Histologic and immunohistochemical predictors of clinical behavior for feline diffuse iris melanoma

SOX-10 and TRP-1 expression in feline ocular and nonocular melanomas

In felines, ocular and nonocular melanomas are uncommon tumors that represent a diagnostic challenge for pathologists, especially when amelanotic. To date, the immunohistochemical diagnostic panel in cats is based on specific melanocytic markers (Melan-A and PNL2) and a nonspecific but sensitive marker (S100). In human medicine, SOX-10 is reported to be a sensitive antibody for the detection of melanoma micrometastasis in the lymph node. TRP-1, an enzyme involved in melanogenesis, has recently been used in humans and dogs as a specific melanocyte marker. The aim of this study was to evaluate the cross-reactivity and the expression of SOX-10 and TRP-1 antibodies in feline normal tissue and melanocytic tumors. Thirty-one cases of ocular, cutaneous, and oral melanomas were retrospectively evaluated and confirmed by histopathological examination and by immunolabeling with Melan-A and/or PNL2. SOX-10 nuclear expression in normal tissues was localized in epidermal, subepidermal, hair bulb, and iridal stromal melanocytes and dermal nerves. In melanomas, nuclear expression of SOX-10 was detected in ocular (11/12; 92%), oral (6/7; 86%), and cutaneous sites (12/12; 100%). TRP-1 cytoplasmic immunolabeling in normal tissue was observed in epidermal and bulbar melanocytes and in the lining pigmented epithelium of the iris and in its stroma. Its expression was positively correlated to the degree of pigmentation in the tumor and was observed in 75% of ocular (9/12), 43% of oral (3/7), and 33% of cutaneous melanomas (4/12). This study demonstrated the cross-reactivity of SOX-10 and TRP-1 antibodies in feline non-neoplastic melanocytes and their expression in ocular and nonocular melanomas.

Mitotic activity: A systematic literature review of the assessment methodology and prognostic value in feline tumors

Increased proliferation is a driver of tumorigenesis, and quantification of mitotic activity is a standard task for prognostication. This systematic review is an analysis of all available references on mitotic activity in feline tumors to provide an overview of the assessment methods and prognostic value. A systematic literature search in PubMed and Scopus and a nonsystematic search in Google Scholar were conducted. All articles on feline tumors that correlated mitotic activity with patient outcome were identified. Data analysis revealed that of the 42 eligible articles, mitotic count (MC, mitotic figures/tumor area) was evaluated in 39 studies, and mitotic index (MI, mitotic figures/tumor cells) in 3 studies. The risk of bias was considered high for most studies (26/42, 62%) based on small study populations, insufficient details of the MC/MI methods, and lack of statistical measures for diagnostic accuracy or effect on outcome. The MC/MI methods varied between studies. A significant association of MC with survival was determined in 20 of 28 (71%) studies (10 studies evaluated other outcome metrics or provided individual patient data), while 1 study found an inverse effect. Three tumor types had at least 4 studies, and a prognostic association with survival was found in 5 of 6 studies on mast cell tumors, 5 of 5 on mammary tumors, and 3 of 4 on soft-tissue sarcomas. MI was shown to correlate with survival for mammary tumors by 2 research groups; however, comparisons to MC were not conducted. Further studies with standardized mitotic activity methods and appropriate statistical analysis for discriminant ability of patient outcome are needed to infer the prognostic value of MC and MI.

A Pilot Study to Evaluate the Usefulness of Optical Coherence Tomography for Staging Iris Pigmented Lesions in Cats

This study investigated the utility of optical coherence tomography (OCT) for staging iris pigmented lesions in cats. Eighteen cats that underwent OCT examination for unilateral iris pigmented lesion were included. The cats were either suspected of melanosis due to clinical features (n = 8) or had been definitively diagnosed through histopathology with iris melanosis (n = 3), early feline diffuse iris melanoma (FDIM) (n = 4), or mid-stage or advanced FDIM (n = 3). From OCT images, mean iris thickness (MIT) was measured, and the ratio of pigmented lesion to normal iris (PN) was calculated. OCT images depicted the entire iris layer in all eyes with suspected melanosis, iris melanosis, and early FDIM, but observing the entire lesion in mid-stage/advanced FDIM was challenging. No significant difference in MIT was observed among the groups. Conversely, PN ratio was significantly higher (p < 0.05) in early FDIM (1.29 ± 0.16) than in suspected melanosis (1.02 ± 0.10) or iris melanosis (0.99 ± 0.09). Furthermore, OCT imaging revealed hyperreflective lines in 75% of eyes with suspected melanosis and in all the eyes with iris melanosis, corresponding to the pigmented lesions. Our results demonstrate that OCT is capable of detecting subtle differences in iris thickness and features in early-stage FDIM, indicating its potential utility in distinguishing between iris melanosis and early FDIM. Further study is warranted to verify the reliability of such OCT findings.

