Lens, Cataracts and Optics Flashcards
1
Q
Anatomy of lens
A
The lens is a transparent crystalline structure which is suspended in between the vitreous and the iris by the zonular fibres of the ciliary body.
2
Q
Dimensions and Structure of the lens
A
- Spheroid biconvex shape
- Composed mainly of water (65%) and crystallins (34%)
- Lens is avascular and receives nutrients from the aqueous humour
- The lens contains glutathione and ascorbic acid (antioxidants which prevent cataract formation)
- Adult lens (unaccommodated) is 6mm thick
- 10mm of anterior curvature
- 6mm of posterior curvature
3
Q
Lens Capsule Anatomy
A
- The lens itself is tightly held inside a capsular bag made of type 4 collagen
- The anterior part of the capsule (14 microns) is thicker than the posterior (4 microns)
- The peripheral part of the capsule is thicker than its centre
- The anterior capsule is produced by the lens epithelium whilst the posterior capsule is formed from elongating lens fibres
4
Q
Lens Epithelium anatomy
A
- Lens epithelium is only found underneath the anterior capsule
- A single layer
- The epithelium at the centre is non-mitotic, whereas the epithelium at the periphery is mitotic.
5
Q
Core Ultrastructure of the lens
A
- The lens itself has a cortex and a nucleus
- The nucleus is in the centre and contains older lens fibres
- The cortex contains recently formed fibres
- Lens fibres are placed in a characteristic manner to form 2 sutures when looking at the lens head-on:
- Anterior ‘Y’ shaped suture
- Posterior ‘inverted Y’ shaped suture
6
Q
Zonular Fibres
A
- Arise from the ciliary body and attach as sheets onto the lens capsule
- Made of fibrillin
- Zonular force keeps the capsular bag tense and the lens taut.
In their normal state, the zonular fibres are taut and hold the lens with tension. It is only with accommodation that these fibres relax and allow the lens to thicken from its tense arrangement.
7
Q
Power of the lens
A
- Lens power is the extent to which it can refract light
- Refractive index of 1.386 (around 1.4 centrally)
- Adult (unaccommodated) lens power is around 17D
8
Q
Total power of the eye
A
- Lens 1/3 of the total power of the eye 17D
- Cornea 2/3rds is around 43D
9
Q
How does ageing affect the lens?
A
Ageing leads to a loss in the accommodative power of the lens, this is why adults need reading glasses at around the age of 50.
10
Q
Helmholtz Theory of Accommodation
A
- Ciliary muscles contract
- Ciliary body becomes larger and moves closer to the lens → zonular fibres relax
- Lens thickens (increases power to focus the light from the nearer image)
- Pupils constrict and converge
- Choroid and Ora Serrata stretch forward
11
Q
Accommodative Power at various ages of life
A
With age, the lens hardens and the anterior capsule thickens - its power decreases because it loses the ability to change shape with accommodation
- At birth → 16D
- 25 years → 8D
- 50 years → 2D
12
Q
Loss of power of the eye is known as what?
A
Presbyopia
13
Q
Pathology of catarats
A
Alongside ageing, there is a decrease in the overall biochemical activities of the lens and a loss of antioxidants. This causes the lens to become thicker, weightier and cloudier, resulting in light scatter
2 types of crystallins are specifically affected with age:
* Alpha and gamma decrease
* Beta becomes more dispersed.
14
Q
Grades of cataracts
A
- Immature - partially opaque
- Mature - opaque
- Hypermature - cataractous material is leaking outside the capsular bag and the capsule itself has shrunk, and wrinkling can be seen
- Morgagnian cataract - cortex has liquified and the nucleus has sunk within the capsular bag
15
Q
Classification of Age-related Cataracts
A
16
Q
Classic Cataract Associations
A
17
Q
Congenital Cataracts
A
Most commonly occur bilaterally and follow an AD inheritance pattern
There are many secondary causes of congenital cataracts such as Down’s syndrome and homocystinuria.
18
Q
When should congenital cataracts be opperated on?
