pupils Flashcards
how are the pupils innerveated (parasympathetically)
The pupil constricts by contraction of the sphincter pupillae muscle, which is controlled by innervation by the parasympathetic fibres in the 3rd cranial nerve
The neurotransmitter is acetylcholine
Acetylcholine binds to muscarinic cholinergic receptors
Pupil size is predominantly controlled by parasympathetic tone
how are the pupils innervated sympathetically
The pupil dilates by contraction of the dilator pupillae muscle, which is controlled by innervation from sympathetic fibres carried to the eye via blood vessels
The neurotransmitter is noradrenaline
Noradrenailne binds to alpha receptors
describe the pupil reflexes
The pupil has three reflexes
Reaction to light (constriction-miosis)
To direct illumination in one eye
To consensual illumination in the fellow eye
Reaction to dark (dilatation-mydriasis)
Reaction to a near target
Miosis
Accommodation (focussing by ciliary muscle)
Convergence
describe the mechnaissm of the light reflex
Light enters the eye and stimulates the retina
Impulses pass to both optic tracts at the chiasm
Afferent limb ends at the parasympathetic part of the 3rd nerve nucleus (Edinger-Westphal)
Efferent impulses pass along 3rd cranial nerve
Fibres synapse in ciliary ganglion in orbit
Pupil constriction occurs in both eyes by stimulation of sphincter muscle
What are the key components and mechanisms involved in the control of pupil size?
Sphincter muscle (cholinergic, IIIn): Responsible for constriction of the pupil. It is innervated by the parasympathetic nervous system via cranial nerve III (oculomotor nerve).
Dilator muscle (noradrenergic, sympathetic): Responsible for dilation of the pupil. It is innervated by the sympathetic nervous system.
Afferent limb: Involves the pathway of sensory input. It includes the retina, optic nerve, optic chiasm, and optic tract.
Central integration: The integration of sensory input and generation of appropriate motor responses. It occurs in the mesencephalon and involves the Edinger-Westphal nucleus (E-W nucleus).
Efferent limb: Involves the pathway of motor output. It includes cranial nerve III (oculomotor nerve) and the sphincter muscle for pupil constriction.
how to check pupil reactions
Check that pupils react to light:
Shine into left eye and observe left eye
Shine into left eye and observe right eye
Shine into right eye and observe right eye
Shine into right eye and observe left eye
Check pupils react to a near target
if you are asked to examine the patients pupils what should you do
Anisocoria (pupils are different sizes)
Afferent pupillary defect (problem in the retina or optic nerve)
Efferent pupillary defect (problem with the 3rd nerve or sphincter muscle)
Light near dissociation (poor or absent response to light but a normal response to near)
what is the direct and consensual papillary refelx
shine the light from your pen torch into patietns pupil to observe for pupillary reaction
a normal direct pupillary reflesx involves constriction of the pupil that the light is being shone into
consensula shine light into one pupil observe pupil constrictiion in the other eye
what does the swinging light test asess
Direct the light from the flashlight into one eye while observing the response of both pupils. Note the initial size and reaction of the illuminated pupil.
Quickly move the light to the other eye, shining it into that pupil while observing both pupils simultaneously.
Repeat this process, swinging the light back and forth between the two eyes several times.
Observations: During the swinging of the light:
Normal Response: Both pupils should constrict briskly when illuminated and dilate when the light is removed.
Abnormal Response (RAPD): In a patient with RAPD, the affected pupil may paradoxically dilate (or fail to constrict as briskly) when the light is swung to that eye. This is because the affected eye is less responsive to light due to a defect in the afferent visual pathway.
what do the different pupillary tests assess
The direct pupillary reflex assesses the ipsilateral afferent limb and the ipsilateral efferent limb of the pathway.
The consensual pupillary reflex assesses the contralateral efferent limb of the pathway.
