Classifications, screening and genetics of the glaucomas (Dave Edgar)

Question 1

what is the definitions of the glaucomas

Answer 1

Glaucoma (or more correctly but less conveniently “the glaucomas”) is the name given to a group of ocular conditions that produce a characteristic optic neuropathy

glaucomas - a group of ocular conditions which give the same end point
optic neuropathy - pale optic disc with increased cup to disc ratio and loss of NRR

Question 2

what 2 categories are the glaucomas classified into

Answer 2

• primary - the condition is not associated with any other ocular disorder
• secondary -secondary to another ocular or non-ocular disorder e.g. steroid induced glaucoma

Question 3

which type of glaucoma is usually recognised quite early on

Answer 3

secondary recognised earlier than primary

Question 4

list the 3 types of primary glaucomas and how much each accounts for in primary glaucomas

Answer 4

• Primary angle closure glaucoma
  About 15% of the primary glaucomas in UK
• Primary (or Chronic) open angle glaucoma (POAG or COAG)
  About 85% of the primary glaucomas in UK

Congenital glaucoma

Question 5

what is unknown about Primary (or Chronic) open angle glaucoma

Answer 5

Mechanism for the disease

Question 6

what is the possible definition for Primary (or Chronic) open angle glaucoma and what is a very important risk factor

Answer 6

A form of chronic injury to axons of the retinal ganglion cells and other optic nerve tissues, mainly at the level of the optic disc and lamina cribrosa, frequently associated with characteristic, but not diagnostic, visual field defects

IOP, which is often raised above average levels, is a very important risk factor - BUT NOT CHARACTERISTIC OF GLAUCOMA

Question 7

what are the 2 main theories on the pathogenesis of glaucomatous optic neuropathy in open angle glaucoma and describe what happens in each one

Answer 7

• Mechanical (or IOP mediated)
  Axon damage within the optic nerve head is caused by pressure-induced deformation of the lamina cribrosa
• Vasogenic (or vascular)
  The quality of blood supply to the ONH is impaired which leads to hypoxia and reduced nutrition to ONH, and then to axon death

Question 8

what happens to the lamina cribrosa in the mechanical theory on the pathogenesis of glaucomatous optic neuropathy in open angle glaucoma

Answer 8

the bending and bowing of the plates causing deformation of the lamina cribrosa

Question 9

describe the appearance of a typical visual field defect defect in moderate open angle glaucoma and explain why it is characteristic but not diagnostic of glaucoma

Answer 9

• Classic arcuate shape with sharp cut off on horizontal nasal quadrant called nasal step
• can get the same in other defects e.g. multiple sclerosis or congenital nerve defect

Question 10

in POAG, which area of the visual field is affected and which vf test is best to pick up on early signs

Answer 10

• central 24 degrees is affected

- central 24-2 is the vf test used because most diseases affects the optic nerve and central vf

Question 11

list the 8 risk factors of POAG, starting from highest to lowest

Answer 11

POAG is multifactorial. Risk factors include:
Age++++
IOP+++
Race++
Family history+
Myopia+
Genetics+
Corneal thickness
Smoking?

Question 12

give 3 points as to how IOP and POAG are linked

Answer 12

• Prevalence of POAG increases as IOP increases
• Risk of subsequent field loss increases with increasing IOP
• Significant reductions in IOP reduce the risk of subsequent nerve damage

Question 13

what is the aim of anti glaucoma drugs

Answer 13

to aim for a target IOP that will cause no further damage

when ophthalmologists decide to treat glaucoma, they look at the patient’s pressures on diagnosis, which is based on individual factors relating to the px, the ophthalmologist will then decide on a target pressure, to stop progression of the disease or to minimise/slow down the progression of the disease

Question 14

what is the prevalence of POAG as stated by Beaver Dam and Roscommon and what did both studies’ results have in common about the patients

Answer 14

Beaver Dam 2.1%
Roscommon 1.87%

both in over 40’s in rural caucasian populations

Question 15

what is the prevalence of POAG in the black and white population by Baltimore

Answer 15

blacks 4.18%
whites 1.29%

from people in inner city populations

Question 16

what does the NICE guidelines state about the prevalence of glaucoma in those over 40 and those over 75 years go age

Answer 16

• approximately 2% of those over 40

- approximately 10% of those over 75

Question 17

what are the 2 classifications of POAG

Answer 17

• normal tension glaucoma (NTG)
  still occasionally referred to as “low tension glaucoma” - HERE IOP IS BELOW 21mmhg
• high-tension glaucoma - HERE IT IOP is above 21mmhg

Question 18

what 2 types of definition is there of normal tension glaucoma (NTG)