A
The timing of surgery for congenital cataracts is based on balancing the risk of glaucoma and amblyopia
- Unilateral congenital cataracts should be removed at 6 weeks
- Bilateral congenital cataracts should be removed at around 10 weeks
19
Q
Biometry
A
There are various machines that conduct biometry such as the IOLmaster
Measurements include the axial length, corneal curvature and anterior chamber depth
The numbers from these measurements are plugged into predictive formulae to find the appropriate power for the lens implant which will replace the cataract
20
Q
Types of Lens Implants
A
There are 2 broad types of IOL: rigid and flexible
Rigid is made of polymethylmethacrylate (PMMA) and is not widely used in the UK because it is associated with more complications, but it is cheap
Flexible IOLs have 3 further subtypes
- Silicone - highest complication rates so not widely used in modern settings
- Acrylic hydrophobic - high refractive index but can cause glare
- Acrylic hydrophilic - best for biocompatibility
21
Q
Consent / Risks associated with cataracts
A
- 0.1% risk of sight-threatening complications such as endophthalmitis, retinal detachment and choroidal haemorrhage
- The commonest intra-op complication is posterior capsular rupture with vitreous loss - 4% risk
- The commonest post-op complications is posterior capsular opacification, 10% risk in 2 years
22
Q
Techniques for cataract surgery
A
Phacoemulsification surgery is the gold standard - It uses ultrasound and suction to break the cataract into small fragments and to remove it from the capsular bag.
Extracapsular cataract extraction (ECCE) is a manual operative method which has been superseded by phacoemulsification. It requires a larger incision, resulting in longer recovery and a higher risk for complications.
ECCE may still be used if the cataract is extremely hard and the surgeon is unable to break it with ultrasound.
23
Q
Phacoemulsification Procedure
A
- Generic preparations with antiseptic, appropriate anaesthesia (typically topical or peribulbar) and mydriasis
- Cleaning the external ocular structures with 5% povidone-iodine (is the most important step in preventing endophthalmitis)
- main incision and 1/2 side ports created on the sclera using a blade or femtosecond laser
- Capsulorhexis - creation of a continuous curvilinear opening in the anterior capsular bag
- Hydrodissection - saline solution injected between the capsule and cortex to allow rotation
- Phacoemulsification - cataract broken with direct contact of the ultrasound tip on the nucleus and fragments aspirated
- IOL insertion - Fill bag with viscoelastic gel and inject IOL
- Remove viscoelastic gel
- Inject intracameral cefuroxime - another important step in preventing endophthalmitis
24
Q
Benefit of the femtosecond laser
A
Is an automated method of creating reproducible incisions
25
What is the name of the ultrasound tip of the femtosecond laser?
What frequency does it vibrate at?
The ultrasound tip is called the phaco tip and it vibrates at 30-60kHz
26
4 main Intraoperative complications of catract surgery
1. posterior capsular rupture
2. zonular dehiscence
3. dropped nucleus
4. choroidal haemorrhage
Vitreous loss can also occur alongside these complications. The vitreous is encapsulated by the hyaloid membrane and vitreous loss occurs when this membrane is damaged and vitreous humour leaks. In these cases, the vitreous material is stained with triamcinolone and a vitrectomy is performed
27
Posterior Capsular Rupture
* Every effort is made to keep the capsular bag intact, so the IOL can be inserted.
* The posterior lens capsule is thinner than the anterior and is prone to rupture
* Typical signs include: sudden deepening of the anterior chamber and momentary pupillary dilation
* If there is a large rupture then it may not be possible to place the IOL implant within the capsular bag. In which case it can be placed anteriorly in the sulcus.
28
What is the most common intraoperative complication
Posterior capsular rupture with vitreous loss
29
Zonular Dehiscence
* Damage to the zonular fibres can make the lens unstable and complicate the surgical procedure
* Iris hooks can be used to secure the affected area and a capsular tension ring can be used to stabilize the lens capsule for the remainder of the procedure
30
What is floppy iris syndrome?
How does it affect cataract surgery
Floppy iris syndrome is when the iris is flaccid.
This complicates surgery because it gets in the way of the operation and isn’t fixed into position.
It is classically associated with alpha-blockers such as tamsulosin (a drug used in the management of prostatic hyperplasia)
31
Dropped Nucleus
* This is when lens nucleus fragments escape the capsular bag. This material is inflammatory
* Small fragments can be addressed with post-operative steroids
* Large fragments may need removal via vitrectomy
32
Choroidal Haemorrhage
* Bleeding from the choroid is a serious sight-threatening complication with a 0.1% risk of occurrence during surgery
* Typical signs include: suddenly elevated intraocular pressure, shallowed anterior chamber, darkening/loss of the red reflex and severe pain
* Management involves the immediate suturing all of wounds + IV acetazolamide/mannitol + topical steroids
33
Early post op complications
1. Corneal oedema
2. Endophthalmitis
34
Most common **Early** Postoperative Complication
Corneal oedema
Managed with topical steroids/NSAIDs. Return to the theatre if there is wound leakage or iris prolapse
35
Endophthalmitis presentation
* 0.1% risk after cataract surgery
* Sight threatening and required immediate treatment
* Typically occurs within the week of surgery
* Presents with pain, worsening vision and hypopyon (pus in the anterior chamber)
36
How is Endophthalmitis diagnosed?
