(3a) Double Sphere Method

Question 1

Process for double sphere method

Answer 1

• control accommodation (fog, dim light etc)
• scope beam along 90, 180, 45, 135
• scope on the patient’s visual axis

Question 2

3 things to observe when conducting DSM

Answer 2

Direction (type of ametropia)
Thickness (spherical/astigmatism)
Speed/brightness (refractive power)

Question 3

Indications of astigmatism during retinoscopy

Answer 3

Thickness variation
-different meridian thickness means different refractive power in the 2 meridians = astig

Skew phenomenon
-reflex not aligned parallel to retinoscopy streak / not on axis

Question 4

How to determine 2 principle meridians using DSM

Answer 4

• no cyl lens needed
• according to thickness variation & skew phenomenon
• neutralise either meridian with sphere lens then record power and axis
• same with the second meridian (perpendicular)
• record into sphere/minus cyl format