Day 12 (1): The Pupil Flashcards
What is the pupil?
- normal: round, centrally-located, symmetric
- size depends on ambient illumination
1. constriction: light stimulus; near target
2. dilation: low-light environment
2 muscles:
1. Iris Sphincter
- contraction –> pupil constriction = MIOSIS
- location: 2 - 3 mm from margin
- innervation: Parasympathetic only via CN3
- Iris Dilator
- contraction –> pupil dilation = MYDRIASIS
- location: iris root
- innervation: Sympathetic only via Oculosympathetic system
What is Hippus or Pupillary Unrest?
- NORMAL, rhythmic pupillary oscillation seen with light stimulation
- balance between opposing innervations by the parasympathetic (sphincter) and sympathetic (dilator) systems
What is the Near Reflex?
- three-component SYNKINETIC reflex that assist in the redirection of gaze from a distant to a nearby object
- synkinetic: voluntary muscle movement causes the simultaneous involuntary contraction of other muscles
Components:
1. pupillary constriction: iris sphincter
2. lens accommodation: ciliary muscles
3. convergence: medial rectus
Enumerate the pathway of the Pupillary Light and Near Reflexes.
Common Afferent Pathway:
1. Retina
2. Optic Nerve
3. Optic Chiasm
4. Distal Optic Tract
Light Pathway
1. Retino-Tectal Tract via the Brachium of the Superior Colliculus
2. Pretectal Nucleus: innervation to both sides
3. EW nucleus
Near Pathway
1. Proximal Optic Tract
2. Lateral Geniculate Nucleus
3. Optic Radiations
4. Striate (Primary Visual) Cortex
5. Parastriate Cortex
6. Thalamomesencephalic Junction
7. EW Nucleus + Main Motor Nucleus of III
Common Efferent Pathway
1. Oculomotor nerve –> MR (near only)
2. Ciliary ganglion
3. Short Ciliary nerve –> Iris Sphincter (light and near), Ciliary Muscles (near only)
What are the components of the AFFERENT pathway of the Pupillary Reflex?
Melanopsin system
- light reflex + near reflex: non-image-forming
- vs Rhodopsin system: image-forming (visual)
A. Common Visual and Pupillary Pathway:
1. RGC: intrinsically-photosensitive
2. Optic Nerve
3. Optic Chiasm
4. Anterior/Distal optic tract
*Lesion:
- POOR vision
- POOR light reflex
- GOOD near reflex: synkinetic reflex; converging the eyes also triggers pupillary constriction and lens accommodation even if there is no vision (ask pt to converge eyes towards nose)
*Note: Vision is NOT a prerequisite for accommodation.
B. Divergence of Visual from Pupillary pathway:
4. Posterior/Proximal Optic Tract
–> Tract to the LGN: visual pathway
–> Retino-tectal tract via the Brachium of the Superior Colliculus: pupillary light pathway
*Lesion in the LGN, Optic radiations and Visual cortex:
- POOR vision
- GOOD light reflex
- GOOD near reflex
C. Pupillary Pathway:
5. Brachium –> Superior Colliculus –> Pretectal nucleus
- located in the dorsal midbrain
- supranuclear control center of the light reflex
- part of the pupillary LIGHT pathway BUT NOT the near reflex (Light-Near Dissociation)
*Lesion:
- GOOD vision
- POOR light reflex
- GOOD near reflex
- Edinger-Westphal nuclei
- NEAR reflex: fibers descends directly here from the Thalamomesencephalic Junction which is the supranuclear control center of the near synkinetic reflex
*Lesions:
- GOOD vision
- POOR light reflex
- POOR near reflex
What is the Superior Colliculus?
- paired multilayered structure located in the dorsal midbrain
- non-mammals: optic tectum/lobe
Layer groups:
1. Superficial Layers
- receive direct input from the retina
- respond ONLY to visual stimuli
- Deeper Layers
- respond to other stimuli such as auditory and somatosensory inputs
- connected to many sensorimotor areas
- contain a population of motor-related neurons, capable of activating eye movements
Function:
- direct behavioral responses toward specific points in body-centered space
- each of the two colliculi contains a 2D topographic map representing half of the visual field in retinotopic coordinates
- activation of neurons at a particular point in the map evokes a response directed toward the corresponding point in space
- receives a strong input directly from the retina BUT is largely under the control of the cerebral cortex
What is Light-Near Dissociation?