Anterior chamber tap and vitreous biopsy are used for diagnosis and microbiology
37
Treatment of Endophthalmitis
* Intravitreal vancomycin + Ceftazidime or Amikacin
* Biopsy and antibiotics are administered simultaneously and can be repeated if there is a poor response
* Severe cases with very low visual acuity are treated with pars plana vitrectomy
38
What is the most important step to prevent endophthalmitis
To clean the external ocular structures with povidone iodine before the surgery. Infective agents are often transmitted from the patients' own skin.
39
Late post op complications
1. Cystoid Macular Oedema (CMO) (Irvine-Gass syndrome)
2. Posterior Capsular Opacification (PCO)
40
Cystoid Macular Oedema (CMO) (Irvine-Gass syndrome)
* Tractional stress of surgery leads to the release of inflammatory mediators that cause oedematous fluid accumulation in perifoveal vessels around the macula
* Typically occurs within weeks of surgery
* Presents with painless blurry vision
* Diagnosis is made with OCT
41
Management of CMO
* Starting with steroid drops
* 2nd step to periocular triamcinolone
* 3rd step to intraocular steroids
* If medications fail then vitrectomy might prove useful
42
Posterior Capsular Opacification (PCO)
43
Posterior Capsular Opacification (PCO)
* 10% risk within 2 years
* Typically manifests as blurry vision some months after surgery
* Treatment with Nd: YAG laser capsulotomy
44
Aside from cataracts, list 2 other types of lens abnormalities
1. Ectopia Lentis
2. Lenticonus/globus
45
Ectopia Lentis
Ectopia lentis is the dislocation of the lens and it can be acquired or congenital. Changes in lens position distort the refraction of light and decrease visual acuity. Anterior dislocation in particular can obstruct the normal flow of aqueous and result in glaucoma.
46
The eye of a patient with homocystinuria. Note the light blue curved line at the centre of the pupil. This is the superior border of the lens, which shows inferior displacement
47
List 4 Congenital Causes of Ectopia lentis
1. Familial ectopia lentis - Superotemporal displacement
2. Marfan’s syndrome - Superotemporal displacement
3. Homocystinuria - Inferonasal displacement
4. Weill-Marchesani syndrome - Anterior inferior displacement
48
What is Congenital aphakia?
The complete absence of the lens.
It is caused by a FOX3 gene mutation.
49
Management of Ectopia lentis
* Minor displacement can be managed with contact lenses and spectacles
* Major displacements are managed with lensectomy
50
What are Lenticonus and globus
Abnormal lumpy protrusion of the lens surface. It causes abnormal distortion of light
51
Lenticonus
* A conal protrusion
* Anterior lenticonus (anterior lens) is associated with Alport's syndrome
* Posterior lenticonus (posterior lens) is associated with Lowe’s syndrome
52
Alport’s syndrome
Disorder of type 4 collagen
53
Lentiglobus
* A hemispherical protrusion
* Retroillumination using a slit-lamp shows a classic oil droplet sign
54
Simple ray diagrams demonstrating refraction and reflection. n1 is a material with a higher refractive index.
55
What is refractive index?
Light rays bend when moving between mediums of different refractive index
Refractive index describes the speed of light through a material, relative to a vacuum
56
What is the refractive index of the cornea?
**1.376**
Means the light travels 1.376 times slower through the cornea than in a vacuum
57
What is the Critical Angle
The critical angle (θc) is the angle of incidence at which light is first reflected instead of refracted
The critical angle for normal spectacles is 41°
58
What is Total Internal Reflection
* Total internal reflection is when the angle of incidence exceeds the critical angle
* Light is not refracted
* Light is reflected back into the medium with the higher refractive index
59
Why is it difficult to visualise the anterior chamber?
How does a gonioscopy allow this
The anterior chamber angle is an important clinical structure in glaucoma.
It cannot be seen directly because the light coming out of it is totally internally reflected by the corneal/air interface.
A gonioscope changes the refractive index of this interface such that the angle becomes visible.
