wk7: ND - Monochromacy Flashcards
Name the 3 kinds of monochromacy
Typical (rod) monochromacy
“Blue cone” monochromacy
Atypical monochromacy
How common is typical monochromacy?
very rare
What should you look for in the clinic when you suspect typical monochromacy? (6)
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
What is the best V.A you can typically expect for rod monochromats? Why?
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
How is photophobia in monochromats different from photophobia in patients with uveitis?
Painless, (whereas painful photophobia in uveitis)
Does photophobia and nystagmus occur in ALL patients with monochromacy?
No. But they are common
Are there any retinal changes associated with monochromacy?
usually none apparent
True/False: there is an increase in myopia prevalence in monochromats, evidenced by squinting and closer viewing distance
FALSE! Very false! There is no association between myopia and monochromacy. Squinting is not because of myopia necessarily
How do monochromats fare with pseudo-isochromatic plates?
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
How do monochromats perform on Farnsworth D15?
on average, they arrange in order of scotopic reflectance
How do monochromats perform on the nagel anomaloscope?
Full range, but Y setting decreases dramatically with increasing red in the R:G mixture because of scotopic wavelength
Which sex is more often affected by typical rod monochromacy?
Both equally affected
How commonly is typical rod monochromacy transmitted to offspring?
rarely
How high is the incidence of consanguinity in affected family for typical rod monochromacy?
high
How is typical rod monochromacy inherited?
Autosomal recessive
What findings in typical monochromacy suggest absence of cones or cones not functioning? (4)
Scotopic VA
Lowered VA
Preference for dim light
Nystagmus (due to scotoma at rod free foveal area, not a moor anomaly)
What are the 2 theories behind the mechanism of typical rod monochromacy?
Absence of cones (“pure rod theory”) or
Cones not functioning (“rod only theory”)
Which theory for the mechanism of typical rod monochromacy has been dismissed via research and why?
Pure rod theory. Research has found presence of morphohlogically intact cones or cone like structures, however vastly fewer in number
What about V.A in typical rod monochromats suggest that it’s not rod only function?
V.A in some monochromacy is 6/18-6/60 whereas with rod vision alone you’d expeect 6//36-6/60.
What did sloan, 1954 find about typical rod monochromats and dark adaptation?
Rod-cone break found in 3 cases during dark adaptation
What did Hecht et al 1948 find about typical rod monochromats during increment threshold and CFF (critical flicker frequency)?
Both data showed rod cone break present
What did Alpern Falls and Lee 1960 find about typical rod monochromats?
Directional sensitivity (SCE) was evident
What did Alpern Falls and Lee 1960, conclude about typical rod monochromats after all this evidence about cone presence?
Cones present but outer segments contain rhodopsin or a visual pigment which has an action spectrum that is indistinguishable from rhodopsin.
What is “blue cone” monochromacy?
Monochromats that have blue cones and rods, but no M or L cones
What is the spectral sensitivity of blue cone monochromats like? (3)
Scotopic wavelength discrimination max at 507nm
Photopic wavelength discrimination max at 445nm
“reverse purkinje shift”
What is a reverse Purkinje shift?
spectral sensitivity moves to shorter wavelengths as you increase illumination, so they become even less sensitive to red light
State the clinical characteristics of blue cone monochromacy (5)
“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)
What is the V.A like in cone (atypical) monochromacy?
Normal V.A
Do atypical monochromats have photophobia and nystagmus?
no
Explain the Principle of Univariance
A photoreceptor can only count the number of photons it absorbs, it cannot tell the wavelength. Therefore, an individual photoreceptor is colourblind
