Myopia - Classification/Types 2/4

A. Etilogical classification:

1. Axial myopia:

• Due to increase antero-posterior (axial) length of eye And others parameters are normal

2. Curvatural myopia:

• Occurs due to increased curvature of the cornea, lens Or both.

3. Positional myopia:

• Due to anterior placement of crystalline lens in the eye.

4. Index myopia:

Due to increase in the refractive index of crystalline Lens associated with nuclear sclerosis.

5. Myopia due to excessive accommodation:

• Occurs in patients with spasm of accommodation.

B. Clinical entity:

1. Simple myopia:

i. The refractive status of the eye with simple myopia is dependent on:

• The optical power of the cornea
• The optical power of the crystalline lens, and
• The axial length.

ii. It’s a  physiological abnormality, not associated with any disease

iii. Simple myopia occurs due when

• Eyes are too long for its optical power, or
• Eyes are too optically powerful for its axial length.

iv. Simple myopia is much more common than the other types of myopia and generally less than 6 diopters

2. Nocturnal myopia

• Nocturnal myopia occurs only in dim illumination, primarily due to increased accommodative response associated with low levels of light.
• Due to insufficient contrast accommodative gets stimulated and light focus in front of retina.

3. Pseudomyopia:

• It occurs due to overstimulation of the eye's accommodative mechanism or ciliary spasm.
• Due to overstimulation accommodaitom the total power of the eye increased and light rays focused in front of retina.

4. Degenerative or pathological myopia

i. A high degree of myopia associated with degenerative changes in the posterior segment of the eye.

ii. The degenerative changes can result in abnormal visual function:

• Decrease in best corrected visual acuity
• Changes in visual fields

5. Induced or acquired myopia:

i. It occurs due to:

• Exposure to various pharmaceutical agents
• Variation in blood sugar levels
• Nuclear sclerosis of the crystalline lens
• Other anomalous conditions

ii. This myopia is often temporary and reversible.

C. Degree of myopia:

1. Low myopia 

• Less then -3D (<3.00 d)

2. Medium myopia 

• -3 to -6D 

3. High myopia 

• More than -6D (>6.00 d)

D. Age of onset:

1. Congenital myopia

• Present at birth and persisting through infancy

2. Youth-onset myopia

• It occurs before 20 years (<20 years of age)

3. Early adult-onset myopia

• It occurs between 20-40 years of age  (20-40 years of age)

4. Late adult-onset myopia

• It occurs after 40 years of age (>40 years of age)

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