Small Animal Ophthalmology: What's Your Diagnosis?
By Heidi Featherstone and Elaine Holt
()
About this ebook
Adopting a case-based approach, chapters are built around common ophthalmic presentations and are directed by questions to test the reader's ability to interpret clinical history, ophthalmic photographs and diagnostic results in order to provide differential diagnoses, diagnostic plans and treatment options.
For veterinary students, this book is an ideal guide to how ophthalmology cases are handled in the clinical setting. For veterinary practitioners, it is an innovative and interesting way to increase their knowledge and skills in clinical ophthalmology.
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Small Animal Ophthalmology - Heidi Featherstone
Acknowledgements
As with any book, the finished work is possible only with the help of many people. We are indebted to numerous colleagues, not only for generously contributing case material, but also for their time and advice. For their ophthalmology expertise, we thank Peter Renwick, Christine Heinrich, John Mould, Mike Rhodes, Barbara Braus, Sue Manning, Elizabeth Giuliano, Ellison Bentley, Marnie Ford and David Gould. Peter Renwick and Christine Heinrich deserve special thanks for their generosity in sharing their library of photographs, built up over many years. John Mould also deserves special thanks for his exceptional photographic skills, and for his readiness to share his photographs. With respect to imaging, we are grateful to Paul Mahoney for his tireless help and advice on advanced imaging techniques, and to Ruth Dennis and Rob White for sharing their case material. Emma Scurrell has provided invaluable expertise, along with enthusiasm, on ophthalmic pathology, supported by excellent gross and histological photographs. Simon Scurrell has worked skilfully and painstakingly on the annotated diagrams. Photographing the eyes of conscious dogs and cats is a challenge that requires excellent handling skills and patience: special thanks to David Hindley, Stephanie Ascott, Tom Buckley, Nicola Millington, Lou Hadley and the nursing team at Willows Veterinary Centre and Referral Service. Sue Jenkins’ non-veterinary editing expertise has been invaluable throughout.
Not least, we would like to thank the animals, their owners, and the referring veterinary surgeons, without whom this book would not have been possible.
List of Abbreviations
ACTH
adrenocorticotrophic hormone (adrenocorticotropin)
ALT
alanine aminotransferase
AP
alkaline phosphatase
CCT
corneoconjunctival transposition
CDV
canine distemper virus
CNS
central nervous system
CT
computed tomography
CTT
corneal touch threshold
DIM
diffuse iris melanoma
EOM
extraocular polymyositis
ERG
electroretinography
FCV
feline calicivirus
FeLV
feline leukaemia virus
FHV-1
feline herpesvirus-1
FIV
feline immunodeficiency virus
FNA
fine-needle aspiration
H&E
haematoxylin and eosin
HF-UBM
high-frequency ultrasound biomicroscopy
ICLE
intracapsular lens extraction
IMR
immune-mediated retinitis
IOP
intraocular pressure
IVIg
intravenous therapy with human immunoglobulin
KCS
keratoconjunctivitis sicca
LIU
lens-induced uveitis
MMM
masticatory muscle myositis
MRI
magnetic resonance imaging
Nd : YAG
neodymium-doped yttrium aluminium garnet
NGE
nodular granulomatous episclerokeratitis
NSAID
non-steroidal anti-inflammatory drug
OD
oculus dextor, right eye
OS
oculus sinister, left eye
OU
oculi unitas, both eyes
PCR
polymerase chain reaction
PDT
parotid duct transposition
PHPV
persistent hyperplastic primary vitreous
PLR
pupillary light reflex
PPM
persistent pupillary membrane
PRA
progressive retinal atrophy
prcd
progressive rod-cone degeneration
q
quisque, every, e.g. q4 hours means ‘every four hours’ (six times daily)
RPE
retinal pigment epithelium
RPED
RPE dystrophy
SARDS
sudden acquired retinal degeneration syndrome
SFT
swinging flashlight test
STT
Schirmer tear test
TEL
third eyelid
TBUT
tear film break-up time
TPA
tissue plasminogen activator
UBM
ultrasound biomicroscopy
VOR
vestibulo-ocular reflex
CHAPTER 1
Abnormalities of Globe Size and Position
Introduction
It can be a challenge to differentiate between a change in size and a change in position of the eye. An abnormally small eye (microphthalmos) may be confused with a normal-sized eye that is recessed in the orbit (enophthalmos); an enlarged eye (buphthalmos) may have a similar appearance to a normal-sized eye that is anteriorly displaced (exophthalmos). Assessing the size of the palpebral fissure, position of the third eyelid (TEL) and corneal diameter, looking at the eye from different angles, careful comparison with the other eye and concurrent clinical signs are helpful in differentiating between these conditions.
