Visual Pathway😊

Neuro-Ophthalmology

Visual Pathway

• Wernicke’s hemianopic pupil → Optic tract

• Meyer’s Loop
• Mayer in Temple

• Baum’s Loop
• Bum → Parietal bum

Retina:

• Covers the posterior part of the eye till ora serrata.

• Deficient anteriorly.

Field Loss Principle:

• Field loss is always opposite to the loss of fibers.
• Example: Temporal fibers injured → nasal field loss.

Pathway Components:

• Retina → Optic Nerve (ON) → Optic Chiasm (OC) → Optic Tract (OT) → Lateral Geniculate Body (LGB) → Optic Radiations (OR) → Visual Cortex (VC).

Visual Cortex

Fiber Types:

• Nasal visual field fibers → Temporal fibers.

• Temporal visual field fibers → Nasal fibers.

Visual Field Defects (VFD)

1. Optic Nerve Lesion:

• Affects ipsilateral temporal and ipsilateral nasal fibers.
• Result: Ipsilateral anopia (blindness).

2. Optic Chiasma Lesion:

• Affects bilateral nasal fibers.
• Result: Bitemporal/heteronymous hemianopia.
• Most common cause: Meningioma.

Causes for Bitemporal Hemianopia:

• Mnemonic: 3PC & AC
1. 3rd ventricular CSF blockage
• ventricular dilatation
• optic chiasma compression.
2. Pituitary macroadenoma (>10 mm/1 cm).
3. Craniopharyngioma
4. Anterior communicating artery aneurysm.
5. Cavernous sinus thrombosis

3. Junction of ON & OC (Proximal ON) Lesion:

 

• Affects anterior knee of von Willebrand.
• Inferonasal fibres
• Result: Junctional scotoma.

4. Lateral Optic Chiasma Lesion:

• Affects bilateral temporal fibers (rare, always bilateral).
• Result: Binasal hemianopia.

Contralateral Homonymous Hemianopia Causes:

5. Optic Tract (OT) lesion.

6. LGB lesion.

7. Optic Radiations (OR) lesion.

Inferior Optic Radiations (Meyer’s loop) Lesion:

• Causes contralateral homonymous superior quadrantanopia.

• Also known as “Pie in the sky.”

• Mnemonic: TIPS (Temporal inferior pie in the sky).

9. Superior Optic Radiations (Baum’s loop) Lesion:

• Causes contralateral homonymous inferior quadrantanopia.

• “Pie on the floor.”

10. Occipital Lobe Lesions:

a. Tip of Cortex Lesion:

• MCA affected

• Affects macular representation.
• Result: 
• Contralateral homonymous macular defect.

b. Visual Cortex Except Tip Lesion:

• PCA affected

• Affects ipsilateral temporal and contralateral nasal fibers (macular spared).
• Result: 
• Contralateral homonymous hemianopia with macular sparing.

Differences in Homonymous Hemianopia Lesions:

• Congruity increases as the lesion moves posteriorly in the visual pathway.

• Optic Tract → Teacher from reailway track
• Student becomes abnormal (Pupil abnormal)
• Nashich povum (Atrophy)

• Optic radiation → Prakasham parathunna teacher
• from Congress (Congruous)
• Pupil become normal (Normal pupil)
• Nashichu povilla (No atrphy)

• LGB → Computer sir
• Student will be normal (Pupil normal)
• But life will be distroyed (Atrophy)

Pupillary Reflex

• Mechanism: Light causes pupil to constrict or dilate.

• Types: Direct and consensual.

Lesions of Pupillary Light Reflex

• Pupillary Pathway:
• Afferent Pathway (Ipsilateral)
• Optic nerve
• Optic tract (pupillomotor fibers exit before LGB)
• Pretectal nucleus (center of light reflex)
• Efferent Pathway (Bilateral)
• Bilateral Edinger-Westphal Nucleus (EWN, accessory CN III nucleus) →
• Bilateral CN III (inferior fibers) → Bilateral ciliary ganglion → Bilateral short ciliary nerve → Bilateral sphincter pupillae constriction.
• Note:
• Optic nerve, optic chiasm, and optic tract are common to both pupillary light reflex and visual pathways.

