PUPILS
Light reflex
The light reflex is mediated by the retinal photoreceptors and subserved by
four neurones.
• First (sensory) connects each retina with both pretectal nuclei in the
midbrain. Impulses originating from the nasal retina are conducted by fibres
that decussate in the chiasm and pass up the opposite optic tract to terminate
in the contralateral pretectal nucleus. Impulses originating in the temporal
retina are conducted by uncrossed fibres (ipsilateral optic tract) that
terminate in the ipsilateral pretectal nucleus.
• Second (internuncial) connects each pretectal nucleus to both Edinger–
Westphal nuclei. Thus a uniocular light stimulus evokes bilateral and
symmetrical pupillary constriction. Damage to internuncial neurones is
responsible for light–near dissociation in neurosyphilis and pinealomas.
• Third (preganglionic motor) connects the Edinger–Westphal nucleus to the
ciliary ganglion. The parasympathetic fibres pass through the oculomotor
nerve, enter its inferior division and reach the ciliary ganglion via the nerve to
the inferior oblique muscle.
• Fourth (postganglionic motor) leaves the ciliary ganglion and passes in the
short ciliary nerves to innervate the sphincter pupillae. The ciliary ganglion is
located within the muscle cone, just behind the globe.
ﺩ. ﺯﻳﺎﺩ ﻛﺎﻣﻞ ﺍﳉﻨﺎﺑﻲ
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N ear reflex
The near reflex, a synkinesis rather than a true reflex, is activated when gaze
is changed from a distant to a near target. It comprises accommodation,
convergence and miosis. Vision is not a prerequisite. Although the final
pathways for the near and light reflexes are identical (i.e. third nerve, ciliary
ganglion, short ciliary nerves), the centre for the near reflex is ill-defined.
A fferent pupillary defect
Absolute afferent pupillary defect
An absolute afferent pupillary defect (amaurotic pupil) is caused by a complete
optic nerve lesion and is characterized by the following:
• The involved eye is completely blind (i.e. no light perception).
• Both pupils are equal in size.
• When the affected eye is stimulated by light neither pupil reacts.
• When the normal eye is stimulated both pupils react normally.
• The near reflex is normal in both eyes.
Relative afferent pupillary defect
A relative pupillary defect (Marcus Gunn pupil) is caused by an
incomplete
optic nerve lesion or severe retinal disease, but never
by a dense cataract. The
clinical features are those of an amaurotic
pupil but more subtle. Thus the
pupils respond weakly to stimulation
of the diseased eye and briskly to that of
the normal eye. The
difference between the pupillary reactions of the two
eyes is highlighted
by the ‘swinging flashlight test’ in which a light source is
alternatively switched from one eye to the other and back, thus
stimulating
each eye in rapid succession. For example, A right eye relative defect is
characterized by the following:
• When the normal left eye is stimulated, both pupils constrict
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• When the light is swung to the diseased right eye, the stimulus delivered to
the constriction mechanism is reduced and both pupils dilate instead of
constricting.
• When the normal left eye is again stimulated ,both pupils constrict once
more.
• When the diseased right eye is stimulated, both pupils dilate.
It should be remembered that in afferent (sensory) lesions, the pupils are
equal in size; anisocoria (asymmetrical pupil diameter) implies disease of the
efferent (motor) nerve or the iris itself.
Horner syndrome (oculosympathetic palsy)
Anatomy
The sympathetic supply involves three neurones:
• First (central) starts in the posterior hypothalamus and descends, uncrossed,
down the brainstem to terminate in the ciliospinal centre of Budge, located
between C8 and T2.
• Second (preganglionic) passes from the ciliospinal centre to the superior
cervical ganglion in the neck. During its long course, it is closely related to the
apical pleura where it may be damaged by bronchogenic carcinoma (Pancoast
tumour) or during surgery on the neck.
• Third (postganglionic) ascends along the internal carotid artery to enter the
cavernous sinus where it joins the ophthalmic division of the trigeminal nerve.
The sympathetic fibres reach the ciliary body and the dilator pupillae muscle
via the nasociliary nerve and the long ciliary nerves.
Causes:
Most commonly, isolated (without additional neurological features)
postganglionic Horner syndrome is microvascular in aetiology, but this cannot
be assumed.
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Presentation
The majority of cases are unilateral. Causes of bilateral involvement include
cervical spine injuries and autonomic diabetic neuropathy. Painful Horner
syndrome, especially of acute onset, should raise the possibility of carotid
dissection.
• Mild ptosis (usually 1–2 mm) as a result of weakness of Müller muscle, and
miosis due to the unopposed action of the sphincter pupillae with resultant
anisocoria .
• Pupillary constriction to light and near stimuli is normal.
• Hypochromic heterochromia (irides of different colour, the Horner being
lighter) may be seen if congenital or longstanding.
• Reduced ipsilateral sweating.
Pharmacological tests
Apraclonidine or cocaine is used to confirm the diagnosis.
Hydroxyamphetamine and adrenaline may be used to differentiate a
preganglionic (abnormal first- or second-order neurone) from a postganglionic
lesion (abnormal third-order neurone).
