Pupil assessment (9)

what is the pupil

an aperture in the iris

regulates retinal illumination 

very small (miotic) in brightly lit conditions and large (mydriatic) in dim illumination 

covered by a membrane up to the 8th month gestation

what 2 muscles does the iris contain

sphincter pupillae

dilator pupillae 

both innervated by thr autonomic nervous system : controlled by the central neural pathways that are influenced by retinal illumination , viewing distance, attention and alertness 

what is the sphincter pupillae

a ring of muscle at pupil margin

what is the dilator pupillae

a radial muscle type fibres running from the sphincter pupillae to the iris root

sphincter muscle

circular muscle

anchored to adjacent stroma and retains its function even if severed

contraction of the sphincter causes pupil to constrict in miosis.

muscle is innervated by the parasympathetic system

Afferent: to the brain - optic nerve → chiasm → optic tract → pretectal nucleus

Efferent: from the brain - ciliary gaglion → 3rd nerve → sphincter pupillae

section of iris: shows sphincter 

dilator: myoepithelium which is muscle and epithelial properies 

Afferent pathway

going from retina down the optic nerve, half goes into the brain on one side, other goes to the brain on the opposite side

fibres of sphincter muscle doesnt go too far into the brain . hits pre tectal nucleus

crosses over- so any light shon on one eye should affect both

efferent pathway

ciliary ganglion from the brain, passes the 3rd nerve then to the sphincter pupillae

dilator - innervation pathway 

dilator pupillae 

innervated by sympathetic system 

arises from the superior cervical ganglion 

lack of stimulation of sphincter causes pupils to get bigger 

starts from the central nervous system in the neck ( cervical ganglion)

both pathways in the eye

pupillary reflec pathway

pupil responses

pupil response- dilation

this is when pupil gets larger 

called dilation/mydriasis 

stimulated by sympathetic nervous system or lack of stim from sphincter 

• associated with low light 

• associated with mydriatic drugs eg tropicamide or phenylphrine , or amy sympathetic NS stimulant

• associated with excitement or fear 

pupipl response- constriction

pupil gets smaller 

miosis 

induced by the parasympatheic action on the sphincter muscle 

associated with bright light 

miotic drugs

what are the 2 types of pupil responses

direct- seen when the light enters the eye

consensual- seen when light falls in the fellow eye

pupils should react as a pair

if shine light in one eye and that pupil constricts its called direct

when shine light in one pupil and look at the other eye, thats consensual

the pupil- the near triad

accomodation - pupil constriction change when looking at distance vs near

convergence

pupil constriction

pupil size 

it is governed by a balance between the sypathetic and parasympathetic input 

methods of assessment of the pupil

done in normal room illumination

px remove glasses

look at a letter on the chart

use a spotlight if they have a vision of less than 6/18 or theyre hyperopic , to avoid stimulating accom

sit in front of the patient, dont block their view

check for size shape and location

checkng pupil size

both pupils should be equal in size

bright light: 3-6 mm in da=iameter

dim light: 4-8mm in diameter

the pupil size will show normal fluctuations known as hippus

pupil size nomally decreases with age

pupil shape 

both pupils should be round  or even slightly oval 

location of pupils

both pupils should be central in the iris

measuring pupil size

can use a ruler or pupil gauge

have ruler on forehead and as close to thr px eye as possible

reduce the room illumination but keep emough light to se the pupil margin

use a UV burton lamp if eyes are dark

do in bright and dark

repeat the pupil measurements

duchek pupil gauge 

peoce or plastic, cardboard with holes in it 

distance between holes get larger 

start from bottom 

hold the gauge as close to the eye as possible 

look through the bottom pair of holes 

the holes appear to overlap through which you can see the distance target 

move the card down. As you lool through the holes that are further apart the images will overlap less 

at some point the images will only just touch ( no black in between them) ,and youll only be able to see the distance target . this is the pupil size 

anisocoria

unequal pupil size in normal room illumination

down to physiology

direct and consensual reflexes

px fixates ahead at the chart or spot of light

shine a pen torch into the right eye (5-10cm)

position the light at the inferior temporal side

watch the right eye for constriction

note down speed and degree of response

this is direct response

do the same but watch the left pupil for constriction : consensual response

may need a burton lamp

repeat with light entering the left eye

observing the direct and consensual dilation

shine the light in the right eye as before 

observe the pupil response in both eyes when the light is removed 

it should be equal 

now repeat shining the light in the left eye 

observe the diation when the light is removed 

should be equal and smooth