• Synapse #1 → Pretectal

• Synapse #2 → EWN

• Synapse #3 → Ciliary ganglion

• Decussation #1 → Chiasma

• Decussation #2 → Posterior Commissure

• Short ciliary N → Postganglionic Parasympathetic fibres → Myelinated
• Light fibres → 3 - 5 %
• Near fibres → 95 %
• So atropine → cycloplegic + Mydriatic

• All Parasympathetic comes from NCC. Eg
• Ciliaris muscle
• Short ciliary N

Afferent Pathway Lesions:

• Mnemonic:
• Wrap koduth → Wernickes, RAPD, AAPD
• ATP kitti → ARP, tonic, Pharmacological

Absolute/Total Afferent Pathway Defect (AAPD):

• ON lesion

• Findings: Isocoria (pupils equal in size).

• Light on normal eye: Bilateral pupils constrict.

• Light on lesion side: Bilateral pupils do not constrict.

• Note: Wernickes pupil → optic tract lesion

Relative Afferent Pathway Defect (RAPD)/Marcus Gunn Pupil:

• ON lesion on Swinging flash light test

• Findings: Partial defect, nerve fatigue, paradoxical pupillary dilatation.

• Light on normal eye: Bilateral pupils constrict.

• Light on lesion side: Bilateral pupils dilate

• Optic Neuritis

Efferent Pathway Lesions:

Muscles of Iris

• Sphincter Pupillae:
• Function: Miosis (constriction).
• Nerve Supply: Parasympathetic via CN III (inferior branch of inferior oblique) → short ciliary nerve.

• Dilator Pupillae:
• Function: Mydriasis (dilatation).
• Nerve Supply: Sympathetic via hypothalamus.

Causes of Small Pupils (Miosis)

• Pontine hemorrhage

• Horner’s syndrome

• Old age

• Argyll Robertson pupil

• Drugs / poisons
• Opiates
• Organophosphates

Causes of Dilated Pupils (Mydriasis)

• Holmes–Adie (myotonic) pupil

• Third nerve palsy

• Atropine

• Carbon monoxide

• Ethylene glycol

False Localizing Sign – Ipsilateral Blown Pupil

• Uncal herniation:
• Herniation of uncus (medial temporal lobe)

• Progression:
• Affect Parasympathetic fibers of ipsilateral cranial nerve III
• Called "ipsilateral blown pupil" or "false localizing sign"
• C/L corticospinal tract/crus cerebri compressed
• I/L UMN palsy

• Kernohan’s notch phenomenon:
• False localizing sign
• Hemiparesis appears ipsilateral to lesion
• instead of expected contralateral

Key signs:

• Ipsilateral pupil dilatation → Hutchinson pupil

• Ipsilateral UMN palsy

Horner’s Syndrome/Oculosympathetic Palsy

Definition:

• Paralysis of sympathetic pathway of dilator pupillae.

Course and Lesions of Sympathetic Supply:

• A lesion at any point leads to oculosympathetic palsy.
• First Order Neuron (Central):
• Course: Hypothalamus → ciliospinal bulge (C2-T1).
• Causes:
• Brain stem lesions (tumor)
• syringomyelia
• diabetic autonomic neuropathy
• Wallenberg syndrome
• Second Order Neuron (Pre-ganglionic):
• Course: Ciliospinal bulge → superior cervical ganglion in neck.
• Causes:
• Pancoast tumor
• carotid and aortic aneurysm
• neck lesions.
• Third Order Neuron (Post-ganglionic):
• Course: Ascends to cavernous sinus → supplies pupillae via long ciliary nerve.
• Causes:
• Cluster headache,
• internal carotid artery dissection,
• otitis media,
• cavernous sinus mass.