- differential response of the pupils to light stimulus and near object
+ light reflex: IMPAIRED
+ near reflex: INTACT - more commonly affects the afferent pathway
- may sometimes affect the efferent pathway in chronic cases due to aberrant regeneration
Causes:
1. Afferent Pathway: lesions anterior to LGN
- most common cause
- near reflex is synkinetic: converging the eyes (by ask pt to converge eyes towards nose) also triggers pupillary constriction and lens accommodation even if there is no vision
+ Unilateral: RAPD
+ Bilateral, asymmetric: RAPD
+ Bilateral, symmetric: BAPD
- Dorsal Midbrain:
- disruption of light reflex fibers which pass through the pretectal nucleus with sparing of the near reflex fibers which descend directly to the EW nucleus from the Thalamomesencephalic Junction
+ Dorsal Midbrain Syndrome
+ Argyll-Robertson Pupil
+ Pineal Tumor - Efferent Pathway
- especially at the level of the ciliary ganglion which normally innervates the ciliary muscles and the iris sphincter in a 30:1 nerve fiber ration
- due to aberrant regeneration, the fibers originally intended for the ciliary muscles innervate the pupils instead
- when the near synkinetic reflex is triggered, the stronger innervation goes instead to the iris sphincter causing prominent constriction
+ EWN, CN3: BOTH poor; (+) RRAPD
+ CG, SCN (acute): BOTH poor; (+) RRAPD
+ CG, SCN (chronic): (+) LND
What are the components of the PARASYMPATHETIC EFFERENT pathway of the Pupillary Reflex?
Parasympathetic pathway:
- pupillary constriction or MIOSIS
- Edinger-Westphal nuclei
- Oculomotor nerve (CN 3)
- Ciliary ganglion: within orbital muscle cone between the LR and the ON
- Short Ciliary nerve
- Ciliary Muscles and Iris Sphincter (30:1)
- for every 1 fiber to the iris sphincter, the ciliary muscles receive 30 fibers
What are the components of the SYMPATHETIC EFFERENT pathway of the Pupillary Reflex?
Sympathetic pathway:
- pupillary dilation or MYDRIASIS
- 3-neuron pathway
First-Order Neuron (Pre-ganglionic)
- Hypothalamus –> Midbrain –> Pons –> Medulla –> Ciliospinal Center of Budge (level of C8 - T2)
- at midbrain: close to the Trochlear nucleus
Second-Order Neuron (Pre-ganglionic)
- Ciliospinal Center of Budge –> exits sympathetic trunk –> Lung apex –> loop around Subclavian artery in the Brachial plexus –> Cervical Plexus –> Superior Cervical Ganglion (near the mandibular angle at the bifurcation of the common carotid artery)
Third-Order Neuron (Post-ganglionic)
- Superior Cervical Ganglion –> along with ICA (in its adventitia) –> Skull base –> Cavernous Sinus –> along with Abducens nerve –> transfers to Ophthalmic division of Trigeminal nerve –> SOF via the Nasociliary nerve –>
- Long Ciliary nerve:
- Iris Dilator
- Muller’s muscle: superior lid
- Inferior tarsal muscle: inferior lid - Sudomotor (facial sweating) and Vasomotor (vasodilation) fibers branch off at the Superior Cervical Ganglion
Clinical Correlation:
1. Lesions in the 1st and 2nd order neurons upto the Superior Cervical Ganglion:
(+) facial anhidrosis
- Lesions in the 3rd order neurons from the ICA to the orbit: (+) facial sweating
How is the pupil examined?
Set-up:
- dark room
- bright pen light
- fixating at distance
Parameters:
1. Size (mm)
2. Shape
3. Centration
4. Symmetry: anisocoria (unequal pupil size)
5. Light response: direct and consensual
6. Dilation in the dark
7. Constriction at near
Slit-Lamp Examination
- done if with abnormal findings
1. posterior synechiae
2. signs of uveitis
3. iris tears
4. shape irregularities
5. iris segmental contractions
6. iris transillumination defects
7. pigment dispersion
8. iris tumors
9. lens subluxation
How is the light response of the pupil examined?
- shining light in one eye should cause BOTH pupils to constrict EQUALLY
- Direct Response
- pupillary response in the ILLUMINATED eye - Consensual Response
- pupillary response in the FELLOW eye
What is a Relative Afferent Pupillary Defect?
