine. Hydroxyamphetamines (paredrias) are used to differentiate preganglionic from postganglionic lesions. Epinephrine can be used to assess denervation hypersensitivity.

Cocaine 4% is instilled into both eyes.

• Result: normal pupil dilates, Horner pupil does not.
• Explanation: Noradrenaline released by the postganglionic sympathetic endings is reuptaken, which stops its action. Cocaine blocks reuptake, so noradrenaline accumulates and causes pupil dilation. In Horner's syndrome, noradrenaline will be released, so cocaine has no effect. Thus, cocaine confirms the diagnosis of Horner's syndrome.

Hydroxyamphetamine 1% is instilled into both eyes.

• Result: With a preganglionic lesion, both pupils will dilate, whereas with a postganglionic lesion, Horner's pupil will not dilate. (The test is performed the day after the effects of the cocaine have worn off.)
• Explanation: Hydroxyamphetamine increases the release of norepinephrine from postganglionic nerve endings. If this neuron is intact (lesion of the first or second order neuron, and also a normal eye), NA will be released and the pupil will dilate. If the third order neuron (postganglionic) is damaged, dilation cannot occur, because the neuron is destroyed.

Adrenaline 1:1000 is instilled into both eyes.

• Result: in a preganglionic lesion, neither pupil will dilate because adrenaline is rapidly broken down by monoamine oxidase; in a postganglionic lesion, the Horner pupil will dilate and ptosis may temporarily decrease because adrenaline is not broken down due to the absence of monoamine oxidase.
• Explanation: A muscle deprived of motor innervation exhibits increased sensitivity to the excitatory neurotransmitter released by the motor neuron. In Horner's syndrome, the muscle that dilates the pupil also exhibits "denervation hypersensitivity" to adrenergic neurotransmitters, so that even low concentrations of adrenaline cause a noticeable dilation of the Horner pupil.

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