Metastatic extradural melanoma of the lumbar spine in a cat

A 7-year-old neutered male Domestic shorthair cat, with a 1.5-year history of left eye enucleation secondary to a diffuse iris malignant melanoma, was evaluated for progressive onset of pelvic limb paresis and ataxia with severe thoracolumbar hyperaesthesia and dysorexia. Neurological examination localised a lesion to the T3-L3 spinal cord segments. Magnetic resonance imaging of the thoracolumbar spine showed a well-defined extradural T1-weighted hyperintense non-contrast-enhancing mass, initially suggesting a potential haemorrhagic component. Exploratory surgery revealed a brownish extradural lumbar mass. Histologic examination concluded to a melanoma, most probably metastatic given the animal's previous medical history. This report highlights the importance of collecting a complete medical history, which can help in obtaining a preliminary differential diagnosis in cats with clinical signs of myelopathy. Although the location of this metastasis is particularly unusual both in human and veterinary medicine, making optimal treatment challenging for neurosurgeon, our increased understanding of immune and tumour cell biology during the past decade is likely to improve the future treatments of feline melanoma and its metastases.

Canine and feline uveal melanocytic tumours: Histologic and immunohistochemical characteristics of 32 cases

Objective

Gross, histopathological, and immunohistochemical characteristics of uveal melanocytic neoplasms in dogs and cats were investigated.

Samples

Thirty‐two enucleated globes with uveal melanocytic neoplasms, 27 from dogs and 5 from cats, were examined.

Procedures

Morphological characteristics of uveal melanocytic neoplasms in dogs and cats were evaluated with anti‐PNL2, anti‐Melan‐A, anti‐Ki‐67, anti‐caspase‐3, and anti‐BAP1 immunomarkers. Statistical analysis was performed to compare canine melanocytomas and melanomas.

Results

The 32 uveal neoplasms were classified as melanocytomas (19/27 in dogs) or melanomas (8/27 in dogs, 5/5 in cats). Most tumours (84%) were located in the anterior uvea. Neoplastic cells were classified as epithelioid, spindle‐shaped, mixed, or special type (balloon and signet ring cells). The percentage of cells with melanin, melanin concentration within cells, anisocytosis and anisokaryosis, mitotic count, lymphocytic inflammation, necrosis, vascular invasion, and glaucoma were also characterized. Anisocytosis, percentage of neoplastic cells with melanin, mitotic count, and indices (proliferation and apoptotic) varied significantly between canine uveal melanomas and melanocytomas; in general, melanomas had greater cell variability, were less pigmented, and had a higher mitotic count. The melanocytic origin of the neoplasms was confirmed by positive anti‐PNL2 immunolabelling (29/32) and positive anti‐Melan‐A immunolabelling (3/32). In canine uveal melanomas, anisocytosis and anisokaryosis correlated with less pigmentation and minimal pigmentation correlated with a high percentage of immunolabelling for caspase‐3.

Conclusions

Uveal melanocytomas were more common in dogs, and uveal melanomas were more frequent in cats. Anisocytosis, percentage of neoplastic cells with melanin, and mitotic count are important histologic characteristics of malignancy to evaluate in uveal melanocytic neoplasms. The proliferation and apoptotic indices are relevant when comparing malignant tumours with benign tumours.

