OPTICS . . . CONT.

REFRACTION

technique of determining the lenses needed to correct the optical defects of the eye (ametropia)

two techniques used

1)Flash/Streak Retinoscopy

•refractive error determined objectively by use of retinoscope and lenses

2)Manifest (Acceptance)

•subjective trial of lenses used to refine retinoscopy findings

•a typical lens prescription would contain

•sphere power in D (diopters)

•cylinder power in D to correct astigmatism

•axis of cylinder (in degrees)

•add (reading lens) for presbyopes

•e.g. –1.50 + 1.00 x 120 degrees, add +2.00

REFRACTIVE EYE SURGERY

permanently altering the refractive properties of the cornea

used for correction of myopia, hyperopia, and astigmatism

most commonly using excimer laser system

ablation of the corneal surface - photorefractive keratectomy (PRK)

ablation of the stroma below a flap of corneal tissue (LASIK)

THE ORBIT

EXOPHTHALMOS (PROPTOSIS)

eyeball protrusion

Etiology

hyperthyroidism: Graves' disease (unilateral or bilateral, the most common cause in adults)

orbital cellulitis (unilateral, most common cause in children)

primary or secondary orbital tumours

orbital/retrobulbar hemorrhage

cavernous sinus thrombosis or fistula

sinus mucoceles

Diagnosis

exophthalmometer (Hertel): measure at lateral canthi

CT head

ENOPHTHALMOS

retracted globe

often due to "blow-out" fracture (see Ocular Trauma section)

PRESEPTAL CELLULITIS

infection of soft tissue anterior to orbital septum

Etiology

stye, chalazion, acute meibomitis

insect bite

secondary to lid laceration or lacrimal system inflammation

Symptoms and Signs

tender and erythematous lids

may have adenopathy and fever

normal VA, pupil, EOMs

no exophthalmos or RAPD

Management

topical and systemic antibiotics

warm compress

ORBITAL CELLULITIS

inflammation of orbital contents posterior to orbital septum

common in children, but also in the aged and immunocomprimised

Etiology

secondary to sinusitis, facial and/or tooth infections and trauma