Download pp_Basic-Review-of-the-Eye-as-an-Optical

Examination Techniques for
Accuracy and Efficiency
Basic Review of the Eye as an
Optical System
A VOSH-Florida Presentation
•
•
•
•
•
•
•
•
•
•
Axial length
Cornea
Lens
Emmetropia
Myopia
Hyperopia
Astigmatism
Accommodation
Presbyopia
Ocular Motility and Alignment
• Axial Length: 22mm – 25mm
– At birth, the axial length is ~17mm and
increases by ~5mm by age 3.
– Changes of ~1mm occur between age 3-13,
with stability usually by age 15.
• Anterior Chamber Depth:
– 20 year old: ~4.0mm
– 70 year old: ~3.5mm
– Changes are due to an increasing thickness
of the lens and may result in increased
myopia.
• Cornea :
– There is a normal distribution of corneal
refractive powers.
– Most normal corneas will produce 42 – 44
diopters of refractive power (~9.4mm – 6.7mm
of curvature).
• Lens:
– Produces ~20 diopters of refractive power by
age 15.
Ocular Refractive Status
• Emmetropia - incident parallel rays of light
converge to focus on the retina.
• Myopia - incident parallel rays of light
converge to focus in front of the retina.
• Hyperopia - incident parallel rays of light
converge to focus behind the retina.
• Emmetropia:
– Incident parallel rays of light converge to
focus on the retina.
• Emmetropia – Clinical Symptoms:
– Patient is happy.
– Discuss sunglasses and cosmetic contact
lenses!
• Myopia :
– Incident parallel rays of light converge to
focus in front of the retina.
• Uncorrected Myopia – Associated Clinical
Symptoms :
– Distance blur
– Headaches/fatigue (squinting)
– Photophobia/glare
– Night vision problems (blur/glare)
– Pseudo-diplopia (ghost images)
• Hyperopia:
– Incident parallel rays of light converge to
focus behind the retina.
• Uncorrected Hyperopia – Associated
Clinical Symptoms :
– Headaches
– Fatigue when reading
– Blur when reading, constant or intermittent
– Distance blur later in day
– Night myopia
– Esotropia - accommodative strabismus
• Astigmatism:
– generally defined as a non-spherical or toric
anterior corneal curvature, resulting in
incident rays of light having multiple points of
focus:
– Regular – Maximum and minimum refractive
angles are separated by 90 degrees.
– Irregular - Maximum and minimum refractive
angles are separated by other than 90
degrees (pterygia, keratoconus, corneal
scars).
– Lenticular – astigmatism induced by a tilting of
the crystalline lens (minimal, insignificant).
• Astigmatism:
• Uncorrected Regular Astigmatism:
– Results in a Circle of Least Confusion, or the
dioptric midpoint between the major lines of
focus.
• Uncorrected Astigmatism – Associated
Clinical Symptoms :
– Headaches/fatigue (squinting)
– Distance blur
– Near blur
– Photophobia/glare
– Night vision problems (blur/glare)
– Pseudo-diplopia (ghost images)
• Presbyopia – the age-related, irreversible
reduction in amplitude of accommodation.
– Clinically significant when the accommodative
demand at the patient’s normal reading
distance (~ 40cm) is 50% of the available
accommodative amplitude.
– Example: ~2.5D accommodative demand at
40cm with 5D or less of accommodative
facility = Presbyopia
– Usually becomes symptomatic at ~40 years of
age.
• Uncorrected Presbyopia – Associated
Clinical Symptoms :
– Near blur
– Headaches/fatigue
– Accommodative spasms/transient distance
blurring
– Intermittent diplopia/pseudo-diplopia at near
• Ocular Motility Disorders – The inability of both
eyes to accurately align or maintain alignment
on the target of interest.
– Types:
• Primary extra-ocular muscle problems
– Paralytic strabismus
– Convergence/Divergence infacility/insufficiency
• Refractive-related problems
– High uncorrected hyperopia (esotropia)
– Significant uncorrected anisometropia
(tropias/amblyopia)
– Significant uncorrected antimetropia
(tropias/amblyopia)
• Ocular Motility Disorders - Associated
Clinical Symptoms :
– Asthenopia/fatigue
– Blur – distance/near
– Night vision problems (blur/glare)
– Diplopia
– Pseudo-diplopia (ghost images)
– Depth perception problems