Marcus-Gunn Pupil
- condition demonstrating a difference in pupillary light reaction between two eyes
- requires two eyes but only one working pupil
- due to defect in the afferent pupillary pathway
- DIRECT RESPONSE: dilation or pupillary escape in the AFFECTED eye “relative” to the normal eye
- tested by the Swinging Flashlight Test
- detected only in:
1. unilateral disease
2. bilateral but asymmetric disease
+ RAPD manifests in the eye with the worse disease
+ if bilateral AND symmetric = Bilateral APD
Causes:
- occurs in lesions affecting the visual pathway in FRONT of the lateral geniculate nucleus
- Optic neuropathy: most common
- most objective clinical sign of optic neuropathy
- if suspect but has (-) RAPD:
+ optic neuropathy RULED OUT
+ optic neuropathy bilateral and symmetric - SEVERE retinal disease: most common
- Maculopathy: trace RAPD only when VA worse than 20/200
- Amblyopia: trace RAPD only when VA worse than 20/200
- Post-chiasmal lesions: contralateral RAPD
- remember that most fibers in the ON decussate to the contralateral tract
- optic tract lesions: (+) vision loss
- tectal RAPD (lesions in the brachium, superior colliculus, pretectal nucleus): (-) vision loss
When is an RAPD not observed?
1. D: no defect
2. P: (+) defect BUT NOT in pupillary pathway
3. A: (+) defect in the pupillary pathway BUT NOT in the afferent side
4. R: (+) defect but no comparison relative to the other eye (bilaterally symmetric disease)
Note:
1. Any problem in FRONT of the retina (cataract, corneal disease, refractive errors, tear film dysfunction) DO NOT produce an RAPD
- ANTERIOR to the pupillary pathway
- there may be vision loss due to poor light transmission but the pupillary light reflex is still intact because the melanopsin system in the retina is still activated by the incoming, albeit disordered, light rays
- Any problem in the LGN and beyond also DO NOT produce an RAPD
- POSTERIOR to the pupillary pathway
- because the pretectal neurons in the retino-tectal tract have already entered the Brachium prior to reaching the LGN
What is the Reverse RAPD?
RAPD by Reverse Testing
- employed when affected eye also has an EFFERENT pupillary defect
+ posterior synechiae
+ trauma
+ oculomotor palsy
+ pharmacological mydriasis
- also utilizes the Swinging Flashlight Test
- CONSENSUAL RESPONSE: dilation or pupillary escape in the NORMAL eye
REMEMBER:
The reverse RAPD is present every time because both pupils dilate when the light swings back to the abnormal pupil but:
1. RAPD: clinician observes the AFFECTED eye for dilation (DIRECT)
2. Reverse RAPD: clinician observed the NORMAL eye for dilation (CONSENSUAL)
What is Anisocoria?
- UNequal pupil size
RAPD vs Anisocoria
- RAPD: AFFERENT pathway problem
- Anisocoria: EFFERENT pathway problem
Differentials:
1. Check pupillary reaction to light
2. Measure pupil size in both bright and dim illumination
A. EQUALLY reactive to light OR anisocoria greater in the DARK
- problem in the SYMPATHETIC (dilation) system
- light: both constricts due to intact parasympathetic system
- dark: normal eye dilates while affected eye remains constricted –> anisocoria
1. Physiologic Anisocoria
2. Horner’s Syndrome
3. Intermittent Pupillary Mydriasis (overactive)
B. NOT equally reactive OR anisocoria greater in LIGHT
- problem in the PARASYMPATHETIC (constriction) system
- dark: both dilates due to intact sympathetic system
- light: normal eye constricts while affected eye remains dilated –> anisocoria
1. Traumatic iris damage
2. Oculomotor nerve palsy
3. Adie’s Tonic Pupil
4. Pharmacologic Mydriasis
5. Intermittent Pupillary Mydriasis (underactive)
6. Argyll-Robertson Pupil
What is Physiologic Anisocoria?
Essential Anisocoria/Normal Benign Anisocoria
- equally reactive (constricts) to light
- anisocoria in light = anisocoria in dark
- in 20% of population
- size difference: < 0.5 mm
- may come and go and may switch sides
Dx:
- confirmed with old photos: establish chronicity
- eye exam normal (VA, motility, lids)
