Lecture for May 5, 2018, UP-PGH Sentro Oftalmologico Jose Rizal. Basic Course in Diagnostics and Instrumentation.

1. May 5, 2018
Pediatric Ophthalmology Module
PRISMS

2. Prism diopter (PD)
• 1 PD displaces an image 1 cm at a distance of 100 cm
or 1 meter
• 1D lens, 1 cm decentration, 1 PD
100 cm
1 cm

3. Factors Affecting Measurement
• Prism placement:
– plastic prisms:
frontal plane
– glass prisms:
prentice position
• Stacking prisms
• Splitting prisms
From Rosenbaum & Santiago, Clinical Strabismus Management

4. Glass Prisms
• One face perpendicular
to light rays
• Back face parallel to iris
• Prisms in spectacles
• Measure back surface
of spectacles flat
against nose cone of
lensmeter

5. Plastic Prisms / Prism Bar
• Angle of minimum
deviation
• Approximated by
rear face in frontal
plance

6. Inserting Prisms in Glasses
• 1D lens, decentered 1 cm = 1 PD
• Effectively only about 4 cm/4PD can be done
by displacing center (beyond 5 cm already
beyond edge)
• Grind in beyond 4PD each lens(?)

7. Fresnel Prisms
Ffrom Clinical Optics. Book 3, Basic and Clinical Science Course, AAO 2014-2015

8. Stacking Prisms
• Horizontal Prisms
• Vertical Prisms
https://picclick.com/High-Quality-Horizontal-
and-Vertical-Prism-Bar-Set-112276815591.html

9. Splitting Prisms

10. Cover Tests and Endpoints
• Cover Uncover Test
• Simultaneous prism
cover test
• Prism under cover test
• Alternate prism cover
tests
• Phorias, fixation
preference
• Monofixation
• DVD
• Gold standard for any
misalignment

11. Skills Checklist
• Differentiate between glass and plastic prisms
• Prentice vs Frontal plane
• Direction of prisms for ET, XT, HT/HoT
• Stacking prisms
• Splitting prisms
• True vertical vs DVD
• End point of prism testing
• Neutralizing combined deviations:
– XT w HoT in 3rd nerve
– ET with DVD
– XT with HT