Clinical Features:

• Mnemonic: HIMAPLE

• Heterochromia iridis (hypochromic affected eye; congenital cases).

• Inferior eyelid elevation.

• Miosis (dilator pupillae paralysis → unopposed sphincter).

• Anhydrosis (ipsilateral absence of sweating).

• Ptosis (upper eyelid drooping due to Muller’s muscle paralysis).

• Loss of ciliospinal reflex.

• Enophthalmos (sunken eyes).

Diagnosis:

1. Identification:

• 4% Cocaine Test:
• ⛔ reuptake of NE
• Normal pupil dilates (↑NE);
• Horner’s pupil does not dilate (no NE).
• Coconut on head → ↑ Adrenaline (NE)

• 1% Apraclonidine Test:
• Mainly α2 agonist
• Weak α1 agonist (In eye)
• In normal eye
• Minimal or no pupillary change
• In Horner eye
• Denervation hypersensitivity of α1 receptors
• Marked mydriasis

2. Localisation:

• 1% Amphetamine Test:
• ↑ Synaptic release of
• Dopamine (main effect)
• NE
• Pre-ganglionic lesion → Pupil dilates
• Post-ganglionic lesion → does not dilate

• IF Horny
• Aprath (Apraclo 1) poi give
• Cocaine (4 Cocaine) + Amphetmaine (1 Amphe) kodukkum

Optic Atrophy

• Definition:
• Optic neuropathy
• Progressive, characteristic optic disc changes
• Irreversible visual field defects
• +/- raised IOP

Types and Features

• Optic Atrophy
• Primarily bad → Psycho
• He is well dressed (), white () shirt
• Secondarily bad → Thadich alambayi
• He is obese (OD swelling)
• Ill looking () & Dirty shirt ()
• He has disease → ↓ Blood in Retina
• Appear Pallor (Waxy pallor)

Altitudinal defects:

Definition

• Loss of f or lower half of visual field.

• Respects the horizontal meridian.

• Inferior altitudinal defect > Superior

Causes

• Advanced Glaucoma

• Optic nerve head infarction

Optic Neuritis

• Definition: Inflammation of the optic nerve.

• Cause: Most common cause is Multiple Sclerosis.

Clinical Features:

• Mnemonic: 2 CUP mar

• Central scotoma.

• Color blindness (red & green).

• Unilateral loss of vision.

• Pain in eye on ocular movements.

• Marcus Gunn pupil (RAPD).

• Uhthoff sign:
• Worsening of symptoms with increased body temperature (e.g., exercise).
• Mnemonic: Uhthoff → Utto → Up → Temperature

• Pulfrich sign:
• Tachtokinetic dissociation.
• Illusion of depth perception
• Mnemonic: Pullil thotta ariyulla

• Lhermitten’s sign
• Passive neck flexion causing an electric shock-like sensation radiating to the spine and shoulders
• Due to increased sensitivity of the myelin to stretch

Investigation

• MRI

• LP
• NMO (Neuromyelitis optica antibodies → Anti Aquaporin 4 antibody)
• Nemo (NMO) fish → aquatic (Anti Aquaporin 4)
• MOG (Myelin Oligocyte Glycoprotein) antibodies

Treatment:

• Steroids (IV →f/b oral)

• f/b β Interferons

• Glatiramer acetate : 30 mg s/c injection.

Mnemonic:

• Vinnu → calls myre (myelin basic protein), pulle (pulfrich), then spits - thphuuu (uhthoff)

• Olipich (oligodendrocytes) nadakkunavan vannu

• she took gun (marcus gunn) → fire to his eyes

• he got eye pain (pain on movement)

• Kandavar scoot ayi (scotoma)

NEUROMYELITIS OPTICA / DEVIC'S DISEASE

• https://www.notion.so/Neuro-3-2350a7eb0c778023ac7dc562d86f9426?source=copy_link#2350a7eb0c778068b7efe6f96af88a61

NMOSD (Neuromyelitis Optica Spectrum Disorder)
DEVIC'S DISEASE

• DEVIC'S DISEASE

• 20 -40 yrs

• Female: Male ratio = 3:1.

