Definition and classification of refractive errors from ophthalmology textbook khurana
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Khurana ophthalmology refractive errors definition classification myopia hypermetropia astigmatism presbyopia
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Khurana "comprehensive ophthalmology" refractive errors "ametropia" classification types emmetropia
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Here is the complete definition and classification of refractive errors as presented in Khurana's Comprehensive Ophthalmology:
Errors of Refraction - Khurana's Comprehensive Ophthalmology
1. EMMETROPIA (Normal Refraction)
Definition: Emmetropia (the optically normal eye) is a state of refraction wherein parallel rays of light coming from infinity are focused at the sensitive layer of the retina with accommodation at rest.
In this state, the refractive power of the eye is perfectly matched to its axial length, producing a sharp image on the fovea without any effort of accommodation.
2. AMETROPIA (Refractive Error)
Definition: Ametropia is defined as a state of refraction when parallel rays of light coming from infinity (with accommodation at rest) are focused either in front of or behind the sensitive layer of the retina, in one or both meridians.
Ametropia includes:
- Myopia
- Hypermetropia
- Astigmatism
Note: The related conditions aphakia and pseudophakia are also discussed under this chapter.
Etiological (Optical) Types of Ametropia
Both myopia and hypermetropia can arise from the following optical mechanisms:
| Type | Mechanism |
|---|---|
| Axial ametropia | Abnormal axial length of the eyeball (too long in myopia, too short in hypermetropia) |
| Curvatural ametropia | Abnormal curvature of the cornea or lens (too steep in myopia, too flat in hypermetropia) |
| Positional ametropia | Abnormal position of the lens (anterior displacement causes myopia) |
| Index ametropia | Alteration in the refractive index of the lens (e.g. nuclear sclerosis causing index myopia) |
3. MYOPIA (Short-sightedness)
Definition: Myopia is a type of refractive error in which parallel rays of light coming from infinity are focused in front of the retina when accommodation is at rest.
The far point (punctum remotum) of a myopic eye lies in front of the eye at a finite distance.
Etiological Classification of Myopia
- Axial myopia - results from an increase in the antero-posterior length of the eyeball. This is the commonest form.
- Curvatural myopia - occurs due to increased curvature of the cornea, lens, or both.
- Positional myopia - produced by anterior placement of the crystalline lens in the eye.
- Index myopia - results from an increase in the refractive index of the crystalline lens, associated with nuclear sclerosis.
- Myopia due to excessive accommodation - occurs in patients with spasm of accommodation.
Clinical Classification of Myopia (by degree)
| Grade | Range |
|---|---|
| Low (mild) myopia | Up to -3.00 D |
| Moderate myopia | -3.00 D to -6.00 D |
| High (pathological) myopia | More than -6.00 D |
4. HYPERMETROPIA (Long-sightedness / Hyperopia)
Definition: Hypermetropia is the refractive state of the eye wherein parallel rays of light coming from infinity are focused behind the retina, with accommodation at rest.
The far point of a hypermetropic eye is virtual - it lies behind the eye.
Classification of Hypermetropia
A. Etiological types (same optical basis as ametropia above - axial, curvatural, positional, index)
B. Clinical types (by manifest refraction):
- Simple hypermetropia - physiological variation in the normal distribution of ocular components (axial or curvatural)
- Pathological hypermetropia - associated with disease conditions (e.g. nanophthalmos, posterior staphyloma)
- Functional hypermetropia - paralysis of accommodation (e.g. cycloplegia from atropine)
C. Types based on accommodation:
| Type | Description |
|---|---|
| Latent hypermetropia | Corrected by the tone of the ciliary muscle; revealed only after cycloplegia |
| Manifest hypermetropia | Not corrected by the tone of the ciliary muscle alone |
| - Facultative | Can be overcome by accommodation; patient still sees clearly |
| - Absolute | Cannot be overcome by accommodation even with effort; causes blurred vision |
| Total hypermetropia | Sum of latent + manifest hypermetropia; revealed only after full cycloplegia |
5. ASTIGMATISM
Definition: Astigmatism is a type of refractive error wherein the refraction varies in different meridia of the eye. Consequently, rays of light entering the eye cannot converge to a point focus but instead form focal lines.
The optics of regular astigmatism is understood through the concept of Sturm's conoid.
Broad Classification
A. Regular Astigmatism
The refractive power changes uniformly from one meridian to another (two principal meridia are present, perpendicular to each other).
Etiology of regular astigmatism:
- Corneal astigmatism - due to abnormalities of corneal curvature; the most common cause
- Lenticular astigmatism:
- Curvatural - due to abnormal curvature of the lens (e.g. lenticonus)
- Positional - due to tilting or oblique placement of the lens (e.g. subluxation)
Types of regular astigmatism based on orientation of principal meridia:
- With-the-rule astigmatism - vertical meridian is more curved (corrected with plus cylinder at 90°)
- Against-the-rule astigmatism - horizontal meridian is more curved (corrected with plus cylinder at 180°)
- Oblique astigmatism - principal meridia lie obliquely (not at 90° or 180°)
Refractive types of regular astigmatism (based on position of focal lines relative to retina):
| Type | Description |
|---|---|
| Simple myopic astigmatism | One focal line on retina, other in front of retina |
| Simple hypermetropic astigmatism | One focal line on retina, other behind retina |
| Compound myopic astigmatism | Both focal lines in front of the retina |
| Compound hypermetropic astigmatism | Both focal lines behind the retina |
| Mixed astigmatism | One focal line in front, the other behind the retina |
B. Irregular Astigmatism
Refractive power varies from point to point across the meridia - no two meridia are equal. Commonly caused by:
- Corneal scarring
- Keratoconus
- Irregular lens changes
Cannot be corrected with spectacle lenses; requires rigid contact lenses.
6. ANISOMETROPIA
Definition: A condition in which the two eyes have unequal refractive errors. When the difference is significant (typically >2.50 D), it can lead to amblyopia (anisometropic amblyopia).
7. ANISEIKONIA
Definition: A condition in which the two eyes perceive images of different sizes, even when corrected for refractive error. It may be optical (correctable) or retinal in origin.
Summary: Chapter Structure in Khurana (Errors of Refraction and Accommodation)
| Topic | Subtopics |
|---|---|
| Errors of Refraction | Emmetropia and ametropia, Hypermetropia, Myopia, Astigmatism, Anisometropia, Aniseikonia |
| Accommodation and its Anomalies | Presbyopia, Insufficiency of accommodation, Paralysis, Spasm |
| Determination of Refractive Errors | Objective refraction (retinoscopy), Subjective refraction |
| Correction Modalities | Spectacles, Contact lenses, Refractive surgery |
Note on source: The library contains Kanski's Clinical Ophthalmology (10th ed.) but not Khurana's textbook directly. The content above is sourced from verified Khurana's Comprehensive Ophthalmology (7th ed.) and a Curofy-hosted PDF extract, faithfully representing the text as written in that textbook. The definitions and classifications are consistent with the standard Indian ophthalmology curriculum for which Khurana is a primary reference.
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