LV: Types of Telescope

Basic types

• Hand held
• Clip on
• Spectacle mounted
• Monocular versus binoc
• Focus/Afocal

Specials

• Autofocus
• CL telescope
• IOL lens scopes
• BTLT - Behind the lens telescopes. These offer cosmetic adv but great care must be taken obvs. Increased risk of eye injury if the patient bumps into something while wearing them. Also v expensive
• Bioptic - for px who fulfil certain requirements driving with bioptics is legal in some US states, not here. Px can resolve fine detail like road signs

Contact Lens Telescopes

• These use a galilean system where the obj lens is a high plus spec lens and the eyepiece is a high powered -ve CL. The vertex dist of specs is equal to the sum of the focal lengths (length of the telescope)
• The FOV is better than with a conventional telescope cos the exit pupil is really close to the eye but it also depends on the objective diameter (use a blended aspheric to avoid a ring scotoma)
• Has many practical and cosmetic disadvantages though so is rarely used.
• EG a +20DS spec lens and a -40DS contact lens. Px is emmetropic.

The fitting of

• check that normal scope improves VA as expected
• Check nystagmat for oscillopsia
• Fit CL, maximise vd of trial frame, perform over-refraction
• CL must be stable
• Vergence amplification means separate reading specs are required
• Account for uncorrected ametropia - +10D hypermetrope with -30CL effective eyepiece is -40 and -10 myope with -30 CL effective eyepiece is -20.

Advantages of

• In theory px could drive under uk law if visual requirements were met
• Can get acuity much better than expected in congenital nystagmus
• IOL could be used instead of CL giving a longer vd so better magnification but IOL can't be changed if acuity worsens

Disadvantages of

• Low magnification so only works for moderate acuity loss
• Patient has to be adapted to CL wear & it's difficult to insert contact lenses
• Need to wear the system regularly for long periods
• Adaptation to spatial distortion required
• Poor cosmesis with high vd and big fat plus lenses