CASE STUDY 1
History
A 12-year-old female neutered domestic shorthaired cat is presented because of a sudden redness in the right eye. The left eye had looked abnormal for several weeks but appeared comfortable. The cat has recently lost weight and is lethargic.
Questions
1. Describe the abnormalities in Figs. 1.1a, b, and c.
Fig. 1.1a
c01f001aFig. 1.1b
c01f001bFig. 1.1c
c01f001c2. What differential diagnoses should be considered for this presentation?
3. What tests could you perform to make the diagnosis?
Answers
1. What the Figures Show
Fig. 1.1a The left eye appears larger than the right eye; a wide palpebral fissure, increased corneal diameter and clearly visible medial and lateral regions of the limbus are consistent with buphthalmos. There is a generalised corneal opacity which is most dense axially; a tapetal reflection is not visible. In the right eye, the green tapetal reflection is obstructed ventrally by a red/black irregular opacity which appears to be in front of the iris, and there is a similar coloured opacity overlying the iris at the 9 o’clock position. The pupil is moderately dilated.
Fig. 1.1b In the left eye, the Purkinje images are disrupted. There is generalised corneal vascularisation and a stippled area of fluorescein stain uptake axially. The conjunctival vessels overlying the sclera on the lateral aspect of the globe are congested. The iris is difficult to see well but appears darker (medially) and possibly thickened. In the right eye, there is hyphaema; the regions of the iris that are visible appear normal.
Fig. 1.1c Oblique view from the lateral aspect of both eyes. In the left eye there is an irregular contour and anterior protrusion of the cornea (OS > OD). There is increased exposure of the sclera and conjunctiva, and episcleral congestion. The anterior chamber is obliterated by abnormal iris tissue which appears to be displaced anteriorly. In both eyes fluorescein dye is visible on the periocular hair at the medial canthus.
2. Differential Diagnoses
Given the history and the appearance of the left eye, the following conditions should be considered:
Chronic glaucoma In contrast to the dog, primary glaucoma in the cat is rare, and secondary glaucoma is more common. The most common causes of secondary glaucoma in the cat are chronic idiopathic lymphocytic-plasmacytic uveitis and primary intraocular neoplasia, most notably diffuse iris melanoma. Typical clinical signs include buphthalmos, conjunctival and episcleral congestion, corneal oedema, mydriasis, and impaired or absent vision. Buphthalmos can be difficult to discern in the cat and assessment of the size of the palpebral fissure can be helpful because it becomes wider as the size of the eye increases. Glaucoma in cats is typically insidious in onset and is often difficult to recognise. This is in contrast to canine primary glaucoma which is characterised by peracute pain, episcleral congestion, marked corneal oedema, mydriasis and blindness (Ch.6, case 2).
Exophthalmos Anterior displacement of the globe within the orbit. Common causes of exophthalmos in the cat include orbital neoplasia, orbital cellulitis/abscess and orbital trauma (haematoma, emphysema, fracture, foreign body). Primary malignant neoplasia and abscesses secondary to dental disease are more likely in old cats, whereas head trauma and orbital foreign bodies are more common in young cats (Ch.12, case 2).
