• The orbit contains:
o Globe (eyeball)
o Extraocular muscles
o Optic nerve (CN II)
o Ophthalmic artery & vein
o Fat pad (protective)
• 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)
• 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)
• 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
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
• 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
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Author & Educational Disclaimer
Author:
Dr Phillip Cockrell BM FRCP DipClinEd
Dr Phillip Cockrell is a UK Consultant Physician in Internal Medicine, currently working at Queen Alexandra Hospital, Portsmouth University Hospitals NHS Trust. He has previously worked as a registrar across Intensive Care Medicine, Gastroenterology, Cardiology, Stroke Medicine, Acute Medicine, and Respiratory Medicine.
He has held senior leadership roles including Associate Clinical Director of the Acute Medical Unit, Clinical Director of Internal Medicine, and Chief of Medicine. Dr Cockrell has over 15 years’ experience in postgraduate medical education, having lectured extensively across the MRCP syllabus and contributed to MRCP revision teaching and course development.
Dr Cockrell holds a Bachelor of Medicine (BM), Fellowship of the Royal College of Physicians (FRCP), and a Diploma in Clinical Education (DipClinEd). His teaching approach is based on structured consolidation of complex medical topics to support efficient and effective revision for postgraduate examinations.
Purpose of this content:
The material on this page is intended solely for educational purposes to support revision for the MRCP (UK) Part 1 examination. It reflects examination-relevant principles of internal medicine and is designed to aid learning and pattern recognition.
Medical disclaimer:
This content is designed for postgraduate medical examination revision and does not constitute medical advice, diagnosis, or treatment guidance and must not be used as a substitute for professional clinical judgement, local guidelines, or specialist consultation. Clinical decisions should always be made in the context of individual patient circumstances and current national guidance.