Feline Uveal Melanoma Review: Our Current Understanding and Recent Research Advances

Melanocytic neoplasia is the most common form of ocular tumour in cats, accounting for 67% of cases in an analysis of 2614 cases of primary ocular neoplasia. Feline diffuse iris melanoma (FDIM) is by far the most common form of ocular melanocytic neoplasia, with limbal melanomas and atypical melanoma (melanoma affecting the choroid or ciliary body) infrequently recognised. Early lesions begin as flat areas of pigmentation of the iris, known as iris melanosis. This melanosis is a precursor lesion that can become FDIM when pigmented cells infiltrate the anterior iris stroma, commonly alongside a transition in cell morphology. The differentiation between FDIM and benign iris melanosis is only recognisable though histologic examination, with no in vivo means of identifying the malignant transformation. The behaviour of FDIM is variable and difficult to predict. Some FDIM lesions have a more benign progression and can slowly grow or remain static for years without affecting the ocular or systemic health of the individual, whilst other tumours behave aggressively, invading the ocular structures and significantly affecting the life expectancy of cats through metastatic disease. This makes management and timely enucleation of these cases challenging in practice. This article aims to review our current knowledge of FDIM.

Current Therapeutics and Future Perspectives to Ocular Melanocytic Neoplasms in Dogs and Cats

Neoplasms of melanocytic origin are diseases relevant to dogs and cats' ophthalmic oncology due to their incidence, potential visual loss, and consequent decrease in life quality and expectancy. Despite its non-specific clinical presentation, melanocytic neoplasms can be histologically distinguished in melanocytomas, which present benign characteristics, and malignant melanomas. The diagnosis often occurs in advanced cases, limiting the therapeutic options. Surgery, cryotherapy, radiotherapy, photodynamic therapy (PDT), and laser are currently available therapeutic strategies. As no clinical guidelines are available, the treatment choice is primarily based on the clinician's preference, proficiency, and the owner's financial constraints. While surgery is curative in benign lesions, ocular melanomas present a variable response to treatments, besides the potential of tumour recurrences or metastatic disease. This review presents the currently available therapies for ocular melanocytic neoplasms in dogs and cats, describing the therapeutic, indications, and limitations. Additionally, new therapeutics being developed are presented and discussed, as they can improve the current treatment options.

Tumor-infiltrating lymphocytes (TILs) in feline melanocytic tumors: A preliminary investigation

The presence and the role of tumor-infiltrating lymphocytes (TILs) in different types of tumors, but particularly in melanoma, has become more and more investigated during the last decade, both in human and veterinary medicine. Melanocytic tumors are quite rare in cats, with diffuse iris melanoma being the most commonly diagnosed in this species. The aim of this study was to characterize the lymphocytic infiltration in feline melanocytic tumors and to analyze their association with the histological features of malignancy recognized in these tumors, as well as with the expression of the most commonly used immunohistochemical markers. Thirty-eight feline melanocytic tumors were retrospectively selected; histological and immunohistochemical characterization of the tumors (histologic criteria of malignancy; S100, Melan A, and PNL2 expression) and of TILs (presence/absence, density, distribution, and grade; CD3, CD20 expression) were performed and associations between them tested. Results showed that TILs grade increased with cellular pleomorphism (P < 0.05) and, within the group of cutaneous melanocytic tumors, also with the mitotic count (P < 0.05). On the other hand, TILs grade was inversely associated with the percentage of neoplastic cells positive for Melan A (P < 0.05) and PNL2 (P < 0.05). Both CD3⁺ and CD20⁺ lymphocytes increased significantly with TILs grade and in association with mitotic count, when stratified in low/high quantity. This preliminary study suggests that TILs in feline melanoma may be associated with histologic features of malignancy and loss of melanocytic-specific markers, such as Melan A and PNL2. Further studies, with a larger cohort and follow-up information, are recommended.

Iris biopsy to investigate feline iris hyperpigmentation

OBJECTIVE

To evaluate the benefit of iris biopsy in cats with iris hyperpigmentation to differentiate melanosis from early feline diffuse iris melanoma (FDIM).

METHODS

The medical records of cats with unilateral iris hyperpigmentation that had undergone iris biopsy between February 2013 and September 2016 at Willows Veterinary Centre & Referral Service were reviewed.

