Computed Tomographic Grading of Ear Diseases in Guinea Pigs (Cavia porcellus)

ABSTRACT

Ear disease affecting the external and middle ears is a significant health concern in guinea pigs, which can be clinically silent and, if left untreated, can lead to severe and irreversible complications. Computed tomography (CT) is the primary imaging modality used to diagnose otitis in other species, particularly otitis media (OM). This study aimed to develop a CT‐based grading scale to assess the severity of CT changes in guinea pigs with otitis. As a prerequisite to this, a detailed, subjective CT‐based anatomical characterization of the normal guinea pig ear was conducted to establish a foundational reference framework essential for the construction and application of the grading system. The study included head CT of 60 guinea pigs from 2 institutions. After reviewing normal CT anatomy, a novel grading system was created for otitis externa (OE) and OM, with a scale of I–II for OE and I–V for OM, and applied to the guinea pig population by five reviewers. The level of agreement among reviewers was statistically assessed, revealing an excellent intra‐observer agreement and a moderate inter‐observer agreement between evaluators, with higher reliability for OM (κ = 0.84) than OE (κ = 0.75). This novel CT‐based grading scale provides a systematic approach for evaluating the severity of ear disease in guinea pigs. However, the complexity of guinea pig ear anatomy and the variability in the structure of the tympanic bullae among healthy individuals can make grading particularly challenging.

1. Introduction

Ear disease is a significant health concern in guinea pigs, encompassing otitis externa (OE), media (OM), and interna (OI). If left untreated, it can lead to severe and irreversible complications ranging from neurological deficits to recurrent general health disorders, such as gastrointestinal stasis, which can result in the death of the animal [1, 2]. Guinea pigs serve as important models for human otologic studies due to their ear anatomy, particularly the labyrinth system, which closely resembles that of humans [3]. OM has been recognized as a major disease in laboratory guinea pigs since 1976 [4, 5, 6, 7, 8], with reported prevalence rates of 25%–40% [9, 10, 11]. However, its description in veterinary literature remains limited, and the much lower prevalence of 0.8% reported in pet guinea pigs is likely an underestimation [1, 12, 13, 14].

Understanding the normal ear anatomy of guinea pigs is crucial to effectively diagnose and classify OM. CT imaging, as an advanced diagnostic tool, provides detailed cross‐sectional images of the ear, allowing for precise visualization of the external, middle, and osseous structures of the inner ear. Although studies in other species, particularly rabbits [1, 12], have focused on the use of advanced imaging to describe ear structures, limited data are available on the normal ear anatomy of guinea pigs on computed tomography (CT).

Otitis in guinea pigs is often caused by bacterial infections, though other factors, such as trauma, allergies, respiratory problems, and dental disease, can also play a role [13, 15]. The condition often involves the middle ear and can be accompanied by OE and/or OI. Diagnosing otitis in guinea pigs can be challenging, as they are commonly asymptomatic [1, 9, 13, 16]. Current diagnostic methods include otoscopy and radiography. However, these techniques are limited in assessing the severity and extent of the disease. For example, otoscopic examinations have been used to observe changes in the tympanic membrane's appearance [6]. Unfortunately, this examination is not accurate, as an intact or normal‐looking tympanic membrane does not exclude middle ear disease [17, 18, 19]. This is partly due to the different routes of transmission for OM, which can occur following the progression of OE through the tympanic membrane, secondary ascending infection via the Eustachian tube from respiratory tract and dental infections, or by hematogenous spread [17, 18, 20].

In rabbits, a well‐established grading scale exists to assess the severity of otitis on the basis of CT findings [20]. However, no such grading system currently exists for guinea pigs. The lack of a standardized grading scale for otitis in guinea pigs limits the ability to compare clinical cases, evaluate treatment efficacy, and give a prognosis. This article proposes a novel grading scale for otitis in guinea pigs, adapted from rabbits but tailored to the unique anatomical features and clinical manifestations observed in guinea pigs.

The primary objective of this study is to develop a novel grading scale for otitis in guinea pigs, similar to the one used in rabbits, which will allow for a standardized, reliable assessment of otitis severity. To support this aim, a detailed characterization of the normal CT anatomy of the external and middle ear structures was conducted as an essential prerequisite, providing the necessary anatomical baseline for accurate interpretation and application of the proposed grading system.

2. Materials and Methods

2.1. Case Selection

A multicenter retrospective study was conducted from January 2014 to May 2023 within the database of the Ecole Nationale Vétérinaire d'Alfort teaching hospital and between January 2017 and May 2023 from the referral hospital, CHV Fregis. Guinea pigs with any clinical complaint were included in the study if they had one available head CT with at least a bone algorithm, with or without a pre‐ and post‐contrast soft tissue algorithm. Repeat scans from the same individual were excluded. In cases where images of other regions were acquired, only the head was assessed. For each patient, medical records were analyzed to identify the age, sex, presenting complaint, and the presence or absence of ear disease on the basis of physical examination findings and CT results, and other CT findings were also retrieved from the conclusion of the original CT report, as well as any other investigations, notably, culture with bacterial susceptibility tests.

2.2. CT Image Acquisition

CT studies of the head were performed with the animal placed in sternal recumbency under general anesthesia or sedation, depending on the clinician's preference, before and, when available, after intravenous injection of nonionic iodinated contrast medium (2 mL/kg of iohexol at 300 mgI/mL; Omnipaque, GE Healthcare, SAS, Velizy‐Villacoublay, France). The post‐contrast phase acquisition began approximately 60 s after medium injection. At the teaching hospital, a 64‐slice scanner (Brilliance CT; Philips Healthcare, Amsterdam, the Netherlands) was used for studies performed prior to December 2019 (n = 16), and an 80‐slice scanner (Aquilion Lightning SP 80; Canon Medical Systems Corporation, Otawara‐Shi, