Anatomy And Physiology

The optic nerve is the second cranial nerve.
It is a purely sensory nerve responsible for vision and the afferent limb of the pupillary light reflex.
The nerve is formed by the axons of the ganglionic cell layer of the retina.
The optic nerves leave the orbital cavity through the optic foramen.
The right and left nerves unite to form the optic chiasma in the middle cranial fossa.
In the optic chiasma, the nerve fibres from the nasal side of the retina cross to the opposite side.
The nerve fibres from the temporal side of the retina continue on the same side.
Most fibres reach the lateral geniculate body.
A few fibres leave the optic tract to pass to the oculomotor nuclei via the pretectal nucleus and superior colliculus for pupillary reflexes.
From the lateral geniculate body, the fibres pass as the optic radiation.
The optic radiation terminates in the primary visual cortex of the occipital lobe.

Visual Pathway

graph TD
    %% Anatomical Pathway
    R[Retina] --> ON[Optic Nerve]
    ON --> OC[Optic Chiasm]
    OC --> OT[Optic Tract]
    OT --> LGN[Lateral Geniculate Nucleus]
    LGN --> ML[Meyer's Loop - Temporal]
    LGN --> BL[Baum's Loop - Parietal]
    ML --> VC[Visual Cortex]
    BL --> VC

    %% Lesions & Deficits
    L1[Lesion: Optic Nerve] -.->|Ipsilateral Anopsia| ON
    L2[Lesion: Optic Chiasm] -.->|Bitemporal Hemianopsia| OC
    L3[Lesion: Optic Tract] -.->|Contralateral Homonymous Hemianopsia| OT
    L4[Lesion: Meyer's Loop] -.->|Contralateral Superior Quadrantanopsia| ML
    L5[Lesion: Baum's Loop] -.->|Contralateral Inferior Quadrantanopsia| BL
    L6[Lesion: Visual Cortex] -.->|Homonymous Hemianopsia + Macular Sparing| VC

    classDef anatomy fill:#e1f5fe,stroke:#03a9f4,stroke-width:2px,color:#000;
    classDef lesion fill:#ffebee,stroke:#f44336,stroke-width:2px,stroke-dasharray: 5 5,color:#000;
    
    class R,ON,OC,OT,LGN,ML,BL,VC anatomy;
    class L1,L2,L3,L4,L5,L6 lesion;

Assessment From History

Elicit the onset and progression of the vision loss.
Differentiate between sudden non-progressive loss and insidious progressive loss.
Ask about difficulties in reading letters on a blackboard or a television.
Inquire if the child complains of double vision or blurring of words.
Elicit any history of the child bumping into objects or failing to judge the periphery, suggestive of visual field defects.
Ask for associated complaints like redness of eyes, eye discharge, floating spots, or pain upon moving the eyes.

Clinical Examination

Inspection

Examine the external aspects of the eye by looking from the front, side, and above.
Observe the eyebrows, eyelids, cornea, iris, sclera, and conjunctiva.
Look for ptosis, epicanthal folds, and any proptosis.
Note the presence of squint or nystagmus.

Testing Procedure

Gross Vision

Check if the infant recognizes the parents or smiles at them.
Observe if the child exhibits stranger anxiety.
Note if the child recognizes and reaches out for familiar objects.
Test the menace reflex by suddenly bringing a finger towards the eyes without creating an air current.
Normal vision results in blinking.

Visual Acuity

Test each eye separately while occluding the other eye.
In infants, assess the ability to fixate and follow a moving target like a bright toy or human face.
Test distant vision in older children using the Snellen chart placed at a distance of six metres.
Test near vision using Jaeger's chart placed at thirty centimetres.
For severe visual impairment, assess the ability to count fingers.
If unable to count fingers, assess the appreciation of hand movements.
If hand movements are not appreciated, shine a light to assess the perception of light.

Colour Vision

Test the ability to identify basic colours like red, blue, and green.
Use Isochromic charts like Ishihara charts.
Use Holmgren's wools to ask the child to match coloured wools.

Field Of Vision

Perform the confrontation test for children above three years of age.
Sit face-to-face with the child at a distance of one metre.
Ask the child to close one eye while you close your opposite eye.
Ask the child to fixate their gaze on your open eye.
Bring a wiggling finger or a red pin from the periphery to the centre in all four quadrants.
Compare the child's point of perception with your own visual field.
Use automated perimetry for older children above seven years.

Pupillary Reflexes

Perform the examination in a dimly illuminated room.
Ask the patient to fix their gaze on a distant object to prevent the accommodation response.
Shine a bright light into one eye to test the direct light reflex.
Observe the constriction of the pupil in the illuminated eye.
Observe the constriction of the pupil in the opposite eye to test the consensual light reflex.
Perform the swinging flashlight test to detect a Relative Afferent Pupillary Defect (RAPD).
Swing the light rapidly between the two eyes and hold it for three seconds on each side.
Check the accommodation reflex by asking the child to look at a distant object and then quickly focus on a near object.

Fundus Examination

Perform ophthalmoscopy in a dark room.
Use the right eye to examine the child's right eye, and the left eye for the left eye.
Look for the red reflex first.
Examine the optic disc for colour, margins, and the physiological cup.
Look for signs of papilloedema, such as blurred margins, hyperaemia, and loss of venous pulsations.
Look for signs of optic atrophy, where the disc appears chalky white or pale.
Examine the macula for a cherry red spot.

Age-Specific Key Points

Neonates: The newborn turns their head towards a diffuse light source.
Six Weeks: The blink to menace response is present.
Three Months: The eyes converge for finger play.
Four Months: The infant can follow objects through 180 degrees.
Five Months: The infant reaches for a toy within their visual field.
Six Months: The infant moves both eyes together in all directions. Normal visual acuity is approximately 6/30.
Two Years: Normal adult visual acuity of 6/6 is typically achieved.

Example For EXAM

Optic Nerve Assessment:

Visual Acuity: Fixing and following to light present in both eyes independently.
Colour Vision: The child is able to correctly identify primary colours on the Ishihara chart.
Visual Field: Confrontation test reveals normal peripheral visual fields in all four quadrants bilaterally. No hemianopia or quadrantanopia appreciated.
Reflexes: Direct and consensual pupillary light reflexes are brisk and equal bilaterally. Accommodation reflex is present. Swinging flashlight test is negative for RAPD. Menace reflex is intact.
Fundus: Red reflex is present bilaterally. Optic disc margins are well-defined with normal physiological cupping. No evidence of papilloedema, pallor, or retinal haemorrhages. Macula appears normal with no cherry red spot.
Interpretation: Second cranial nerve (Optic nerve) is intact and functioning normally on both sides.

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