Given the appearance of the right eye, the following conditions should be considered:
Systemic hypertension Sustained systemic hypertension is commonly associated with ocular manifestations which primarily involve the posterior segment but also affect the anterior segment. Abnormalities in the posterior segment involve the retina, choroid and vitreous humour and appear as retinal oedema and bullae, retinal and intravitreal haemorrhages, retinal detachment and increased tortuosity of the retinal arterioles. Intraocular haemorrhage can occur as a result of haemorrhage from the iris (Fig. 1.1d), ciliary body, retina, and choroid. Extensive hyphaema can lead to the formation of anterior and posterior synechiae and secondary glaucoma.
Coagulopathy and platelet disorders Ocular haemorrhage can be a clinical sign of a coagulopathy or a platelet disorder. Ocular haemorrhage typically occurs when the platelet count is <50 000 cells/µl.
Uveitis When there is a breakdown of the blood-aqueous barrier during inflammation, red blood cells can enter the anterior chamber (hyphaema). The blood may form either a homogenous layer in the ventral anterior chamber or a clot, as in this cat.
Trauma Ocular haemorrhage may result from both blunt and penetrating ocular trauma (Ch.12, cases 2 and 3).
Pre-iridal fibrovascular membrane (PIFM) The formation of fibrovascular membranes on the anterior iris is usually a consequence of intraocular inflammation, haemorrhage and/or hypoxia due to the release of vasoactive substances. Hence the formation of PIFMs is common in eyes with chronic uveitis, intraocular haemorrhage, retinal detachment, glaucoma, and neoplasia. The newly formed blood vessels within the membranes are fragile and can cause spontaneous and recurrent hyphaema. PIFMs can extend into the filtration angle and result in secondary glaucoma. Fibrovascular membranes are not restricted to the surface of the iris – they can also form on the retina and optic disc and in the vitreous.
Neoplasia (primary or secondary) Intraocular haemorrhage may occur in eyes affected with primary or secondary neoplasia, either originating from a PIFM or as a result of the direct effect of neoplasia (e.g. adverse effect on clotting function).
Congenital anomalies These include persistent hyaloid artery and persistent hyperplastic primary vitreous, both of which are rare conditions in the cat.
Fig. 1.1d Aneurysm within the lateral region of the major arterial circle in the left eye of a cat with systemic hypertension.
c01f001d3. Appropriate Diagnostic Tests
Ocular reflexes
Pupillary light reflex – the left pupil is not visible. Negative consensual OS (from left to right eye); positive direct OD, albeit slow and incomplete.
Dazzle reflex – negative OS, positive OD
Palpebral reflex – positive OU, OS < OD
Corneal reflex – positive OU, OS < OD
Menace response – negative OS, equivocal OD
In this cat, these results are consistent with blindness, reduced corneal sensation and lagophthalmos in the left eye, and reduced vision in the right eye.
Examination with a focal light source – in the left eye, slit-lamp biomicroscopy reveals extensive superficial and deep corneal vascularisation, and generalised corneal oedema and fibrosis which is most marked axially.
Ophthalmoscopy – in the right eye, this reveals an extensive dorsal retinal detachment, most marked within the medial quadrant, and multiple retinal haemorrhages of different sizes throughout the tapetal fundus and ventral to the optic disc (Fig. 1.1e).
Schirmer tear test – 4 mm/min OS, 10 mm/min OD
Fluorescein dye – negative staining OD, positive staining in the superficial axial cornea OS. This is indicative of suboptimal ocular surface health in the left eye, most likely because of the lagophthalmos.
Tonometry – IOP 35 mmHg OS, 20 mmHg OD
Fig. 1.1e Extensive dorsal retinal detachment and multiple retinal haemorrhages throughout the tapetal fundus and ventral to the optic disc.
c01f001eThere is increased resistance to retropulsion of the left eye; retropulsion of the right eye is normal. The remainder of the ophthalmic examination reveals no additional abnormalities. A general physical examination reveals an underweight body condition and mild dental disease.