• Demyelinating disease.

• Disease is Astrocytopathy
• Damage to astrocyte foot processes causes cerebral edema and death.
• Astrocyte foot processes: 
• Do not form blood brain barrier.
• BBB → Formed by endothelium of Brain capillaries

Cause

• Can be secondary (2°) to Multiple Sclerosis

Death

• Spinal cord involvement at C3, C4, C5

• Phrenic nerve supply to diaphragm is affected.
• Causes respiratory failure.

• NOTE: In many neurological diseases
• parkinsonism, Alzheimer’s, VCJD
• death is often due to pneumonia from being bedridden and aspirating.

Antibodies:

• NMO IgG antibodies / Anti-Aquaporin-4 antibody
• Damages water channel,
• Causes cerebral edema

• Anti-MOG
• Myelin oligodendrocyte glycoprotein antibody.

Manifestation:

1. Optic neuritis
• Sudden onset blindness
• bilateral
• Increased (↑) chance of relapse
• More frequent than in Multiple Sclerosis alone
• History of recurrent Optic Neuritis
• most important to differentiate from MS

2. Longitudinal extensive transverse myelitis:
• >3 consecutive spinal segments.
• Symptoms:
• Pin, needle sensations,
• Root pain/radicular pain below level.
• Acid or boiling pain

3. Spectrum disorder manifestations:
• A: Area postrema syndrome (protracted vomiting).
• B: Brainstem syndrome (3rd cranial nerve or any other nerve palsy).
• C: Cerebral syndrome (seizures, encephalopathy).
• D: Diencephalic syndrome (hypothalamus affected)
• Hunger affected → Anorexia, Weightless
• N: Narcolepsy (damage to Reticular activating system).

NOTE

• Multiple sclerosis:
• Relapsing/remitting course,
• mostly unilateral ocular deficit.

Diagnosis criteria for NMOSD

• At least
• 1 clinical core feature + Anti aquaporin 4 antibody.

• If antibody not identified:
• 2 clinical core features + Gadolinium enhanced MRI.

• MRI:
• Optic neuritis.
• MRI spine: >3 segments involved.
• Cloud like brain lesions (parenchyma or brain stem).

Treatment:

• β-Interferon, Glatiramer: Contraindicated.

• Methylprednisolone.

• No improvement: Plasmapheresis.

Prevention:

• Immunomodulators +
• Mycophenolate, Azathioprine

• Steroids

Important Information

• Therapeutic Plasmapheresis indicated in:
• HUS.
• TTP.
• NMOSD.
• GBS

Papilledema

• Definition: Swelling of optic disc due to ↑↑ ICP

• ↑↑ ICP → Blurring of disc margin → Transient vision loss / Visual field defect → Paton’s lines

• Amaurosis fugax (transient vision loss)

• Signs:
• Blurring of disc margins (earliest sign)
• Enlargement of blind spot (visual field defect)
• Paton’s lines (retinochoroidal folds)
• Edema of optic nerve head
• Blockage of axoplasmic flow
• Axonal swelling
• Collection of extracellular fluid
• Painless progressive loss of vision
• Color vision, visual acuity, and pupillary reaction are normal,
• until optic atrophy sets in.
• Paton → Papilledema

• Treatment:
• Control ICP

• NOTE:
• LP C/I when Papilledema present

Myelinated Nerve Fibres / Medullated Nerve fibres

• Normally, optic nerve myelination stops at
• Lamina cribrosa.

• If myelination continues beyond optic disc
• Leads to the expansion of the blind spot.

• Ophthalmoscope
• Whitish patch with feathery margins.

• Description: Myelination of optic disc and retinal nerve fibers.

• Effect: 
• No loss of vision;
• Increases speed of conduction.