Light Reflex Pathway And Defects

Optic Nerve
Optic Chiasma
Optic Tract
Visual Pathway – Optic Tract to LGB to Optic Radiation
Pupillomotor Fibers leaves the optic tract before it relays into LGB. The fibers then go to Pretectal Nuclei (Center for light reflex)
Pretectal Nuclei (Center for light reflex)
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
Edinger Westphal Nucleus (EWN)
3rd Cranial Nerve (Oculomotor nerve)
Ciliary Ganglion
Short Ciliary Nerve
Sphincter Pupillae

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

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“

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

(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

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

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).

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