The degree of resistance to retropulsion of the eye varies amongst species and between breeds. The normal feline globe is generally retropulsed less than the normal canine globe because of close apposition between the globe and the orbit in the cat. The degree of retropulsion in brachycephalic breeds is less than in other breeds because of the shallow orbit, in both cats and dogs.
Further Diagnostic Tests
B-mode ocular ultrasound – this is indicated to evaluate the posterior segment when the anterior segment is opaque, and to take measurements. Axial globe length is the distance between the centre of the cornea and the posterior pole.
The ultrasound scan reveals an axial globe length of 22 mm OS and 19 mm OD (within normal limits), which confirms buphthalmos in the left eye. Additional abnormalities in the left eye include hyperechoic material spanning the anterior chamber (consistent with blood, fibrin or anterior synechiae), hyperechoic material within the vitreous (consistent with vitreal degeneration, intravitreal haemorrhage, neoplasm), and convex iris leaflets which contact the anterior lens capsule (consistent with iris bombé, Ch.9, case 4) (Fig. 1.1f).
Laboratory tests – results of routine haematology, biochemistry (including electrolytes), urine analysis and thyroid function are consistent with chronic renal failure.
Systemic blood pressure measurement – indirect assessment with a Doppler sphygmomanometer (ultrasonic detection device) reveals a systolic blood pressure of 220 mmHg (upper limit for systolic blood pressure in the cat is 160–170 mmHg).
Fig. 1.1f Horizontal B-mode ultrasound scan (left eye). There is hyperechoic material spanning the anterior chamber (white asterix) and within the vitreous body (blue asterix); convex iris leaflets contact the anterior surface of the lens (arrow). In this cat, these changes are consistent with intraocular haemorrhage, anterior synechiae, and iris bombé.
c01f001fDiagnosis
Based on the information available, a diagnosis of systemic hypertension is made. The ocular manifestations are hyphaema and hypertensive retinopathy in the right eye, and glaucoma secondary to intraocular haemorrhage in the left eye.
Treatment
The preferred first-line treatment for feline systemic hypertension is amlodipine besylate (a calcium channel blocker) at a dose of 0.625–1.25 mg per os q24 hours. The aim of treatment is to lower the systolic blood pressure to a safe range, i.e. ≤160–170 mmHg. Some cats need more frequent dosing (amlodipine besylate q12 hours) and others require the addition of benazepril to become normotensive. Adverse effects of amlodipine besylate are uncommon but include azotaemia, lethargy, hypokalaemia, reflex tachycardia and weight loss.
Symptomatic treatment for hyphaema can be considered with topical corticosteroid therapy, e.g. 1% prednisolone acetate q8–12 hours. Enucleation of the left eye in this cat is indicated because of pain, irreversible blindness and to prevent complications arising from progressive corneal disease. A transconjunctival procedure is performed. In addition to the clinical findings, ocular histopathology reveals retinal detachment and confirms the clinical diagnosis of glaucoma secondary to extensive intraocular haemorrhage caused by the systemic hypertension (Fig. 1.1g).
Fig. 1.1g (A) H&E section of a normal feline retina demonstrating a normal retinal arteriole (arrow) (×200). (B) H&E section of the detached retina in this hypertensive cat demonstrating two abnormal retinal arterioles (arrows) (×400). The arteriolar walls are diffusely expanded by a bright pink homogenous matrix. This is referred to as hyalinisation and results from the leakage of plasma products into the vessel wall secondary to endothelial damage. Note the haemorrhage dissecting the vessel wall (asterix) and the perivascular haemorrhage. Thickening of the vessel walls has resulted in extreme narrowing of the blood vessel lumens which are now barely visible; this degree of narrowing could result in ischaemic injury. The histopathological diagnosis is retinal and choroidal hyalinising arteriolosclerosis with intraocular haemorrhage characteristic of hypertensive retinopathy.
Reproduced with permission from EJ Scurrell.
c01f001gRepeated thorough physical and ophthalmic examinations as well as blood pressure measurements are recommended, e.g. every 3–6 months.
