In the embryo, the lens originates from the ____________; the ____________ surrounds the lens prior to the immune system being competent because...
surface ectoderm; lens capsule; the lens is not perceived to be as part of self, and exposure to the lens proteins will result in an inflammatory response
What are the components of the lens? (5)
capsule, epithelium, cortex, nucleus, zonules
Describe growth of the lens throughout life and the clinical implications of this.
anteriorly, below the lens capsule, there is a lens epithelium that grows continuously throughout life--> lens can't get larger, so it just gets denser--> all animals > 6yo have lenticular sclerosis
Describe the barrier of the lens capsule.
- semi-permeable membrane
- anteriorly, the lens is bathed in aqueous humor, from which it receives nutrition and removes waste
- posteriorly, it is in contact with the vitreous
Describe the vascularity of the lens and the physiologic implications of this. Does this have clinical implications?
- the lens is AVASCULAR
- it relies on the aqueous humor for the supply of nutrients and removal of waste products
- abnormalities of the aqueous (anterior uveitis) can result in lens abnormalities
Describe diagnostic procedures of the lens. (4)
- dilate the pupil with tropicamide
- penlight exam: animals with cataracts should have normal PLR (cataracts will never interfere with the afferent arm of the PLR)
- fundic exam:abnormalities of the retina can lead to cataracts or prevent cataract sx in the future
- +/- biomicroscopy: referral only
Patients with rapid onset, bilateral cataracts MUST be examined for ___________.
Describe the center of rotation concept for examining the eye.
- abnormalities anterior to the center of rotation move in the same direction as the eye
- abnormalities posterior move in the opposite direction
Describe lenticular sclerosis clinically.
- all dogs and cats >6 years old have lenticular sclerosis (diagnosed based on signalment!)
- it should not prevent examinations of the fundus
- no txt required
A cataract is any ___________, and potential etiologies include... (7)
- loss of lens transparency
- Hereditary: not always present at birth
- Traumatic: penetrating injury, blunt injury, inflammation resulting from trauma
- Metabolic: most likely DM
- Toxic: not a huge differential
- Nutritional: Esbilac in orphaned puppies
- Radiation: gamma treatment of malignant neoplasia of the head
- Electric: shock from wires, etc
Traumatic cataracts resulting from penetrating injuries often result in...
a localized, non-progressive cataract
Traumatic cataracts resulting from blunt trauma and inflammation usually result in...
a more severe, progressive cataract.
Describe the pathophysiology of diabetic cataracts.
over abundance of glucose in aqueous humor--> glucose metabolism shifts to the sorbitol pathway, where glucose is instead reduced to sorbitol--> sorbitol cannot exit the lens, so it accumulates--> osmotic shift--> water drawn into lens, resulting in swelling and disruption of the fibers--> bilateral, rapid onset cataracts (not necessarily symmetrical)
What are the most common locations of damage from gamma radiation to the head? (4)
- eyelids and conj (inflammation, necrosis, alopecia, depigmentation)
- cornea (ulceration)
- lens (equatorial cataracts)
- retina (hemorrhage, uveitis)
What are the different ways to describe the location of cataracts? (5)
- cortical (likely to progress b/c very actively growing part of lens)
- nuclear (always congenital, not always hereditary)
- equatorial (most likely to progress b/c most metabolically active part of lens)
What locations of cataracts are in metabolically active/ actively growing parts of the lens and are more likely to progress?
Many toxic, radiation, and metabolic cataracts will appear first at the __________.
What are the different stages of progression of cataracts? (4)
- incipient: very small area of the lens affected with a cataract
- immature: larger but still permits examination of the fundus with pupil dilation; some visual disturbance
- mature: involves entire lens, fundus cannot be seen, will result in blindness
- hypermature: describe the state of lens fibers (not amount affected); undergoing liquefication and leaking into aqueous--> lens-induced uveitis
Describe hypermature cataract pathophysiology.
cataract undergoing liquefication--> lens capsule is semi-permeable, so lens protein enters aqueous humor--> lens is not recognized as part of self, so body mounts an inflammatory response--> lens-induced uveitis
The significance of lens-induced uveitis is... (2)
- uveitis may require treatment to prevent discomfort and secondary changes, such as glaucoma
- an eye with LIU has an increased risk associated with cataract sx
What is the only treatment for cataracts?
- SURGERY (phacoemulsification) +/- artificial lens implantation (always recommended if want return to almost normal vision)
- this is a referral procedure and is not to be done in private practice
Dogs are more likely to get cataracts from __(2)__ causes; cats are more likely to get cataracts from __________ causes.
- hereditary, metabolic
What is the "dazzle reflex"?
- subcortical reflex; shine a bright light in the eye and it’s so physically abrasive that the patient has to look away
- abnormal if absent
What is vitreous degeneration?
- vitreous is usually hypoechoic on US
- with VD, vitreous becomes hyperechoic and swirls on US (increased risk factors)
What is a medical management option for people who can't afford cataract surgery?
topical intraophthalamic NSAIDs (delay and prevent PIFM, control lens-induced uveitis)
Adjunctive diagnostic procedures should include __(2)__ prior to surgery to
ensure and normal, functioning posterior segment.
ultrasound and electroretinography
What are the steps of cataract surgery?
incision--> capsulorhexis--> +/- hydrodissection--> phacoemulsification--> cortical aspiration--> implant artificial lens--> closure
Primary lens luxation occurs commonly in _________, which are predisposed breeds.
terriers (there's a genetic test- homozygotes WILL be affected b/w age 4-8; consider lens removal before lens luxates in these animals)
How do you know if it's anterior or posterior lens luxation?
- anterior: lens covers part of the iris
- posterior: can see the back of the eye without dilating pupil
What are causes of secondary lens luxation? (3)
trauma, glaucoma, uveitis
What are clinical signs of lens luxation? (6)
- aphakic cresent
- corneal edema (focal or diffuse depending on anterior or posterior)
- anterior: painful, epiphora, blepharospasm, redness
- posterior: maybe no overt clinical signs
All patients with lens luxations MUST have what diagnostic test done?
measure intraocular pressure!
What is treatment for anterior luxation of the lens?
- surgical emergency if acute and primary
- ciliary sulcus fixation with a new lens sutured in place to posterior iris
What is the treatment for posterior lens luxation? (4)
- surgical removal
- miotic agents (trap then lens in the back and monitor pressures until glaucoma results)
- +/- no therapy
- usually eventually needs enucleation or prosthesis
What are complications of lens luxation? (4)
diffuse corneal edema, glaucoma, anterior uveitis, pain