Myopia
Definition
Optical condition of the myopic eye
Classification
Symptoms
Signs of the myopic eye
Risk factors for the development of myopia
Prognosis
Night myopia
Correction
Myopia
■  Definition: Parallel rays of light are focused in front of the retina (when the eye is at rest).
Optical condition of the myopic eye
• Distant objects cannot be seen clearly, only divergent rays will meet at the retina, parallel rays of light are focused in front of the retina.
• The far point (punctum remotum) of the myopic eye - the farthest point, at which objects can be seen distinctly - it is in the finite distance in front of the eye at the optical axis (in the emmetropic eye it is at infinity)
• The higher the myopia - the shorter the distance
•   The near point (punctum proximum) of myopic eye is greatly shifted to the eye
Classification of the myopia
Aetiology:
• Axial myopia - the greatest majority of cases, it is due to an increase in the antero-posterior diameter of the eye
• Curvature myopia - myopia refractiva - may be associated with an increase in the curvature of the cornea or one or both surfaces of the lens _ I
• Myopia combinated
Axial myopia
• The greatest majority of cases
• The normal optical power and an increase in the antero-posterior diameter of the eye
• d > 24 mm, whereas cp = 58,64 D
• The effect of the refraction:
• the change of axial lenght 0,34 mm - ID
Curvature myopia
• Does not appear as much
• The cause - increased refraction of the optical system at its normal length
• I = 24 mm a (p > + 58,64 D
• Classification of curvature myopia:
- index myopia
- radius myopia HOMEWORK
Classification of the myopia
■ Classification according to the degree (according to diopter (optical power))
■ Low myopia (myopia simplex, myopia levis) (< -3,0 D)
■ Medium myopia (myopia modica) (- 3,25 D to - 6,0 D)
■ High myopia (myopia gravis) (> -6,0 D)
■ Severe myopia (> -10,0 D)
In the presence of pathological changes in high myopia - myopia gravis degenerativa (2-3% in general population)
■ These forms of myopia practically do not change with the termination of an individual's growth
■ The prognosis of myopia - it depends upon the age of the patient
|The risk factors for the development of myopia
• Genetic factors:
• Human clinical data also support the genetic bases for myopia. Myopic parents tend to have myopic children in higher proportions than no-myopic parents
• The prevalence of myopia in children with two myopic parents is 30-40 %, 20-25 % in children with only one myopic parent and to less than 10 % in children with no myopic parents.
• Ethnicity (Chinese, Japanese)
• Environmental factors
• Association with near work
• Computers, tablets
• Night lighting
Symptoms of the myopia
• Blurred distance vision x good near vision
• Half shutting of the eyes - greater clarity of stenopaeic vision
Q |
• Advantage: In middle life, when the accomodation fails, presbyopic problems come later (not require spectacles for reading)
The night myopia
A refractive change occurring in reduced illumination, which is very variable in degree, but rarely exceed - 0,5 D, but it can achieve - 4,0 D.
Causes of night myopia:
• Spherical aberration
• Chromatic aberration
• HOMEWORK
• Error in accommodation occuring in dim light
The prognosis of myopia
In simple myopia - the prognosis is good. The error usually does not progress beyond 6-8D and stabilizes by the age of 20 years.
In pathological myopia -the visual prognosis is always guarded. In all cases, the possibility of progressive visual loss due to degenerative changes and danger of complication.
Myopia (Nearsightedness)
Prescription: the lowest possible power of minus lens (concave lens) by which can be accorded VA 6/6 (1,0)
Guidelines for correcting low degrees of myopia up to -6 D
• Low degrees of myopia (up to - 6 D) - full correction Guidelines for correcting high myopia
• High myopia - the full correction can rarely be tolerated Modes of prescribing concave lenses
• Glasses
• Contact lenses
• Next possibilities of correction of the myopia
• Surgery for myopia
• Non-corneal interventions
Negative Lens for Correction
Parallel light rays from a distant object /