The swinging light test is used to detect relative afferent limb defects.
what pathologies may the different pupillary tests pick up
Direct Pupillary Reflex:
Ipsilateral Afferent Limb: This part of the pathway involves the sensory input from the retina to the brainstem. Abnormalities in this limb may indicate dysfunction in the retina, optic nerve, optic chiasm, or optic tract on the same side as the stimulated eye. Conditions affecting these structures include optic neuritis, optic nerve compression (e.g., from a tumor), or ischemic optic neuropathy.
Ipsilateral Efferent Limb: This part involves the motor output from the brainstem to the sphincter muscle of the iris on the same side. Dysfunction in this limb may indicate pathology affecting cranial nerve III (oculomotor nerve) or the Edinger-Westphal nucleus in the midbrain. Causes may include oculomotor nerve palsy, midbrain lesions, or compression of cranial nerve III.
Consensual Pupillary Reflex:
Contralateral Efferent Limb: This limb involves the motor output from the brainstem to the sphincter muscle of the iris on the opposite side. Abnormalities in this limb may indicate dysfunction affecting the contralateral side of cranial nerve III or the Edinger-Westphal nucleus. Causes may include lesions or compression affecting the contralateral oculomotor nerve or midbrain structures.
Swinging Light Test (Relative Afferent Pupillary Defect - RAPD):
Relative Afferent Limb: This test is specifically designed to detect defects in the afferent limb of the pupillary pathway. It assesses the response of both pupils to a swinging light stimulus. A relative afferent pupillary defect (RAPD) indicates decreased or impaired pupillary constriction in response to light in one eye compared to the other. This defect suggests dysfunction in the afferent visual pathway of the affected eye, typically before the optic chiasm. Causes may include optic neuritis, optic nerve ischemia, retinal disorders, or optic nerve compression
afferent puillary defect affecting the right eye would present as
Caused by a problem in the retina or optic nerve of the right eye, for eg.
Pupils equal in size before illumination
Neither pupil responds to stimulation of right eye
Both pupils respond to stimulation of left eye
Both pupils react to near stimulation
what might an efferent pupillary defect affecting the right eye cause
Caused by a problem with the 3rd nerve or sphincter muscle
Right pupil will usually be larger than the left pupil
The right pupil will not respond to either eye being illuminated
The left pupil reacts to illumination from either eye
what are the causes of an anisocoria
Physiological (a difference of 1-2mm, normal reactions)
Horner’s syndrome (smaller pupil, ptosis)
Adie’s pupil (larger pupil, efferent defect)
III n palsy (larger pupil , efferent defect, ptosis, divergent eye)
Drugs
Pilocarpine (makes the pupil smaller)
Atropine (makes the pupil larger)
Iris damage (glaucoma, iritis, surgery, trauma)
what would you see in horners syndrome: oculo- sympathetic paralysis
Physiological (a difference of 1-2mm, normal reactions)
Horner’s syndrome (smaller pupil, ptosis)
Adie’s pupil (larger pupil, efferent defect)
III n palsy (larger pupil , efferent defect, ptosis, divergent eye)
Drugs
Pilocarpine (makes the pupil smaller)
Atropine (makes the pupil larger)
Iris damage (glaucoma, iritis, surgery, trauma)
what is the cocaine test in horners
Cocaine is an indirect sympathomimetic
It prevents the re-uptake of noradrenaline
In a normal eye, cocaine dilates the pupil
In Horners, the pupil fails to dilate as no noradrenaline
what are the courses of horners syndrome
A right sided Horner’s syndrome is caused by a lesion on the right side
Horner’s caused by brainstem and spinal lesions is associated with other neurological problems e.g. brainstem stroke
Apical lung lesions e.g. Pancoast’s tumour, cervical rib
Neck surgery e.g. thyroidectomy
Internal carotid artery dissecting aneurysm
Cavernous sinus and orbital disease
Headache syndromes: cluster headaches, Raeder’s paratrigeminal neuralgia