All enucleated globes should be submitted for ocular histopathology. For routine diagnostic purposes, fixation of the globe in 10% formalin is generally appropriate, although confirmation with the chosen laboratory is recommended. Prior to fixation, as much excess extraocular tissue as possible should be removed, and the optic nerve should be left as long as possible.
Prognosis
The prognosis is good for retinal detachment secondary to systemic hypertension in that most retinas reattach if antihypertensive therapy is successful at lowering the blood pressure sufficiently. The prognosis for vision is variable because it depends on the extent and duration of retinal detachment prior to treatment, as well as the severity of any associated haemorrhage. There is evidence to suggest that the feline retina begins to degenerate within the first week of detachment. However, most cats only present when they are severely visually impaired or blind, by which stage both eyes are affected. The retinal pathology is often chronic in the eye that is affected first but is only noted when the fellow eye develops significant disease. Even if vision is not restored, continued treatment of the systemic hypertension is imperative to minimise progressive disease of other target organs (brain, heart, kidney).
Discussion
Systemic hypertension is a relatively common disease in cats older than 10 years and is usually associated with chronic renal failure, and less frequently, with hyperthyroidism and Conn’s syndrome. As the eye is a target organ for hypertensive damage, the most common reason for presentation is acute blindness secondary to retinal detachment. Neurological deficits may be present and are generally the result of cerebrovascular disease. Prolonged hypertension initially leads to arteriolar vasoconstriction, manifested as narrowing and increased tortuosity of the retinal arterioles, and finally to compromise of the vascular integrity. This in turn leads to intraocular haemorrhage as well as retinal oedema and an accumulation of serous fluid which separates the neurosensory retina from the underlying retinal pigment epithelium. The ocular changes progress over several months and early diagnosis of ‘at risk’ cats is important in preventing blindness. Ideally any geriatric cat should have an annual blood pressure assessment together with a complete ocular examination including fundic examination. Cats with e.g. renal disease or hyperthyroidism should be monitored particularly closely.
Further Reading
See Appendix 2.
CASE STUDY 2
History
A 9-month-old male Labrador Retriever is presented because both eyes have looked different for several months. There has been no evidence of ocular discomfort and the dog catches balls well. The dog has received his primary vaccination course and routine anthelmintic treatment and is reported to be clinically well.
Questions
1. Describe the abnormalities and pertinent normal features in Figs. 1.2a and b.
Fig. 1.2a
c01f002aFig. 1.2b Right eye has received a topical mydriatic agent (tropicamide).
c01f002b2. What differential diagnoses should be considered for this presentation?
3. What tests could you perform to make the diagnosis?
Answers
1. What the Figures Show
Fig. 1.2a Left eye – is normal and shows an iris colour variation consisting of a mid brown outer zone and a dark brown pupillary zone. Right eye – has a small palpebral fissure; there is protrusion of the TEL. The pupil is small which creates a subtle anisocoria (OD < OS). The iris is slightly dark compared to the left eye. A tapetal reflection is not visible.
Fig 1.2b Both eyes are shown – the right pupil has been artificially dilated with tropicamide. Right eye – there is a structure which comprises multiple strands of iris tissue. The strands originate from the iris collarette and join at a single focal point. A cataract is present, resulting in leukocoria (white pupil).
2. Differential Diagnoses
Given the history and appearance of the right eye, the following conditions should be considered:
Microphthalmos This is a congenital anomaly in which the eye is abnormally small and deeply set within the orbit and has a range of concurrent defects including persistent pupillary membrane (PPM) remnants, cataract, retinal dysplasia, staphyloma, and nystagmus.
Nanophthalmos A congenital anomaly in which the eye is abnormally small but otherwise normal.
Phthisis bulbi Acquired end-stage atrophy of the eye following severe inflammation, ocular trauma or glaucoma. Typical features include an absence of visible signs of inflammation, an opaque cornea which prevents intraocular examination and marked hypotony (Fig. 1.2c).
