L6 aqueous humour and IOP

Question 1

describe the aqueous humour (in terms of position and RI)

Answer 1

• it is a clear fluid
• it does not move in random directions
• it is bounded anteriorly by the endothelium (the inner surface of the cornea), except at its far periphery where it is reaching the trabecular meshwork
• posteriorly, it is bounded by the lens within the pupillary aperture, by the anterior surface of the iris, and peripherally by the anterior face of the ciliary body

refractive index: 1.336

Question 2

list the functions of the aqueous humour

Answer 2

1. provide positive pressure, give shape to the globe
2. provide nutrients and remove waste products for cornea, trabecular meshwork, crystalline and anterior vitreous
3. maintain intra-ocular pressure

Question 3

where is the aqueous humour formed?

formation

Answer 3

the site of aqueous humour production is in the inner non-pigmented ciliary epithelium in the ciliary process (pars plicata region) in the ciliary body

Question 4

what is the source of aqueous humour?

(formation）

Answer 4

the ciliary arterial system/ ciliary arterial blood

- it is derived from blood plasma flowing in the ciliary arteries

Question 5

what are the 3 processes that lead to the circulation of aqueous humour?
(circulation)

Answer 5

i. diffusion
- > the movement of molecules along the concentration gradient
- lipid-soluble particles are transported via diffusion

ii. ultra-filtration
- > the movement of smaller molecules across a semi-permeable membrane using hydrostatic force
- smaller water-soluble particles are transported via ultra-filtration

iii. active transport
- > the movement of larger molecules against the concentration gradient using cellular energy (ATP)
- larger water-soluble particles are transported via active transport

Question 6

describe the circulation of the aqueous humour

circulation

Answer 6

• the 3 processes (Diffusion, ultra-filtration and active transport) occur in the non-pigmented epithelium in the ciliary processes
• the molecules are transported out of the cell membranes into the posterior chamber
• water gets added and the aqueous humour moves from the posterior chamber to the anterior chamber (via the pupil)
• aqueous humour circulates in the anterior chamber, providing nutrition and removes waste products from the cornea, iris and crystalline lens

Question 7

describe the aqueous humour drainage system

Answer 7

• drainage is via the anterior chamber angle
• aqueous humour exits the eye from the anterior chamber into the trabecular meshwork

the angle structures are:
- Schwalbe line (js a structure, nt rly impt)
1. trabecular meshwork 
2. schlemm's canal 
3. sclera spur 
4, ciliary body 

** ^
if all structures are visible up to sclera spur = wide angle

if only the trabecular meshwork is seen partially or none of the angle structures are seen = angle considered close

Question 8

describe the uveoscleral pathway

Answer 8

• it is the alternate aqueous outflow
• > involves the ciliary muscle
• aqueous drains through the face of the ciliary body in the region just posterior to the sclera spur, in the apex of the anterior chamber
• aqueous flows through the ciliary body into the suprachoroidal space

Question 9

describe the blood-aqueous barrier

Answer 9

definition:
it is the anatomical mechanism that prevents the exchange of materials between the chambers of the eye and the blood vessels

Question 10

what constitutes the blood-aqueous barrier?

Answer 10

** - the tight junctions of the non-pigmented epithelium of the ciliary body

• the junctions of the iris tissues
• iris blood vessels

Question 11

what are the functions of the blood-aqueous barrier

Answer 11

1. to maintain the transparency of the aqueous humour
2. to restrict the movement of sodium, larger water-soluble ions, proteins, and other large and medium-sized molecules into the aqueous humour

Question 12

what is the intraocular pressure (IOP)

Answer 12

the pressure within the eye maintained by the aqueous humour

Question 13

describe normal vs when to suspect IOP

Answer 13

• > normal
• normal IOP range: 10-21 mmHg
• normal diurnal variation: 4-6mmHg (max value in the morning)
• inter-ocular variation: <3mmHg
• > when to suspect glaucoma
• IOP > 21mmHg
• diurnal variation: 8-10mmHg
• inter-eye variation >3mmHg

Question 14

what are the ocular factors for increased IOP (factors that result in resistance to damage)

Answer 14

1. closed anterior chamber angle
2. open-angle with
   i. shrinkage/swelling/blockage in the trabecular meshwork
   ii. narrowing of the canal of schlemm
   - resistance to aqueous humour drainage
   iii. raised episcleral venous pressure