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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
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What are the components of the lens? (5)
capsule, epithelium, cortex, nucleus, zonules
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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
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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
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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
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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
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Patients with rapid onset, bilateral cataracts MUST be examined for ___________.
diabetes mellitus
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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
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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
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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
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Traumatic cataracts resulting from penetrating injuries often result in...
a localized, non-progressive cataract
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Traumatic cataracts resulting from blunt trauma and inflammation usually result in...
a more severe, progressive cataract.
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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)
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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)
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What are the different ways to describe the location of cataracts? (5)
- capsular
- 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)
- axial
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What locations of cataracts are in metabolically active/ actively growing parts of the lens and are more likely to progress?
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Many toxic, radiation, and metabolic cataracts will appear first at the __________.
equatorial location
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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
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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
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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
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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
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Dogs are more likely to get cataracts from __(2)__ causes; cats are more likely to get cataracts from __________ causes.
- hereditary, metabolic
- inflammatory
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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
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What is vitreous degeneration?
- vitreous is usually hypoechoic on US
- with VD, vitreous becomes hyperechoic and swirls on US (increased risk factors)
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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)
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Adjunctive diagnostic procedures should include __(2)__ prior to surgery to
ensure and normal, functioning posterior segment.
ultrasound and electroretinography
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What are the steps of cataract surgery?
incision--> capsulorhexis--> +/- hydrodissection--> phacoemulsification--> cortical aspiration--> implant artificial lens--> closure
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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)
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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
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What are causes of secondary lens luxation? (3)
trauma, glaucoma, uveitis
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What are clinical signs of lens luxation? (6)
- iridodenesis
- phacodenesis
- aphakic cresent
- corneal edema (focal or diffuse depending on anterior or posterior)
- anterior: painful, epiphora, blepharospasm, redness
- posterior: maybe no overt clinical signs
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All patients with lens luxations MUST have what diagnostic test done?
measure intraocular pressure!
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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
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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
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What are complications of lens luxation? (4)
diffuse corneal edema, glaucoma, anterior uveitis, pain
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