Miopia

01Pathophysiology

Myopia occurs when parallel light rays entering the eye at rest converge anterior to the retina rather than directly on it. This creates a blurred retinal image for distant objects while near vision remains intact, because divergent rays from close objects naturally focus further back.

The most common mechanism is axial myopia, where the anteroposterior length of the globe is disproportionately long relative to the refractive power of the cornea and lens. Each additional millimeter of axial length corresponds to roughly 2.5 to 3.0 diopters of myopia. Less commonly, refractive myopia results from excessive curvature of the cornea or an increase in the refractive index of the lens (as seen in nuclear sclerotic cataracts, where the lens becomes denser and bends light more sharply).

The reason a concave (minus) lens corrects the problem is straightforward: a diverging lens shifts the focal point posteriorly, placing it back onto the retinal plane. This is why the prescription is written with a negative sign (e.g., $-3.00D-3.00\backslash \; D$).

In pathologic (degenerative) myopia, typically defined as myopia exceeding $-6.00D-6.00\backslash \; D$ or axial length beyond 26 mm, the progressive stretching of the sclera leads to thinning of the retina and choroid. This mechanical stretch explains the downstream complications: lattice degeneration, posterior staphyloma, lacquer cracks in Bruch's membrane, and choroidal neovascularization. It also increases lifetime risk of retinal detachment and macular hole, both of which are commonly tested associations.

During childhood, visual input plays a regulatory role in eye growth. Prolonged accommodation for near work is hypothesized to stimulate axial elongation through biochemical signaling cascades involving the retina, choroid, and sclera. Conversely, outdoor light exposure is protective, likely through dopamine release from the retina, which inhibits scleral remodeling and axial growth.

02Classification and Clinical Manifestation

Classification by Mechanism

A commonly tested clinical pearl: "second sight" in elderly patients refers to a temporary improvement in near vision caused by nuclear sclerotic cataract increasing the refractive index of the lens, producing an acquired myopic shift. The patient happily discards reading glasses but develops worsening distance vision. This is a classic vignette distractor.

03Diagnostic Workup

The workup begins with a Snellen chart, which establishes the baseline visual acuity deficit. If the patient's distance vision improves with a pinhole occluder, this confirms a refractive error rather than a retinal or optic nerve problem. The pinhole test is a quick bedside maneuver that narrows incoming light to a small pencil beam, eliminating peripheral rays and effectively bypassing the refractive error.

The next step is refraction, either manually with a phoropter or objectively with an autorefractor. In children, cycloplegic refraction is essential because young patients have strong accommodative tone that can mask hyperopia or exaggerate myopia (pseudomyopia). Cycloplegic agents (cyclopentolate 1% or atropine 1%) paralyze the ciliary muscle and reveal the true resting refractive state.

In patients with high or pathologic myopia, a dilated fundus exam is mandatory to evaluate for sight-threatening complications. Lattice degeneration appears as circumferentially oriented, thinned retinal patches with white lines (sclerosed vessels) and is the most common predisposing lesion for rhegmatogenous retinal detachment. If macular pathology is suspected, OCT provides cross-sectional imaging to detect traction, schisis, or neovascular membranes.

04Management and Treatment

Acute considerations: If a highly myopic patient presents with sudden onset of flashes, floaters, and a curtain-like visual field defect, this is a retinal detachment until proven otherwise. The next best step is an emergent dilated fundus exam and urgent ophthalmology referral. Surgical repair (pneumatic retinopexy, scleral buckle, or pars plana vitrectomy) is time-sensitive.

Progression control in children is one of the most testable management concepts. Current evidence supports low-dose atropine (0.01% to 0.05%) as the pharmacologic intervention with the best efficacy-to-side-effect ratio. Higher concentrations (0.5% to 1%) are more effective at slowing elongation but cause photophobia and loss of accommodation, making them poorly tolerated. Treatment is typically continued for at least 2 years, and a rebound effect (accelerated progression) may occur after cessation, particularly with higher doses.

Refractive surgery is reserved for adults with stable refraction for at least 12 months, confirmed by serial refraction measurements. Contraindications to LASIK include keratoconus (or forme fruste keratoconus on topography), residual stromal bed thickness below 250 microns, active ocular surface disease, pregnancy or breastfeeding (hormonal corneal changes), and autoimmune conditions affecting the cornea.

05Differential Diagnosis and Distractors

The most common trap in a vignette is confusing pseudomyopia with true myopia in a pediatric patient. The key discriminator is that pseudomyopia disappears on cycloplegic refraction. Another frequent distractor is the elderly patient with "improving near vision" who actually has a nuclear sclerotic cataract rather than true refractive improvement.

06Traps and High-Yield Pearls

The most common way students lose points on myopia-related questions is by failing to recognize the complications of high myopia when the vignette buries the refractive history in the social or past medical history section. A patient presenting with acute flashes and floaters who "wears thick glasses" or has a refraction of $-9.00D-9.00\backslash \; D$ is at elevated risk for retinal detachment, and the next best step is always an urgent dilated fundus exam, not reassurance or a prescription change.

Another frequent pitfall is the "second sight" vignette: an elderly patient who can suddenly read without glasses. Students may incorrectly attribute this to an improvement in refractive status, but the correct answer is a nuclear sclerotic cataract causing an acquired myopic shift. The lens is getting denser and bending light more, temporarily aiding near focus while degrading distance acuity and contrast.

For pediatric questions, remember that the question may test your knowledge of myopia progression control. Low-dose atropine, orthokeratology, and peripheral defocus lenses are evidence-based interventions. Outdoor time (at least 2 hours per day) is the most commonly cited modifiable environmental protective factor. If the vignette describes a child with rapidly progressing myopia and asks for the best intervention to slow progression, atropine 0.01% to 0.05% nightly is the answer, not simply updating the glasses prescription.

Finally, always recall the lens mnemonic: myopia = minus lens = concave = diverges light. The focal point is in front of the retina, so you need to push it back. Hyperopia is the opposite: plus lens, convex, converges light. Confusing the lens type is a basic but surprisingly effective distractor on board-style questions.