Answer 18

• qualitative definition:
  would be “a glaucomatous optic neuropathy with IOP within the statistically normal range”
• quantitative definition:
  a glaucomatous optic neuropathy, but the IOP never exceeds 21 mm Hg

a glaucomatous optic neuropathy with a mean IOP over a 24 hour period of less than 21 mm Hg and peak pressures of less than 24 mm Hg

a type of COAG where IOP has rarely been recorded above 21 mm Hg
This is the NICE definition

all quantitative definitions mention 21mmHg

Question 19

where did the value of 21mmHg come from and what does research say about the distribution of IOP

Answer 19

• 21mmHg is the mean IOP (16mm Hg) + 2 Standard Deviations
  SD = 2.5mmHg, 2SD = 5mmHg

IOP is NOT normally distributed

If IOP was normally distributed then about 2.5% of the population would have IOPs > 21mm Hg - but some say the actual figure is closer to 8% (based on normal IOP in the bedford population)

statistics show low value, 21mmHg = not as reliable figure

Question 20

how many % of patients have IOP in the normal range at diagnosis and what do you call this type of glaucoma

Answer 20

• 40 - 50% of patients have IOP in the normal range at diagnosis
• Normal tension glaucoma
• IOP may rise with time but not at a faster rate than in normals - so basically you can’t even assume if you missed a px who had glauoma first time round you would pick it up at next appt bc the iop would increase.

Question 21

what is the risk of normal tension glaucoma causing POAG

Answer 21

• Risk of P(C)OAG increases with IOP, BUT risk is there at levels at or below 21mmHg (and most patients have IOPs

Question 22

what is high tension glaucoma

Answer 22

a glaucomatous optic neuropathy with IOP outside the statistically normal range normal tension glaucoma (NTG)

Question 23

what are the 3 types of primary angle closure glaucoma and how much % of primary glaucomas does it make up in the UK, also what is the prevalence of this type of glaucoma in caucasians aged over 40

Answer 23

• acute
• sub-acute
  or
• chronic
• makes up 15% of primary glaucomas in the UK
• prevalence of 0.4% in Caucasians aged over 40

Question 24

in which parts of Asia is primary angle closure glaucoma more prevalent in compared to POAG and give values for both types from 2 countries

Answer 24

• In China (Beijing) POAG = 0.03%, CAG = 1.4%
  (But this is only one study)
• In India one study (Vellore) POAG = 0.4%, CAG = 4.3%
  (Other studies disagree)

so proportions of open and closed angle glaucoma differs in the world - BASICALLY IDEA IS THAT IN ASIAN COUNTIRES PCAG IS MORE COMMON THAN POAG

Question 25

what is the prevalence of congenital glaucoma of the primary glaucomas in the UK and why is this type of glaucoma important

Answer 25

• makes up less than 0.5% of primary glaucomas in UK
• Important because:
  is responsible for 2.5 - 10% of all registered child blind

Question 26

what is another name of congenital glaucoma and why does it develop

Answer 26

• infantile - alternative name buphthalmos
• develops as result of a maldevelopment in the outflow system of the eye in utero (of aqueous to escape the eye not developed properly)

juvenile - occurs later in childhood

Question 27

list 4 points that describes what ocular hypertension OHT is

Answer 27

• IOP >21 mm Hg
• Normal optic discs
• Normal visual fields
• Open anterior chamber angle

i.e. dont have glaucoma

we are keen to monitor these people though bc high IOP is a risk factor for glaucoma

Question 28

what is the prevalence of OHT in the adult population and what does NICE (2009) state about OHT

Answer 28

• Prevalence of OHT in the adult population is between 2.7 and 7.5%
• NICE (2009) state that between 3 to 5% of those over 40 have OHT

Question 29

what is the prevalence of genetics in glaucoma and what has there found to be when it comes to family history and glaucoma

Answer 29

• 10 – 50% of POAG patients report a family history
• there is undoubtedly significant under-reporting of family history
• It is likely that many genes are involved
  They may interact to cause the variations that occur in individuals with COAG
• Several genes have been identified:
  But these are responsible for uncommon types of glaucoma

Question 30

describe the genetic link between monozygotic twins and glaucoma

Answer 30

if a monozygotic twin develops POAG there is a 98% chance of concordance [i.e. both affected] between the twins = strong genetic trait

Question 31

describe the detection of POAG and list the 3 main tests that are used, explain the reliability of these screening tests

Answer 31

POAG is asymptomatic until the late stages of the disease

Three main tests used are:

• Measurement of Intraocular Pressure
• Assessment of visual fields
• Examination of the Optic Disc

In any screening test there are invariably errors and misclassifications, even if the examiner is highly skilled

• No test is perfect
• There are numerous factors affecting the accuracy of tonometry, fields, and disc assessment