Optic Nerve

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.