RESULTS

Seven cats with unilateral iris hyperpigmentation were included in this retrospective study. The biopsy procedure was performed under general anesthesia (n = 7) with neuromuscular blockade (n = 6) following pre-operative topical miotic therapy (n = 5). One to six biopsy samples per eye were harvested from areas of hyperpigmentation. The samples were partial thickness (n = 4 eyes) and full thickness (n = 3 eyes). Complications were minor: mild intra-operative hemorrhage (n = 4), fibrin clot (n = 2), corneal ulcer (n = 1), post-operative ocular hypertension (n = 1), dyscoria (n = 1), and pseudopolycoria (n = 2). The first biopsy was diagnostic in six cats; a repeat biopsy was necessary in one cat. Histopathology was consistent with melanosis in five cats and with early FDIM in two cats. Screening for signs of metastatic disease (thoracic computed tomography and abdominal ultrasonography) was negative in the two cats with a preliminary diagnosis of early FDIM. Subsequent enucleation and histopathology confirmed the initial diagnosis in both cases.

CONCLUSIONS

Iris biopsy in cats with iris hyperpigmentation can be beneficial to differentiate melanosis from early FDIM and thereby help to justify the decision for early enucleation.

Bleaching melanin in formalin-fixed and paraffin-embedded melanoma specimens using visible light: a pilot study

In fluorescence microscopy, light radiation can be used to bleach fluorescent molecules in formalin-fixed and paraffin-embedded (FFPE) samples, in order to increase the ratio between signal of interest and background autofluorescence. We tested if the same principle can be exploited in bright field microscopy to bleach pigmented melanoma FFPE sections together with cell morphology maintenance. After dewaxing and rehydration, serial FFPE sections of a feline diffuse iris melanoma, a canine dermal melanoma, a gray horse dermal melanoma and a swine cutaneous melanoma were irradiated with visible light for 1, 2, 3, 4 and 5 days, prior to Hematoxylin & Eosin staining. Complete bleaching was obtained after 1-day treatment in feline and swine melanomas, while 2 and 3 days were required in canine and equine neoplasms, respectively. In all treated samples, cell morphology was maintained. Photo-induced bleaching combined with immunohistochemistry was tested after a 3-day photo-treatment using five different markers. According to the literature, in all samples neoplastic cells stained positive for vimentin, S100 and PNL2, while negative for FVIII and pancytokeratin. In conclusion, visible light can be effectively exploited to bleach pigmented melanoma FFPE sections prior to perform routine histochemical and immunohistochemical stains.

Nonocular Melanocytic Neoplasia in Cats: Characterization and Proposal of a Histologic Classification Scheme to More Accurately Predict Clinical Outcome

Nonocular melanocytic neoplasia is considered uncommon in cats yet is routinely encountered in diagnostic pathology and recognized to exhibit a wide variation in biological behavior. Accurate prediction of clinical outcomes is challenging with no widely recognized prognostic criteria. Signalment and tumor location were retrospectively evaluated in 324 cats diagnosed with nonocular melanocytic neoplasia. Histologic features were described in 141 neoplasms and outcome data were available in 79 cases. Immunohistochemistry using Melan-A, PNL-2, cyclooxygenase 2 (COX-2), and E-cadherin was performed in a subset (n = 24). Multivariate analysis identified tumor site, mitotic count, and the presence of intratumoral necrosis to be independent predictors of tumor-related death. On the basis of these findings, we propose a novel histologic grading scheme in which nonocular melanocytic neoplasms involving the lips, oral or nasal mucosa, or nasal planum are considered high grade if they fulfill 1 or both of the following criteria: at least 4 mitoses in 10 high-power fields (HPF) or presence of intratumoral necrosis; those arising elsewhere are considered high grade if they fulfill both of the above criteria. Of 79 tumors with outcome data, 43 (54%) were low grade and 36 (46%) were high grade. The grading system had an 80% sensitivity and 92% specificity for predicting tumor-related death in this population of cats. Median survival for cats with low-grade tumors was not reached, and the median survival was 90 days for those with a high-grade tumor. PNL-2 and Melan-A were sensitive markers for feline nonocular melanocytic neoplasia, and although not significantly associated with prognosis, a large proportion expressed COX-2, suggesting a potential therapeutic role for COX-2 inhibitors.