60
Vergence
Vergence is the amount of spreading (divergence) or coming(convergence) together of light.
It is measured in diopters (D), where D is the reciprocal of the distance to the point where light rays would intersect if extended in either direction (focal length). D=1/f(m)
61
Describe the vergence of various rays
* Converging rays have a plus vergence.
* Diverging rays have a minus vergence.
* Parallel rays have 0 vergence
62
How does a lens affect vergence?
A lens changes the vergence of light, whether it’s the crystalline lens, spectacles, contact lenses or IOL implants
63
Compare the vergence of a plus vs minus lens
* A plus lens converges light
* A minus lens diverges light
64
Basic Lens Formula
65
Image qualities of a a minus lens (biconcave)
virtual, erect and diminished
66
Image qualities of a a plus lens (biconvex)
67
What is visual acuity testing
Visual acuity is a measure of the clarity of vision. There are subjective and objective methods of testing.
68
What is the most commonly used **objective** test of visual acuity
Retinoscopy
69
Retinoscopy
70
An Ishihara plate. This is a test for colour blindness. If you cannot see a number in the middle of this plate then you are likely to be red-green colourblind.
71
List 3 subjective measures of testing visual acuity
1. Snellen Chart
2. LogMAR Chart
3. Duochrome Test
72
Snellen Chart
73
LogMAR Chart
74
Duochrome Test
75
Testing visual acuity in children
76
Visual acuity development at various ages
77
What are ophthalmoscopes
Ophthalmoscopes are also known as fundoscopes because they allow you to visualise the fundus of the eye - the interior surface of the retina behind the lens
78
What are the 2 main types of ophthalmoscopes
1. Direct --> provides a magnified view of the central fundus.
2. Indirect --> provides are wider stereoscopic view, up to the ora serrata. (useful in cases where the direct fundus view is obstructed by hazy media or cataracts)
79
A direct ophthalmoscope.
80
Types of Fundoscopy
81
Compare the field of view in indirect ophthalmoscopy
Field of view is largest in cases of high myopia and smaller in hyperopia
82
An ophthalmologist using an indirect ophthalmoscope. The light is attached to the headband and a handheld lens is used to provide a wide view. This lens is typically +20D.
83
What is a slit lamp
Slit lamps are the staple tool of examination in ophthalmology. They allow you to examine the ocular structures in detail.
The patient is dilated during the exam. There are several different illumination techniques which allow the examiner to focus on various structures.
84
A slit lamp. The clinician sits on the right and uses the optics. The patient sits on the left with their forehead on the white band.
85
List 5 illumination techniques
1. Diffuse Illumination
2. Direct Focal Illumination
3. Retroillumination
4. Specular Reflection
5. Sclerotic Scatter
86
Diffuse Illumination
* Wide beam of light diffusely illuminating the eye and external structures.
* Used for general examination
87
Direct Focal Illumination
* This is the most commonly used type of illumination
* It focuses a beam of light directly on the part of the eye being examined
88
Retroillumination
Light is reflected back from structures to posteriorly illuminate structures in front. E.g from the iris to the cornea or the retina to the iris/lens
89
Specular Reflection
* Focuses the view on the corneal endothelium
* Useful in corneal endothelial diseases such as Fuchs’ endothelial dystrophy
90
Sclerotic Scatter
The beam of light from the slit lamp is projected onto the corneal limbus. This light is scattered throughout the cornea and illuminates corneal lesions
91
Compare normal vs abnormal refraction of the eye
In normal function (emmetropia), there is a balance between the refractive powers of each media (cornea, aqueous humour, lens, vitreous humour, retina) so the vision is clear.
Changes in the ocular media result in abnormal refraction (Ametropia) resulting in blurry vision.
92
How can ametropia be furthur classified?
1. Myopia
2. Hyperopia (hypermetropia)
3. Astigmatism
93
What are the 2 most important underlying principles of refractive error
power and axial length
94
What are the 2 most important underlying principles of refractive error
power and axial length
95
Refractive power of the eye
Refractive power is measured in dioptres (D)
Power = 1/focal length of lens (m)
The total refractive power of the eye is around 60D
* Cornea = 43D
* Lens = 17D
96
Compare a higher vs lower refractive power of the eye
An eye which has a higher refractive power than normal will be myopic
An eye which has a lower refractive power than normal will be hyperopic
97
Myopia
As power increases, focal length decreases and the image is focused at a shorter distance (in front of the retina)
Myopes are short-sighted because a higher-powered eye focuses the light in front of the retina, rather than on it.
