Light Reflex Pathway And Defects

Light reflex pathway

• Parasympathetic pathway
• Location – Midbrain at the level of superior colliculus

Pathway

1. Optic Nerve
2. Optic Chiasma
3. Optic Tract
4. Visual Pathway – Optic Tract to LGB to Optic Radiation
5. Pupillomotor Fibers leaves the optic tract before it relays into LGB. The fibers then go to Pretectal Nuclei (Center for light reflex)
6. Pretectal Nuclei (Center for light reflex)
7. Interneurons/ Internuncial neuron – One half curve around periaqueductal gray and go to ipsilateral Edinger Westphal Nucleus (EWN). The other half go through posterior commissure to contralateral EWN
8. Edinger Westphal Nucleus (EWN)
9. 3rd Cranial Nerve (Oculomotor nerve)
10. Ciliary Ganglion
11. Short Ciliary Nerve
12. Sphincter Pupillae

Defects

• Afferent Fibers – Optic Nerve II CN – Sensory
• Efferent Fibers – Oculomotor nerve III CN – Motor

AAPD – ABSOLUTE AFFERENT PATHWAY DEFECTS
TAPD – TOTAL AFFERENT PATHWAY DEFECTS
RAPD – RELATIVE AFFERENT PATHWAY DFECT

Amaurotic pupil/TAPD/AAPD

• Site – Indicates lesion of Optic nerve / Retina on the affected side
• Absence of Direct light reflex ( DLR) on affected side
• Absence of Consensual light reflex (CLR) on other eye
• In diffuse light both pupils are of equal size

Features

• Ipsilateral Anopia( blindness) + (-) DLR + (-) CLR
• Near Reflex is present due to normal eye (Consensual)
• Isocoria (Same size of pupil)

Marcus Gunn pupil/RAPD

• Site – Occurs due to partial optic nerve lesion
• Incomplete Optic nerve lesion/ Severe retinal diseases
• Causes –
  • Optic neuritis
  • Retinal detachment
  • Central retinal artery/vein occlusion/CRAO/CRVO
  • Anterior ischemic optic neuropathy (AION)
  • Primary open angle glaucoma
• It is the earliest sign of optic nerve disease, even in the presence of normal visual acuity in the affected side.

[1. On presentation both pupils are contracted (Isocoria). 2. Light on the left eye in dark room 3. Light on the right eye in dark room (RAPD Eye)]

Swinging Flashlight Test

• This is a flashlight test for RAPD
• Leads to earlier fatigue of the partially lesioned optic nerve (Right eye).
• So, on drawing light from the normal side (left), for a brief moment as both pupil are in the dark, they start to dilate.
• On reaching the right lesioned side, due to fatigue, impulse for constriction is weaker than impulse of already occurring dilation.
• So, both pupils are seen to Dilated
• This phenomenon is known as “Paradoxical dilation/ Pupillary escape phenomenon“

Wernicke’s Hemianopic pupil

• Site – Proximal optic tract
• Causes –
  • Tubercular meningitis
  • Syphilitic meningitis
  • Cerebral artery Aneurysm

Example

(In case of Right optic tract lesion)

• If Light thrown to the temporal half of the retina of the affected side (side of visual loss) and nasal half of the retina of opposite side eye, then I/L Direct and C/L Consensual light reflex will be absent
• If Light thrown to the nasal half of the retina of the affected side (where the vision is present) and temporal half of the retina of opposite side eye, then I/L Direct and C/L Consensual light reflex will be present
• Visual field defect – Left hemianopia

Argyll Robertson Pupil (ARP)

• Site – Rostral midbrain (Aqueductal grey matter/sylvius) or Posterior commissure
• Cause –
  • Neurosyphilis (tertiary syphilis)(tectum lesion)
  • Tabes dorsalis
  • Aortic regurgitation
  • Patient’s Complain – Unprotected intercourse way back + Neurological symptoms ⇒ Neuro syphilis.
  • It is bilateral presentation
  • Pupil do not react to light
  • Near reflex is present

Mnemonic: ARP PRA

• ACCOMMODATION REFLEX PRESENT
• PUPILLARY LIGHT REFLEX ABSENT

Important Points

• Pupils do not constrict with pilocarpine and dilate poorly with atropine.
• Pupils are always seen constricted, as lesion in the Rostral midbrain also damages the supranuclear inhibitory fibers to EWN. EWN keeps on releasing , parasympathetic impulse to sphincter pupillae. Thus, pupils remain constricted, and do not dilate in dark.
• No dilation lag is seen in ARP (Horner syndrome).

Tonic Pupil/ Adie’s Pupil

• Site – Parasympathetic pupillomotor damage (post ganglionic), Ciliary ganglion, Short ciliary nerve
• Causes –
  • Orbital trauma
  • Herpes zoster virus ganglionic (Herpes virus dormant in ganglion)
  • Diabetes (Pan peripheral autonomic neuropathy)
  • Alcoholism

HOLMES ADIE’S PUPIL

• Tonic pupil + Diminished deep tendon reflex
• Absent knee jerk
• Absent light reflex and slow near reflex

Important Points

• Near reflex is present
• Absent light reflex or show “vermiform movement contraction“
• Anisocoria – Affected pupil is larger
• Pupil constricts with 0.125% pilocarpine (Cholinergic super sensitivity)

Example

(If lesion in Right Ciliary ganglion)

On Presentation

• Right I/L pupil – Dilated
• Left C/L pupil – Constricted
• Anisocoria

Light on Left eye

• Right C/L pupil – Do not Constricts. Consensual absent
• Left I/L – Constricts

Light on Right eye

• Right I/L pupil – Do not constricts. DLR absent
• Left C/L – Constricts. Consensual present.

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