Protein expression of KIT, BRAF, GNA11, GNAQ and RASSF1 in feline diffuse iris melanomas

Feline iris melanoma, the most common feline intraocular tumour, has a reported metastatic rate of 19-63%. However, there is a lack of knowledge about its molecular biology. Previous studies have reported that feline iris melanomas do not harbour mutations comparable to common mutations found in their human counterpart. Nevertheless, there are differences in the gene expression patterns. The aim of this study was to investigate the protein expression of B-RAF oncogene serine/threonine kinase (BRAF), G protein subunit alpha q (GNAQ) and 11 (GNA11), KIT proto-oncogene receptor tyrosine kinase (KIT), and Ras association family member 1 (RASSF1) in feline iris melanomas. Fifty-seven formalin-fixed paraffin embedded (FFPE) iris melanomas and 25 FFPE eyes without ocular abnormalities were stained with antibodies against the respective proteins using immunofluorescence. Averaged pixel intensities/μm² and percentage of stained area from total tissue area were measured and the results were compared. Compared to the control group, iris melanomas showed overexpression of BRAF, GNAQ, GNA11 and KIT. The higher expression of BRAF, GNAQ, GNA11 and KIT in feline iris melanomas suggest that these proteins may play a key role in the development of feline iris melanomas and KIT may present a possible target for future therapies in cats with feline iris melanomas.

Circulating cell-free DNA does not harbour a diagnostic benefit in cats with feline diffuse iris melanomas

OBJECTIVES

Feline diffuse iris melanoma (FDIM) is the most common malignant primary intraocular tumour in cats, with reported metastases rates between 19% and 63%. Currently, the only available diagnostic tool for a tentative diagnosis is histopathological examination of the enucleated eye. Therefore, the veterinary ophthalmologist is often faced with the dilemma of whether to enucleate an oftentimes visual eye or to continue monitoring, with the risk of metastases developing. In the past, cell-free DNA (cfDNA) gained more attention in human medicine, especially in the field of oncology. Prior studies have shown the use of cfDNA as diagnostic or prognostic markers in canine and human cancer patients. Therefore, the aim of this study was to investigate cfDNA concentration and integrity in cats with FDIMs compared with cats with benign iris naevi and without ocular abnormalities.

METHODS

cfDNA from plasma of cats with iris melanoma (n = 34), iris naevus (n = 30) and without ocular abnormalities (n = 32) were extracted. Primer and probes for feline amyloid beta precursor protein ( APP) and beta actin ( ACTB) were designed for amplicons of various lengths and quantitative PCRs of extracted cfDNA were performed to measure cfDNA concentration and integrity of the plasma samples. Differences of cfDNA concentrations and integrity levels between the three groups (iris melanoma, iris naevi and controls) were analysed using the Mann-Whitney U-test.

RESULTS

cfDNA concentration and integrity analysis revealed no significant differences between the cats with iris melanoma, iris naevus or the control group ( P >0.01). Cats with metastases showed similar cfDNA concentration and integrity to cats without metastases.

CONCLUSIONS AND RELEVANCE

cfDNA concentration and integrity seem to be insufficient as a diagnostic or prognostic marker in cats with FDIMs.

Consider the eye in preventive healthcare - ocular findings, intraocular pressure and Schirmer tear test in ageing cats

OBJECTIVES

Geriatric health screening in cats is highly recommended. However, information about normal and abnormal findings is scarce, especially regarding the eyes of ageing cats. This prospective study examined the influence of the ageing process on vision and ocular structures in older cats.

METHODS

A total of 209 cats (aged 9-24 years) underwent physical examination, vision assessment, slit lamp biomicroscopy and ophthalmoscopy. Systolic blood pressure (SBP) measurement, Schirmer tear test (STT) reading and rebound tonometry were performed. Systemic disease was not a criterion for exclusion.

RESULTS

Vision was good in 157/209 cats (75.1%) and impaired in 52/209 cats (24.9%). Increasing age and the occurrence of vision impairment were not statistically associated ( P = 0.053). Retinal oedema, retinal haemorrhage and/or retinal detachment (19 cats) and glaucoma (12 cats) were the most common findings in vision impaired cats. Increasing age was significantly associated with the occurrence of lenticular sclerosis ( P = 0.01) and attenuated retinal vessels ( P = 0.02). Increasing age and SBP were significantly associated with the occurrence of retinal detachment, haemorrhage and oedema ( P <0.001 each). In cats without evidence of hypertensive ocular damage, younger cats had a tendency for higher SBP values than older cats, although this difference was not significant. Mean intraocular pressure (IOP) was 16.5 ± 5.0 mmHg. Age did not significantly affect the IOP values ( P = 0.54). Mean STT was 15.8 ± 4.8 mm/min. The STT was found to increase with age ( P = 0.025).