When an object of visual focus is brought nearer, the person sees it clearer. This is because moving the object closer pushes the point of focus further back and onto the retina.
98
Hyperopia
As power decreases, focal length increases and the image is focused at a longer distance (behind the retina)
Hyperopes are long-sighted because a lower-powered eye focuses the light behind the retina, rather than on it
When an object of visual focus is moved further away, the person sees it clearer. This is because moving the object further, pulls the point of focus from behind the retina.
99
Astigmatism
Astigmatism is when the refractive power is not uniform over a structure i.e if the left side of the corneal surface is steeper (higher power) than the right
In astigmatism, light is not accurately focused, so the patient experiences blurry vision
100
What is the visible spectrum of light in the typical human eye
380nm-750nm
101
Axial length of the eye
102
How are refractive erros of the eye managed?
Contact lenses (CLs), spectacles, intraocular lens procedures and laser vision correction (LVC).
Rigid contact lenses typically perform better than LVC, and LVC typically performs better than spectacles.
103
Is LVC offered under the NHS?
**LVC** and **intraocular lens ** procedures are not available on the NHS for patients who can be managed with CLs or spectacles.
They are only provided on the NHS where an underlying eye condition will lead to blindness without LVC.
104
What are the 2 types of lenses?
Compare these
105
What are the 4 types of Laser Vision Correction (LVC) Procedures
106
Intraocular lens procedures
You can clip an artificial lens in front of the human crystalline lens as an alternative to LVC or where LVC is contraindicated
* Visian lens is implanted onto the ciliary sulcus
* Artisan lens is clipped onto the anterior iris
Pseudophakic lens procedures are when the human crystalline lens is removed altogether and replaced with an IOL implant.
107
Myopia
The patient is short-sighted, closer objects are clearer than distant ones. The refracting power of a myopic eye is higher than normal or the axial length of the eyeball is longer than normal. Both of these situations lead to myopia because the light is focused in front of the retina, rather than being focused onto the retina. Myopia can be physiological or pathological
108
Causes of myopia
* Physiological myopia is common and mild
* Pathological myopia is rare and characterised by continuous lengthening of the posterior segment and results in high myopia.
* Degenerative myopia is a distinct pathological myopia
109
Classification of myopia
The severity of myopia is classified according to the strength of the lens that is needed to correct it. I.e a patient might have a total refraction of 63D, and will need -3D of correction. This patient's myopia is classified as -3D.
* <-3D = low myopia
* -6D = high myopia
110
Management of myopia
* -ve D spherical lenses are used to correct myopia
* They are concave and diverge light
111
Lens Correction Calculation - Myopia
112
Hyperopia
Hyperopia (also known as hypermetropia) is far-sightedness. It arises when the axial length of the eyeball is shorter than normal or when the refractive power of the eyeball is less than normal. It can be physiological or pathological.
113
Classification of hyperopia
The severity of hyperopia is classified according to the strength of the lens that is needed to correct it.
* <+2D → low
* +5D → high
114
Management of hyperopia
* +ve D spherical lenses are used to correct myopia
* These lenses are convex and they converge light
115
Lens Correction Calculation - Hyperopia
116
Astigmatism
Astigmatism is when the refractive power is different across the surface of the same medium. E.g the superior cornea might have 49D and the inferior cornea might have 34D. This is going to course extreme distortion of light and result in an unfocused image. A similar case could apply to the lens surface too but is much more frequently discussed in the context of the cornea.
117
Astigmatism
Astigmatism is when the refractive power is different across the surface of the same medium. E.g the superior cornea might have 49D and the inferior cornea might have 34D. This is going to course extreme distortion of light and result in an unfocused image.
118
Classification of Astigmatism
119
Management of astigmatism
Regular astigmatism can be treated with soft toric lenses
* Toric lenses use a cylinder which focuses light on a particular meridian
* This cylinder can be nested within a spherical lens, where the sphere affects all the meridians equally and the cylinder focuses on a particular meridian
Irregular astigmatism is treated with rigid gas permeable contact lenses (RGP CLs)
120
Toric Lens Transposition
121
Presbyopia
The process of accommodation allows the eye to adapt its power based on the desired level of focus, whether it’s to a near object or a far object. Accommodation relies on the ability of the lens to change shape. This function is lost with ageing and this age-related loss of accommodative power is known as presbyopia
122
Pathology of presbyopia
