Monday, February 02, 2009

Causes of Myopic Shift (Acquired Myopia)

By Bradley Dempsey, Medical Student IV
The University of New Mexico School of Medicine
Acquired myopia is an increase in nearsightedness, which in contrast to physiologic (simple) myopia is induced by pathologic processes. The increased myopia may be de novo or may manifest as changes in pre-existing refractive errors, such as an increased myopia or decreased hyperopia. Acquired myopia, like physiologic and congenital forms of myopia, is caused by an error in refraction in which parallel light rays focus in front of the retina; this results in blurred distance vision and clear near vision. Physicians should have a high index of suspicion for underlying pathology in patients that present with a myopic shift. Determining the underlying etiology is essential as many of the causes of acquired myopic are caused by treatable conditions.

In order to evaluate myopic shift, it is useful to consider the anatomic structures that determine the ocular refractive state: the optical power of the cornea, the optical power of the lens, the anterior chamber depth (which determines to the distance between the cornea and lens), and the axial length.

Differential Diagnosis

1. Corneal Power Increase—caused by increased curvature of the cornea:

a. Keratoconus: usually manifests in late puberty, can cause a steepened
cornea. Keratometry, keratoscopy, or corneal topography will demonstrate central thinning of the cornea and irregular rings.

b. Steepened Cornea from contact lens wear (usually transient)

c. Edematous cornea


2. Lens Power Increase—changes in the lens nucleus or shape changes leading to
increased curvature or increased refractive index:

a. Cataracts: an increased density of the lenticular nucleus can cause an increased refractive power of the lens. Patients may notice a newly acquired ability to read up close without glasses, a phenomenon called “second sight”.

b. Acute Hyperglycemia (serum glucose >600 mg/dl): Non-ketotic hyperosmolar
syndrome, presents in diabetics and is usually secondary to stressors such as infection. Undiagnosed diabetics may complain of transient changes in distance vision, presumably caused by changes in lens hydration related to osmotic changes associated with changes in blood glucose levels (myopic increase in lens thickness and intraocular hypotension secondary to hyperosmolarity).

In patients that present with fluctuating distance vision, ask patients about other signs and symptoms of diabetes (polydipsia, polyuria, weight loss). Consider checking blood glucose levels and referring the patient to a primary care physician or endocrinologist if appropriate.

c. Lenticonus

d. Retinopathy of Prematurity (ROP)

3. Lens Repositioning—causing an increased effective lens power:

a. Ciliary muscle shift caused by pre-eclampsia of pregnancy or medications (chlorthalidone, sulfonamides, tetracycline, carbonic anhydrase inhibitors, phenothiazines). It is thought that these medications induce edema of the ciliary body, causing lens repositioning

b. Ciliary muscle spasm, inducing an increased lens curvature: may be secondary to prolonged reading or other near task, leading to excessive accommodation, medications (miotics, alcohol, morphine, antihistamines), inadequate refraction technique, or functional.

c. Lens movement caused by lens dislocation. The lens can be dislocated as a result of trauma, or secondarily from inherited diseases such as Marfan’s syndrome and(autosomal recessive ectopia lentis et pupillae.

4. Changes in Axial Length/Anterior Chamber Depth:

a. Congenital or developmental glaucoma, posterior staphyloma, idiopathic progressive myopia

b. Medications that can cause acute glaucoma may lead to an increased IOP, and thus an increased axial length

c. Scleral buckle surgery for retinal detachment physically changes the axial length leading to a myopic shift.

5. Other:
a. non-physiologic, “functional”

b. error in refractive measurement

References
1. Last Minute Optics, Hunter D, West C, Slack Incorporated,1996.
2. Decision Making in Ophthalmology: an algorithmic approach, van Heuven WAJ, Zwaan J, Second Ed; Mosby Inc., 2000.
3. Basic Ophthalmology for Medical Students and Primary Care Residents, Bradford CA, Seventh Ed, American Academy of Ophthalmology, 1999.
4. Ophthalmic Desk Reference, Collins JF, Raven Press, 1991.
5, Ocular Differential Diagnosis, Roy FH, Seventh Ed, Lippincott Williams and Wilkins, 2002.

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