pupil response 2 - pathway for dilation, investigation, defects and drugs

Question 1

what two things is pupil dilation caused by

Answer 1

• inhibition of the parasympathetic light pathway (for constriction)
• sympathetic stimulation of the dilatator muscle

Question 2

when does pupil dilation usually occur

Answer 2

in the dark

Question 3

where do all sympathetic fibres that are destined for the eye originate in

Answer 3

the superior cervical ganglion (the first sympathetic ganglion), which is very close to the spinal chord

Question 4

list the complete sympathetic innervation that occurs for pupil dilation (8 steps)

Answer 4

1. the superior cervical ganglion receives input from the cells in the brainstem which synapse in T1 (the first thoratic segment)
2. fibres run from the superior cervical ganglion, up the neck as the internal carotid artery nerves to the level of the cavernous sinus
3. at the level of the cavernous sinus, they break up into bundles (plexuses) around the carotid arteries
4. 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
5. most of these sympathetic fibres run alongside the optic nerve to the ciliary ganglion
6. the sympathetic fibres do not synapse in the ganglion, but pass straight through the ciliary ganglion
7. sympathetic fibres then divide into a dozen short ciliary nerves
8. these fibres puncture the sclera and choroid to reach their target muscle = the dilatator muscle

Question 5

list the steps of the marcus gunn test ‘swinging light test’ with a L eye afferent defect in the sensory pathway

Answer 5

1. begin in dim light: both eyes should be equally dilated
2. illumination of the right eye: normal direct and consensual response i.e. the left eye will constrict too
3. 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…

Question 6

from where in the sensory pathway can there be a pathology, which causes an effect to the afferent pathway

Answer 6

• retina
• optic nerve
• optic tract

Question 7

what will be affected in the marcus gunn test, if there was a defect in the efferent pathway

Answer 7

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)

Question 8

how can a afferent pupillary defect be quantified

Answer 8

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

Question 9

what can be the causes of afferent pupillary defect, name 4

Answer 9

• optic neuritis
• amblyopia
• macular disease
• retinal detachment

Question 10

what neutral density filter is used for someone with optic neuritis

Answer 10

1.0 - 1.5 log units

Question 11

what neutral density filter is used for someone with amblyopia

Question 12

what is characteristic of an afferent pupillary defect

Answer 12

both eyes are equally affected

Question 13

a defect to which pathway is an efferent pathway pupillary defect caused

Answer 13

the motor pathway to one eye, so only one eye is affected

Question 14

what does an efferent pathway pupillary defect cause to the eye

Answer 14

often results in anisocoria (unequal pupil sizes)

however

Question 15

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)

Answer 15

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

Question 16

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

Answer 16

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

Question 17

name two conditions of a pupillary defect in the efferent parasympathetic pathway i.e. a sign of a pupil that us abnormally large

Answer 17

• adie’s syndrome

- argyll robertson pupil

Question 18

what is the signs and cause of aide’s syndrome (defect in efferent parasympathetic pathway) where one pupil is abnormally larger

Answer 18

an unresponsive tonic pupil:

due to degeneration of the sphincter, cause of the
degeneration is uncertain, but may be associated with the ciliary ganglion

Question 19

what is the signs and cause of argyll robertson pupil (defect in efferent parasympathetic pathway) where one pupil is abnormally larger

Answer 19

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)

Question 20

what defect causes horner’s syndrome

Answer 20

a pupil defect in the efferent sympathetic pathway, where the sympathetic innervation of the dilatator muscle is compromised

Question 21

what is the appearance of the effected pupil in horner’s syndrome

Answer 21

pupil of affected eye is not dilated at the other eye

Question 22

when do the symptoms of horner’s syndrome arise, and what are they

Answer 22

• 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

Question 23

where is the lesion in the pupil defect in the efferent sympathetic pathway in horner’s syndrome

Answer 23

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

Question 24

what 3 other symptoms is accompanied by horner’s syndrome

Answer 24

• 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

Question 25

recap on the stages of the sympathetic innervation

Answer 25

• 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

Question 26

recap on the stages of the parasympathetic innervation

Answer 26

• 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

Question 27

how does pilocarpine cause constriction (miosis)

Answer 27

it acts as a acetylcholine agonist, activating acetylcholine receptors

Question 28

why is acetylcholine not used as a drug and pilocarpine is instead

Answer 28

acetylcholine can be broken down and pilocarpine cannot be broken down

Question 29

how do anti muscarinics e.g. atropine and tropicamide result in pupil dilation

Answer 29

they block acetylcholine receptors

Question 30

which drug enhances acetylcholine levels and hence result in sphincter activation and pupil constriction and how

Answer 30

anticholinesterases e.g. eserine, physostigmine

by inactivating acetylecholinesterase (stops the destruction of acetylcholine)

Question 31

how does phenylephrine cause pupil dilation

Answer 31

it acts as a direct agonist for nor adrenaline

Question 32

how does hydroxyamphetamine cause pupil dilation

Answer 32

acts by increasing the release of nor adrenaline by causing more vesicles to be released

Question 33

how can the dilatator muscle be relaxed, causing pupil constriction

Answer 33

by applying alpha-adrenergic blockers e.g. thymoxamine

Question 34

how does cocaine cause pupil dilation

Answer 34

it prevents the repute of nor adrenaline, which hangs around for longer

Question 35

list the three agonists of the parasympathetic nervous system for pupil constriction

Answer 35

• pilocarpine
• metacholine
• physostigmine

Question 36

list the thee antagonists of the parasympathetic nervous system for pupil constriction

Answer 36

• atropine
• tropicamide
• botulinum A toxin

Question 37

list the two agonists of the sympathetic nervous system for pupil dilation

Answer 37

• phenylephrine

- hydroxyamphetamin

Question 38

list the two antagonists of the sympathetic nervous system for pupil dilation

Answer 38

• thymoxamine

- dapiprozole