Basic Sciences Relevant to Ophthalmology
Basic sciences relevant to ophthalmology - anatomy, physiology, pharmacology, microbiology and genetics - is the foundation section. Questions are short and factual, and they reward precise numbers.
Core rules
- Anatomy: the annulus of Zinn gives origin to the four recti; the levator palpebrae superioris arises from the lesser wing of the sphenoid above it, and the superior oblique from the body of the sphenoid. The ophthalmic artery is a branch of the internal carotid; the optic nerve has about 1.2 million axons.
- Physiology: aqueous is produced by the ciliary epithelium at about 2.5 microlitres per minute, flows through the pupil, and drains through the trabecular meshwork and Schlemm canal; the tear film has lipid, aqueous and mucin layers.
- Pharmacology: topical timolol is a non-selective beta blocker; atropine is a competitive muscarinic antagonist causing cycloplegia and mydriasis; phenylephrine is an alpha-1 agonist; latanoprost is a prostaglandin F2-alpha analogue.
- Microbiology: Staphylococcus aureus and Pseudomonas aeruginosa are leading causes of keratitis; Pseudomonas is associated with contact lens wear; herpes simplex latency resides in the trigeminal ganglion.
- Genetics: retinoblastoma follows the two-hit hypothesis with RB1 on chromosome 13q14; autosomal dominant diseases include Marfan syndrome and aniridia; X-linked diseases include choroideraemia and retinoschisis.
Worked example
A couple has a child with unilateral retinoblastoma and no family history. The child's RB1 testing shows a constitutional mutation. If neither parent carries the mutation, it arose de novo and the sibling recurrence risk is low, around 1% from parental mosaicism; the child's own offspring, however, have a 50% risk because the mutation is germline. This is the two-hit model: the first hit is germline and the second is somatic. The family is offered genetic counselling and the parents are tested for mosaicism.
Traps
Confusing the origin of the superior oblique and levator with the annulus of Zinn, and forgetting that atropine systemic absorption can cause anticholinergic toxicity in children.