Lens Flashcards
Where is the lens located?
- Behind the iris
What holds the lens in place?
- Zonules from the ciliary processes
Role of the lens
- Accommodation or changes in shape
Characteristics of the lens
- Transparent, avascular, non-innervated
- Biconvex and refractile
What is the lens composed of?
- Protein and water (50-66%) in a relative state of dehydration
Can you see the fundus if the lens isn’t transparent?
- No you cannot
What are the two parts of the lens?
- Nucleus (center) and cortex
Which part of the lens contains the lens fibers?
- The cortex
WHat are the three main pathologies of the lens?
- Loss of lens transparency
- Lens instability
- Lens perforation
Possible causes of loss of lens transparency
- Nuclear sclerosis or cataract
Possible causes of lens isntability
- Luxation or subluxation
What causes nuclear sclerosis?
- Age related change
- Lens fibers grow throughout life, leading the cortex to become expanded and nucleus to become more compressed and dehydrated
Why does the nucleus of the lens change appearance as it ages and becomes dehdyrated?
- Scatters light differently than the cortex
- THis gives it a hazy appearance
What happens if the nucleus of the lens becomes opaque?
- Nuclear cataract
- Can no longer see the fundus
How can you differentiate nuclear sclerosis from a cataract?
- With nuclear sclerosis you can still see the fundus, which you cannot see with cataracts
At what age do dogs, cats, and horses get nuclear sclerosis?
- Dogs over 6 years
- Cats over 9 years
- Horses over 15 years
Time course of nuclear sclerosis
- Progresses with age
Diagnosing nuclear sclerosis
- Fundus is visible, but must dilate to diagnose
- Cortex of lens will be transparent, and nucleus more dense
Visual impairment associated with nuclear sclerosis
- Animal should still be visual, but may be impaired in dim light
What is a cataract?
- Any OPACITY in the lens
Is a cataract ever normal?
- NO NO
What are the four ways to describe cataracts?
- Age of onset
- Location in lens
- Stage of maturity
- Cause
Age of onset for cataracts
- Congenital
- Juvenile (<6 years)
- Senile (>6 years)
What are the stages of maturity for cataracts?
- Incipient
- IMmature
- Mature
- Hypermature
- Morgagnian
Cause of cataracts**
- Primary (hereditary, no other ocular disease) vs secondary (other ocular disorder exists)
Incipient cataract appearance
- <20% of lens involved
- Scattered opacities
Clinical relevance of incipient cataracts
- Not usually a clinical problem
Immature cataract appearance
- > 20% of lens but tapetal reflex is seen
Clinical relevance of immature cataract
- Clinically significant!
This impairs vision - May cause lens induced uveitis
Mature or hypermature cataract appearance
- Lens opactiy that obstructs visualizing the fundus
- No tapetal reflex seen
Is a patient with a hypermature cataract visual?
- No
- Functionally blind
Clinical relevance of mature and hypermature cataracts
- patient is functionally blind
- These may cause lens induced uveitis
What is the fundamental cause of lens induced uveitis
- Lens proteins outside of the lens capsule are “tolerated” by the ocular immune system in small amounts
- Exposure to increased protein levels from cataracts or lens perforations causes an immune reaction (Lens induced uveitis)
Clinical significance of lens induced uveitis
- Must be medically and/or surgically treated to avoid catastrophic damage to the eye
Signs of LIU
- Cataract or instability
- Pain
- Episcleral injection
- Flare
- KPs
- Miosis
- Pigment on lens capsule
- Secondary glaucoma
What are the four things that we must know about cataracts?
- Is it nuclear sclerosis or cataracts?
- What is causing it?
- Is it causing inflammation?
- It needs to be evaluated by an ophthalmologist
Primary causes of cataracts
- Inherited
- Any age onset
- Any location
- Any degree of maturity
- Most cataracts in dogs are inherited
- Uncommon in cats and horses unless senile
What is the cause of most cataracts in normal dogs?
- Often inherited
SEcondary causes of cataracts (three biggies and some other ones)
- Diabetes mellitus***
- Retinal degeneration (PRA in dogs)**
- Uveitis***
- Also: glaucoma, trauma, toxins, nutritional, electrical shock, hypocalcemia, radiation
Pathophysiology of diabetic cataracts**** KNOW THIS *****
Hyperglycemia –> glucose in aqueous increases –> glucose imbibed into the lens –> overwhelms normal glycolysis pathway –> shunted to sorbital pathway –> aldose reductase –> sorbitols –> osmotic gradient –> water enters the lens and swells fibers
Do cats get diabetic cataracts?
- Most adult cats don’t have aldose reductase and do not develop diabetic cataracts
How quickly does diabetic cataracts occur?
- VERY QUICKLY
WHat % of dogs with DM get cataracts within 6 months and 1 year?
- 50% within 6 months
- 75% within 1 year
What is the most common presenting sign for diabetic dogs?
- Sudden blindness due to cataracts
Treatment of dogs with diabetic cataracts
- ALL DOGS should be treated to prevent lens induced uveitis (LIU)
- REFER EARLY for cataract surgery
Intumescent cataract
- Rapid swelling of lens fibers due to water imbibition
- Y-sutures separate and may cause lens capsule rupture leading to severe LIU
- This is an emergency
- Iris will be pushed forward
Treatment of cataracts
- No effective medical treatment
- May give pred acetate but may not be significant enough
- Treatment is determined by cause and condition of eye
When should cataracts be removed, and how should they be removed?
- Removed if causing LIU or affecting vision
- Remove by phacoemulsification
What do you need to make sure is functional before doing phacoemulsification?
- Must know if the retina is functional
- Dazzle and pupil response will tell you good signs
- Want to make sure the posterior segment is healthy
- To do this, you run an electroretinogram
What do an ERG?
- See if the retina is functioning before surgery
Process of ERG
- Sedate and put electrodes near the eye
- Shine a light at the eye
- If they’re functioning, it will send a response
E. cuniculi cataracts - when are rabbits infected?
- In utero
Appearance of E. cuniculi
- White spot on the pupil
- Can have lens capsule rupture
What causes cataracts with E. cuniculi?
- E. cuniculi within the lens
Sequelae of E. cuniculi cataracts
- Lens capsule ruptures
- Phacoclastic lens induced uveitis
- Sx removal of lens is only treatment at this time
Anterior lens luxation - how quickly should you treat?
- THIS IS AN EMERGENCY
- REFER IT RIGHT AWAY
Primary causes of anterior lens luxation, and who gets it?
- Breed predisposition
- Lens zonule defect
- Jack Russell Terriers get it, Border Collies, Heelers, Siamese cats
Secondary causes of anterior lens luxation
- Chronic uveitis
- Trauma
- Glaucoma
- Senility
Age of patients who get anterior lens luxation?
- young to middle aged
Signs of anterior lens luxation
- Pain
- Lens touching the endothelium damages the cells and leads to corneal edema
Sequelae of anterior lens luxation
- Secondary glaucoma likely
- Emergency referral surgery
- MAYBE can save the eye if it just happened
Posterior lens luxation relative to anterior lens luxation
- Less harmful initially and less painful
Sequelae of posterior lens luxation
- Unstable lens can cause uveitis and glaucoma, as well as retinal detachment
Plan for posterior lens luxation
- Keep pupil miotic with drugs to prevent anterior luxation
- Is this a good idea?
- Anti-inflammatory may be enough
Lens subluxation appearance
- Zonules and lens shifting
Cause of lens subluxation
- Can be caused by glaucoma
Sequela of lens subluxation
- Glaucoma
- Pain
- Uveitis
Treatment for lens subluxation
-REFERRAL
