Accommodation. Disorders in accommodation Topic 15
=in emmetropic eye , parallel rays coming from infinity are brought to focus on the retina without using accommodation àAccommodation is the process to converge rays coming from near objects so that they can be focused onto the retina and thus be seen clearly
The far point =the furthest distance away at which an object can be seen clearly .
Near point = the nearest point which the eye can see clearly with maximum
Range of accommodation= the distance between far point and near point
This varies in presbyopia
The accommodation reflex is one that include miosis , convergence and accommodation and occurs and is observed when a near object is viewed as the eyes come together , pupils become constrict + lens thickens (to focus image on retina
Anomalies of accommodation
Are not uncommon & include Presbyopia, insufficiency paralysis &spasm of accommodation
- 1. Presbyopiaà not refractive error but physiological insufficiency of accomm.
- Age related ↓ in amplitude of accom. ; lens (hardening, ↓ elasticity &ciliary muscle ↓
- Symptoms à near vision difficulty & asthenopic symptoms
- Treatment: convex glasses in near work or even spectacles
- 2. Insufficiency
- accommodative power decline is not within physiological limits à not due to age
- Sclerosis of lens ; weakness of ciliary muscle due to causes of muscle fatigue illness, anaemia , toxaemia, malnutrition diabetes mellitus , pregnancy and stress
- Association with primary open-angle glaucoma
- Features: similar to 1 but more emphasis on asthenopic symptoms
- Treatment: of cause , near vision spectacle , accom. exercise after cause removal
- 3. Paralysis
- Complete absence of accommodation
- Drug such as atropine, homatropine etc., Ophthalmoplegia (paralysis of muscles within eye e.g. ciliary &sphincter ) caused by neuritis caused by diphtheria ,syphilis, diabetes, alcoholism cerebral and meningeal diseases
- Symptoms : blurring, photophobia as pupil dilates , signs: range of accom.
- Treatment: self-recovery, dark-glasses , convex lenses
- 4. Spasm of accommodations
- Abnormally excessive accommodation leading to exertion
- Causes : 1 )Strong miotics such as echothiophate and DFP
- 2) Spontaneousà refractive anomaly but trying to compensate with accom. Aggravated in dim light, near work, state of neurosis, stress & anxiety
- Defective vision due to induced myopia but Asthenopic symptoms
- Treatment : atropine & no near work , removal of aggravating factors ; psychotherapy