Enophthalmos An eye that is recessed in the orbit, causes of which include:
Pain Stimulation of the ophthalmic branch of the trigeminal nerve results in globe retraction by the retractor bulbi muscle which leads to enophthalmos and passive TEL protrusion. Conditions such as entropion and corneal ulceration often cause enophthalmos secondary to ocular surface pain. Ocular surface pain is also manifested by blepharospasm and increased lacrimation (Ch.5).
Horner’s syndrome Interruption of the sympathetic innervation of the eye, eyelids and orbital smooth muscle resulting in miosis, anisocoria, ptosis, narrow palpebral fissure, enophthalmos, and TEL protrusion (Ch.10, case 1).
Reduced volume of orbital tissue This can arise because of dehydration, weight loss (reduction in orbital fat) or fibrosis of orbital tissues following orbital inflammation or surgery.
Fig. 1.2c Phthisis bulbi secondary to chronic uveitis in the right eye of a Tibetan Terrier. Note the third eyelid protrusion, increased scleral show, absence of external signs of inflammation, and diffuse corneal fibrosis.
c01f002c3. Appropriate Diagnostic Tests
Ocular reflexes
Pupillary light reflex – positive direct and consensual OU
Dazzle reflex – positive OU
Menace response – positive OS, negative OD
In this dog, these results are consistent with absent vision but some retinal and optic nerve function in the right eye.
Examination with a focal light source – in the right eye, slit-lamp biomicroscopy shows that the structure originating at the iris collarette converges at a focal point on the anterior lens capsule, consistent with a PPM.
Tonometry – IOP 15 mmHg OU
B-mode ocular ultrasound – this is indicated to evaluate the posterior segment when the anterior segment is opaque, and to take measurements. Axial globe length is the distance between the centre of the cornea and the posterior pole. Axial lens length is the distance between the centre of the anterior and posterior lens capsules.
The ultrasound reveals an axial globe length of 20.8 mm OS (within normal limits) and 18.5 mm OD, which confirms microphthalmos in the right eye (Fig. 1.2d). The right lens is hyperechoic and slightly smaller than the left lens (axial length 7.1 mm compared to 7.3 mm, both within normal limits); the hyperechogenicity is consistent with a cataract.
Fig.1.2d Horizontal B-mode ultrasound scan. (A) Right eye – axial globe length 18.5 mm, axial lens length 7.1 mm; hyperechoic lens. This is consistent with microphthalmos and cataract. (B) Left eye – axial globe length 20.8 mm, axial lens length 7.3 mm. C, cornea; L, lens; P, posterior limit of globe; ‘x’ and ‘+’ represent points of measurement for the axial length of lens and globe respectively.
c01f002dThere is no change in the appearance or apparent comfort level of the right eye following the application of topical anaesthetic eye drops, which rules out enophthalmos because of ocular surface pain. The position of the TEL in the right eye does not change following the application of topical 1% phenylephrine, which makes Horner’s syndrome an unlikely cause of the TEL protrusion (Ch. 10, case 1). The remainder of the ophthalmic examination reveals no additional abnormalities and a general physical examination is unremarkable.
A single drop of a topical anaesthetic will anaesthetise the ocular surface (conjunctiva and cornea) within approximately 10 s. Anaesthesia lasts for about 45 min in the normal dog eye (25 min in the cat). The depth and duration of anaesthesia can be increased by the repeat application of the topical anaesthetic, e.g. one drop applied twice over one minute. The application of a topical anaesthetic can be a simple way of differentiating surface ocular pain from pain caused by intraocular or orbital disease.
Diagnosis
Based on the information available, a diagnosis of microphthalmos in the right eye is made.
Treatment
No treatment is indicated for the majority of eyes affected with microphthalmos. In a small number of dogs, recurrent conjunctivitis may develop because of poor tear drainage and/or entropion because of poor eyelid-to-globe apposition. Conjunctivitis is usually managed conservatively with topical lubricant and antibiotic therapy; entropion should be surgically corrected. Congenital