• Apraclonidine: One drop is instilled into both eyes to confirm or refute the
presence of Horner syndrome. Apraclonidine penetrates the blood–brain
barrier, so should be used only with great caution in infants under one year of
age.
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Result: A Horner pupil will dilate but a normal pupil is essentially unaffected.
The ptosis commonly also improves.
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Explanation: Alpha-1 receptors are upregulated in the denervated dilator
pupillae.
• Cocaine 4% is instilled into both eyes.
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Result: The normal pupil will dilate but the Horner pupil will not.
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Explanation: Cocaine blocks the re-uptake of noradrenaline secreted at the
postganglionic nerve ending, which accumulates and causes dilatation of a
normal pupil. In Horner syndrome, there is no noradrenaline being secreted, so
cocaine has no effect.
• Phenylephrine 1% to distinguish pre- and postganglionic lesions.
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Result: In a postganglionic lesion, the Horner pupil will dilate and ptosis may
be temporarily relieved. A central or preganglionic Horner pupil and a normal
pupil will not dilate or will dilate minimally.
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Explanation: In postganglionic Horner syndrome the dilator pupillae muscle
develops denervation hypersensitivity to adrenergic neurotransmitters due to
its dysfunctional local motor nerve.
• Hydroxyamphetamine 1%. Two drops are instilled into each eye. It may be
slightly more sensitive than phenylephrine testing.
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Result: A normal or preganglionic Horner pupil will dilate, but a postganglionic
Horner will not.
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Explanation: Hydroxyamphetamine potentiates the release of noradrenaline
from functioning postganglionic nerve endings. In a lesion of the third-order
neurone (postganglionic) there is no release of noradrenaline from the
dysfunctional nerve.
• Adrenaline 0.1% has an action similar to that of phenylephrine.
Investigation
Specialist neurological or neuro-ophthalmological assessment should be sought
for a confirmed Horner syndrome; an acute presentation should be regarded
as an emergency. In contrast, if the features have been present for over a
year and there are no other localizing signs then the likely diagnostic yield
from further investigation is very low. The mainstay of investigation is
imaging; CT or MR angiography examining the region from the aortic arch to
the circle of Willis will facilitate exclusion of neck (including carotid), apical
lung, thyroid and skull base lesions. MR may be utilized if greater soft tissue
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definition is required, such as to exclude a brainstem stroke. Plain X-rays and
carotid ultrasound imaging have limited utility.
Treatment
Any identified cause should be addressed as appropriate. The ptosis of Horner
syndrome is mild but surgery can be considered at the patient’s discretion;
apraclonidine may be helpful as a temporizing measure.
A die pupil
Introduction: An Adie pupil (tonic pupil, Adie syndrome) is caused by
denervation of the postganglionic parasympathetic supply to the sphincter
pupillae and the ciliary muscle, and may follow a viral illness. It is occasionally
inherited in an AD pattern. Sites of dysfunction are presumed to be the ciliary
ganglion, and, in wider Holmes–Adie syndrome, the dorsal root ganglion involved
in reflex pathways. It typically affects young women and presents in one eye in
80%, though involvement of the second eye typically develops within months or
years.
Diagnosis
• Symptoms. Patients may notice anisocoria, or may have blurring for near due
to impaired accommodation.
• Signs
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Large, regular pupil.
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The direct light reflex is absent or sluggish.
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On slit lamp examination, vermiform movements of the pupillary border are
typically seen.
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Constriction is also absent or sluggish in response to light stimulation of the
fellow eye (consensual light reflex).
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The pupil responds slowly to near, following which re-dilatation is also slow.
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Accommodation may manifest similar tonicity, with slowed and impaired
focusing for near and prolonged re-focusing in the distance.
• Associations include diminished lower limb deep tendon reflexes (Holmes–
Adie syndrome) and other features of autonomic nerve dysfunction such as
excessive sweating (Ross syndrome).
• Pharmacological testing. Instillation of 0.1–0.125% pilocarpine into both
eyes leads to constriction of the abnormal pupil due to denervation
hypersensitivity, with the normal pupil unaffected. Some diabetic patients may
also show this response and very occasionally both pupils constrict in normal
individuals.
• Syphilis serology should usually be checked in patients with bilateral tonic
pupils.
Treatment: is not generally required, but reading glasses may be used to
address near vision difficulties and sunglasses or low concentration pilocarpine
drops can be used to improve photophobia.
M iscellaneous pupillary abnormalities
Physiological anisocoria
: Anisocoria of around 1 mm is
present in around 20% of the normal population. The asymmetry persists to
the same proportion under differing levels of illumination. Exceptionally,
apraclonidine or cocaine testing may be needed to exclude Horner
syndrome.
Pharmacological mydriasis
: Dilatation of one or both pupils
due to instillation of a mydriatic agent can be inadvertent (e.g. use of eye
drops prescribed for someone else or deliberate for the purpose of
malingering. The pupil does not constrict in bright light or on accommodation
and there is no response to any concentration of pilocarpine. There are no
other neurological features.
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ﻛﺎﻣل اﻟﺟﻧﺎﺑﻲ
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