CONCLUSIONS AND RELEVANCE

Although vision impairment is not a clinical sign of old age in cats, age-related changes may contribute to vision-threatening diseases. This study contributes to preventive healthcare by examining the influence of the ageing process on vision and ocular structures in older cats.

Prospective study of ocular and periocular diseases in animals: 188 cases

ABSTRACT: Histopathology of the ocular and periocular tissues submitted for diagnosis and research is still incipient in Brazil, in contrast to veterinary clinical ophthalmology. In this study, ocular and periocular tissues from domestic and wild animal species, mainly from the state of Minas Gerais, Brazil, were evaluated between February 2012 and September 2015. The samples were analyzed grossly and microscopically. Histochemistry and immunohistochemistry were performed on some of the samples. The frequency, type of ocular alteration, affected animal species, and affected ocular or periocular tissues were recorded. One hundred eighty-eight ocular and periocular tissues from domestic and, occasionally, wild animals were examined. Nine animals presented two concurrent alterations, adding up to 197 alterations. Proliferative lesions were the most frequent (92), followed by traumatic (43), inflammatory (37), degenerative (18), developmental (4) and vascular/hemorrhagic diseases (3). The globe was the most affected structure (112), followed by eyelids (52), third eyelid (17), bulbar conjunctiva (14) and retrobulbar region (2). Neoplasms arising from periocular tissues were the most frequent alteration (60), possibly related to a more active surgical service and histopathologic evaluation request. Many animals presented ocular lesions that reflected systemic diseases, which were diagnosed by necropsy and examination of other organs. Particularly in cases of neoplasia, early detection and surgical treatment can prevent systemic involvement. Ocular histopathologic evaluation can provide better characterization and prognosis of the clinical-pathological condition of the patient as well.

Cyclooxygenase-2 expression in the eyes of cats with and without uveitis

OBJECTIVE To characterize the distribution and intensity of cyclooxygenase (COX)-2 expression in the eyes of cats with and without uveitis and to determine whether COX-2 expression is correlated with severity of inflammation. SAMPLES Archived ocular tissue specimens from 51 cats with and 10 cats without ocular disease. PROCEDURES Specimens from only 1 eye were evaluated for each cat. Specimens were stained with H&E stain or immunohistochemical stain for detection of COX-2 and reviewed. For each eye, the type, severity, and distribution of inflammation and the distribution and intensity of COX-2 expression were determined for the uvea and other ocular tissues. Correlation between COX-2 expression and inflammation severity was also assessed. RESULTS COX-2 was not expressed in any nondiseased eye. Of the 51 diseased eyes, 20 had histologic evidence of lymphocytic-plasmacytic uveitis, 13 had neutrophilic uveitis, 11 had diffuse iris melanoma with uveitis, and 7 had diffuse iris melanoma without uveitis. Of the 44 eyes with uveitis, COX-2 was detected in the uvea of 16, including 11 eyes with lymphocytic-plasmacytic uveitis, 4 with neutrophilic uveitis, and 1 with diffuse iris melanoma-induced uveitis. Inflammation was severe, moderate, or mild in 10, 5, and 1 of those eyes, respectively. Cyclooxygenase-2 was detected in the cornea of 21 eyes with uveitis and 1 eye with diffuse iris melanoma without uveitis. Uveitis severity was positively correlated with COX-2 expression in both the uvea and cornea. CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that COX-2 is an inflammatory mediator in feline uveitis but not diffuse iris melanoma.

Eye examination in the cat: Step-by-step approach and common findings

PRACTICAL RELEVANCE

The ability to perform a complete eye examination in the cat is critically important for patients with an ocular disorder or a systemic disease, as well as for the geriatric cat.

CLINICAL CHALLENGES

Cats may need short breaks between portions of the eye exam in order to minimize stress. For the clinician, use of ophthalmic equipment and interpretation of normal vs abnormal findings may take some practice in order to develop proficiency.

AUDIENCE

This review is aimed at veterinary practitioners and outlines all the steps and equipment necessary to perform a complete ophthalmic examination in the cat.

EQUIPMENT

Although some specialized equipment is required, a complete eye exam can be performed with a modest investment in equipment and supplies.

EVIDENCE BASE

This article draws on published references for normal parameters measured in the feline eye exam, as well as the experience of the authors.