wk7: ND - Monochromacy

Question 1

Name the 3 kinds of monochromacy

Answer 1

Typical (rod) monochromacy
“Blue cone” monochromacy
Atypical monochromacy

Question 2

How common is typical monochromacy?

Answer 2

very rare

Question 3

What should you look for in the clinic when you suspect typical monochromacy? (6)

Answer 3

Lowered V.A with no obvious explanation
Painless photophobia
Nystagmus
Reduced sensitivity to red light
Total colour blindness (hx, CV tests)
Other family members may be affected

Question 4

What is the best V.A you can typically expect for rod monochromats? Why?

Answer 4

About 6/18 is the best you can expect. Because remember the cone pathway doesn’t take over so they are relying on rods, which don’t have as fine resolution

Question 5

How is photophobia in monochromats different from photophobia in patients with uveitis?

Answer 5

Painless, (whereas painful photophobia in uveitis)

Question 6

Does photophobia and nystagmus occur in ALL patients with monochromacy?

Answer 6

No. But they are common

Question 7

Are there any retinal changes associated with monochromacy?

Answer 7

usually none apparent

Question 8

True/False: there is an increase in myopia prevalence in monochromats, evidenced by squinting and closer viewing distance

Answer 8

FALSE! Very false! There is no association between myopia and monochromacy. Squinting is not because of myopia necessarily

Question 9

How do monochromats fare with pseudo-isochromatic plates?

Answer 9

They fail but make irregular responses, they don’t follow classic deutan or protan lines. They arrange in the order of reflectances of the tabs rather than anything that has to do with colour

Question 10

How do monochromats perform on Farnsworth D15?

Answer 10

on average, they arrange in order of scotopic reflectance

Question 11

How do monochromats perform on the nagel anomaloscope?

Answer 11

Full range, but Y setting decreases dramatically with increasing red in the R:G mixture because of scotopic wavelength

Question 12

Which sex is more often affected by typical rod monochromacy?

Answer 12

Both equally affected

Question 13

How commonly is typical rod monochromacy transmitted to offspring?

Answer 13

rarely

Question 14

How high is the incidence of consanguinity in affected family for typical rod monochromacy?

Answer 14

high

Question 15

How is typical rod monochromacy inherited?

Answer 15

Autosomal recessive

Question 16

What findings in typical monochromacy suggest absence of cones or cones not functioning? (4)

Answer 16

Scotopic VA
Lowered VA
Preference for dim light
Nystagmus (due to scotoma at rod free foveal area, not a moor anomaly)

Question 17

What are the 2 theories behind the mechanism of typical rod monochromacy?

Answer 17

Absence of cones (“pure rod theory”) or
Cones not functioning (“rod only theory”)

Question 18

Which theory for the mechanism of typical rod monochromacy has been dismissed via research and why?

Answer 18

Pure rod theory. Research has found presence of morphohlogically intact cones or cone like structures, however vastly fewer in number

Question 19

What about V.A in typical rod monochromats suggest that it’s not rod only function?

Answer 19

V.A in some monochromacy is 6/18-6/60 whereas with rod vision alone you’d expeect 6//36-6/60.

Question 20

What did sloan, 1954 find about typical rod monochromats and dark adaptation?

Answer 20

Rod-cone break found in 3 cases during dark adaptation

Question 21

What did Hecht et al 1948 find about typical rod monochromats during increment threshold and CFF (critical flicker frequency)?

Answer 21

Both data showed rod cone break present

Question 22

What did Alpern Falls and Lee 1960 find about typical rod monochromats?

Answer 22

Directional sensitivity (SCE) was evident

Question 23

What did Alpern Falls and Lee 1960, conclude about typical rod monochromats after all this evidence about cone presence?

Answer 23

Cones present but outer segments contain rhodopsin or a visual pigment which has an action spectrum that is indistinguishable from rhodopsin.

Question 24

What is “blue cone” monochromacy?

Answer 24

Monochromats that have blue cones and rods, but no M or L cones

Question 25

What is the spectral sensitivity of blue cone monochromats like? (3)

Answer 25

Scotopic wavelength discrimination max at 507nm
Photopic wavelength discrimination max at 445nm
“reverse purkinje shift”

Question 26

What is a reverse Purkinje shift?

Answer 26

spectral sensitivity moves to shorter wavelengths as you increase illumination, so they become even less sensitive to red light

Question 27

State the clinical characteristics of blue cone monochromacy (5)

Answer 27

“Colour blind” but rudimentary dichromacy at mesopic levels (rods and s cones work)
Reduced luminosity for red in photopic conditions
Nystagmus in some but not all cases
Lowered V.A (similar to typical rod monochromats)
Not easy to differentiate from typical rod monochromacy except by lab investigation (blood workup, DNA)

Question 28

What is the V.A like in cone (atypical) monochromacy?

Answer 28

Normal V.A

Question 29

Do atypical monochromats have photophobia and nystagmus?

Answer 29

no

Question 30

Explain the Principle of Univariance

Answer 30

A photoreceptor can only count the number of photons it absorbs, it cannot tell the wavelength. Therefore, an individual photoreceptor is colourblind