Orbit and Ocular Anatomy

•    The orbit contains:

o    Globe (eyeball)

o    Extraocular muscles

o    Optic nerve (CN II)

o    Ophthalmic artery & vein

o    Fat pad (protective)

Extraocular Muscles and Innervation

•    LR6SO4R3 rule:

o    Lateral rectus → CN VI (abducens)

o    Superior oblique → CN IV (trochlear)

o    Remaining (superior, inferior, medial recti, inferior oblique, levator palpebrae) → CN III (oculomotor)

Visual Pathway

•    Retina (photoreceptors → bipolar cells → ganglion cells)

•    Optic nerve → partial decussation at optic chiasm

•    Optic tract

•    Lateral geniculate nucleus (LGN) of thalamus

•    Optic radiations (Meyer’s loop = inferior fibres, through temporal lobe; superior fibres through parietal lobe)

•    Visual cortex (occipital lobe)

Pupillary Reflexes

•    Light reflex:

o    Afferent limb: CN II

o    Efferent limb: CN III (parasympathetic to sphincter pupillae)

•    Accommodation reflex:

o    Convergence (medial recti), lens thickening (ciliary muscle), pupillary constriction

o    Mediated via parasympathetic fibres

Visual Field Defects

Lesion Site                       Defect

Optic nerve                                  Monocular vision loss

Optic chiasm                                Bitemporal hemianopia

Optic tract                                    Homonymous hemianopia

Meyer’s loop (temporal lobe)       Upper quadrantanopia (“pie in the sky”)

Parietal lobe                                 Lower quadrantanopia (“pie on the floor”)

Occipital cortex                            Homonymous hemianopia with macular sparing

Assessment Tools

•    Snellen chart: visual acuity (distance vision; expressed as fraction, e.g., 6/6)

•    Confrontation testing: visual field deficits

•    Fundoscopy: retina, disc, vasculature

•    Slit lamp exam: anterior segment (cornea, lens, iris)

Extra Revision Pearls

•    CN III palsy clue → "down and out" eye, ptosis, dilated pupil (parasympathetic loss)

•    CN IV palsy clue → vertical diplopia, worse looking down stairs (superior oblique dysfunction)

•    CN VI palsy clue → horizontal diplopia, failure to abduct

•    Meyer’s loop clue → temporal lobe epilepsy may cause "pie in the sky" field defect

•    Macular sparing clue → due to dual blood supply (PCA + MCA collaterals) in occipital cortex