Astigmatism and Presbyopia Flashcards
When parallel rays of light enter the eye (with accommodation relaxed) and do not come to a single point focus on or near the retina.
Astigmatism
The sum of corneal astigmatism and residual astigmatism
Total Astigmatism
When the two principal meridians are perpendicular to each other
Regular Astigmatism
When the two principal meridians are not perpendicular to each other
Irregular Astigmatism
When both principal meridians are focused either in front or behind the retina (with accommodation relaxed)
Compound Astigmatism
The sum of the two axes of the two eyes equals approximately 180
Symmetrical Astigmatism
The sum of the two axes of the two eyes does not equal approximately 180
Assymetrical Astigmatism
Clinical Tests
Visual acuity tests – distance and near Autorefraction Keratometry Retinoscopy Monocular subjective refraction, including Jackson cross cylinder
in which the cornea exhibits a variation of curvature throughout different meridians.
Corneal Astigmatism
in which two principal meridians exist at right angles to each other, one of the greatest and one of the least curvature.
Regular Astigmatism
in which either the two principal axis are not at right angles to each other or the curvature of any one meridian is not uniform.
Irreggular Astigmatism
both eyes are with-the-rule or both are against-the-rule
Homonymous
one eye is with-the-rule and the other is against-the-rule
Heteronymous
in which the astigmatism is due to unequal curvature of the lens surfaces or layers
Lenticlar Astigmatism
How is astigmatism diagnosed?
Visual Acuity
Keratometry
Refraction
Attributes all of the loss in acc. to biomechanical changes in the lens capsule & lens and none to the ciliary muscle
H-H-G Theory
The amount of ciliary muscle contraction needed to produce a unit change in acc. progressively increases with age. Thus as one ages, the reduced amplitude is due to progressive weakening of the ciliary muscle itself.
D-D-F Theory
Causes of sclerosis
Nutritional changes
Action of UV rays
Exposure to intense infrared radiation
adult patients eventually report visual difficulties when faced with gradually declining accommodative amplitude and near task demands.
Functional Presbyopia
the condition in which virtually no accommodative ability remains.
Absolute Presbyopia
the earliest stage at which symptoms or clinical findings document the near vision effects of the condition
Incipient Presbyopia
Condition which can still be overcome by a hard or forced ciliary effort.
Facltative Presbyopia
accommodative ability becomes insufficient for the patient’s usual near vision tasks at an earlier age than expected.
Pre-mature Presbyopia
the condition in which near vision difficulties result from an apparent decrease in the AA in dim light.
Nocturnal Presbyopia
is the maximum increase in optical power that an eye can achieve in adjusting its focus from as far as possible (beyond infinity for a longsighted eye) to the nearest possible.
Amplitude of Accommodation
Corrections for presbyopia with the use of plus lens or surgery
Bifocals Reading glasses Progressive Addition lenses (PAL) Multifocal CL Surgical Reversal of presbyopia with Scleral Expansion Bands Laser Thermal Keratoplasty
