pupil response 2 - pathway for dilation, investigation, defects and drugs Flashcards
what two things is pupil dilation caused by
- inhibition of the parasympathetic light pathway (for constriction)
- sympathetic stimulation of the dilatator muscle
when does pupil dilation usually occur
in the dark
where do all sympathetic fibres that are destined for the eye originate in
the superior cervical ganglion (the first sympathetic ganglion), which is very close to the spinal chord
list the complete sympathetic innervation that occurs for pupil dilation (8 steps)
- the superior cervical ganglion receives input from the cells in the brainstem which synapse in T1 (the first thoratic segment)
- fibres run from the superior cervical ganglion, up the neck as the internal carotid artery nerves to the level of the cavernous sinus
- at the level of the cavernous sinus, they break up into bundles (plexuses) around the carotid arteries
- the sympathetic route of the ciliary ganglion now runs along with the ophthalmic artery as it branches from the carotid and enters the orbit via the optic foramen and runs along next to the optic nerve
- most of these sympathetic fibres run alongside the optic nerve to the ciliary ganglion
- the sympathetic fibres do not synapse in the ganglion, but pass straight through the ciliary ganglion
- sympathetic fibres then divide into a dozen short ciliary nerves
- these fibres puncture the sclera and choroid to reach their target muscle = the dilatator muscle
list the steps of the marcus gunn test ‘swinging light test’ with a L eye afferent defect in the sensory pathway
- begin in dim light: both eyes should be equally dilated
- illumination of the right eye: normal direct and consensual response i.e. the left eye will constrict too
- but illumination of left eye: results in a lesser response in both the stimulated and unstimulated eye = pupils don’t close down as much
so illuminating one eye, does illuminate the other…
from where in the sensory pathway can there be a pathology, which causes an effect to the afferent pathway
- retina
- optic nerve
- optic tract
what will be affected in the marcus gunn test, if there was a defect in the efferent pathway
only one eye will be affected and would have different pupil sizes in both eyes
e.g. if shine the light in one eyes, the other eye will not react like its supposed to (no consensual reflex)
how can a afferent pupillary defect be quantified
by putting neutral density filters infront of the ‘good eye’ until the response is the same as the one elicited through the ‘bad eye’
so you knock the pupil response down until the good eye is as bad as the bad eye
what can be the causes of afferent pupillary defect, name 4
- optic neuritis
- amblyopia
- macular disease
- retinal detachment
what neutral density filter is used for someone with optic neuritis
1.0 - 1.5 log units
what neutral density filter is used for someone with amblyopia
what is characteristic of an afferent pupillary defect
both eyes are equally affected
a defect to which pathway is an efferent pathway pupillary defect caused
the motor pathway to one eye, so only one eye is affected
what does an efferent pathway pupillary defect cause to the eye
often results in anisocoria (unequal pupil sizes)
however
what pathway is generally affected in the efferent pathway, if one pupil is abnormally larger than the other pupil, and where in this pathway can there be a defect (name 4 places)
the parasympathetic pathway leading to the sphincter muscle may be less effective
the defect can be in:
- the 3rd CN
- or somewhere beyond the ciliary ganglia, if its going to the sphincter muscle
- the midbrain edinger westphal nucleus
- or lesion in sphincter muscle itself
what pathway is generally affected in the efferent pathway, if one pupil is abnormally constricted compared to the other pupil, and where in this pathway can there be a defect
the sympathetic pathway leading to the dilatator muscle may be less impaired
the defect can be anywhere in the sympathetic pathway:
- internal carotid nerves
- nerve going to and through the ciliary ganglion etc
causes efferent defect anywhere in the sympathetic system
name two conditions of a pupillary defect in the efferent parasympathetic pathway i.e. a sign of a pupil that us abnormally large
- adie’s syndrome
- argyll robertson pupil
what is the signs and cause of aide’s syndrome (defect in efferent parasympathetic pathway) where one pupil is abnormally larger
an unresponsive tonic pupil:
due to degeneration of the sphincter, cause of the
degeneration is uncertain, but may be associated with the ciliary ganglion
what is the signs and cause of argyll robertson pupil (defect in efferent parasympathetic pathway) where one pupil is abnormally larger
pupillary light near dissociation, where the pupil is unresponsive to light, but still has a near response so constricts at near (near - far dissociation):
most common lesion is in the midbrain near the edinger westphal nucleus, the disruption of the fibres just before the EWN which disrupts the fibres that come from the retina and not the fibres that are responsible for the near response
characteristic of neurosyphilis (now very uncommon)
what defect causes horner’s syndrome
a pupil defect in the efferent sympathetic pathway, where the sympathetic innervation of the dilatator muscle is compromised
what is the appearance of the effected pupil in horner’s syndrome
pupil of affected eye is not dilated at the other eye
when do the symptoms of horner’s syndrome arise, and what are they
- the problem is in low light levels
- the pupil dilates less and more slowly
- the the light, the pupil can constrict like normal
- the pupil takes a long time to go back to dilated state when turning the light off
where is the lesion in the pupil defect in the efferent sympathetic pathway in horner’s syndrome
it is due to a lesion anywhere in the sympathetic pathway leading to the dilatator
- it can be before or after the superior cervical ganglion
- could be lung cancer affecting the pre-ganglionic fibres
- anything else that is affecting the sympathetic pathway to the dilatator muscle
what 3 other symptoms is accompanied by horner’s syndrome
- ptosis: due to loss of innervation of muller’s muscle which is also sympathetically inverted
- facial anhidrosis: don’t sweat on the affected side
- salivary glands don’t produce much saliva
recap on the stages of the sympathetic innervation
- pre ganglionic fibres synapse with the superior cervical ganglion where they release acetylcholine
- then long post ganglionic fibres work up to the dilatator muscle and here they synapse and release noradrenaline
- noradrenaline binds onto alpha receptors which makes the dilatator contract = pupil dilates
recap on the stages of the parasympathetic innervation
- pre ganglionic fibres synapse in the ciliary ganglion and release acetylcholine
- they go with the shirt ciliary nerves to the sphincter muscle, where they release acetylcholine
- acetylcholine binds onto muscarinic receptors which makes the sphincter muscle contract = pupil constricts
how does pilocarpine cause constriction (miosis)
it acts as a acetylcholine agonist, activating acetylcholine receptors
why is acetylcholine not used as a drug and pilocarpine is instead
acetylcholine can be broken down and pilocarpine cannot be broken down
how do anti muscarinics e.g. atropine and tropicamide result in pupil dilation
they block acetylcholine receptors
which drug enhances acetylcholine levels and hence result in sphincter activation and pupil constriction and how
anticholinesterases e.g. eserine, physostigmine
by inactivating acetylecholinesterase (stops the destruction of acetylcholine)
how does phenylephrine cause pupil dilation
it acts as a direct agonist for nor adrenaline
how does hydroxyamphetamine cause pupil dilation
acts by increasing the release of nor adrenaline by causing more vesicles to be released
how can the dilatator muscle be relaxed, causing pupil constriction
by applying alpha-adrenergic blockers e.g. thymoxamine
how does cocaine cause pupil dilation
it prevents the repute of nor adrenaline, which hangs around for longer
list the three agonists of the parasympathetic nervous system for pupil constriction
- pilocarpine
- metacholine
- physostigmine
list the thee antagonists of the parasympathetic nervous system for pupil constriction
- atropine
- tropicamide
- botulinum A toxin
list the two agonists of the sympathetic nervous system for pupil dilation
- phenylephrine
- hydroxyamphetamin
list the two antagonists of the sympathetic nervous system for pupil dilation
- thymoxamine
- dapiprozole
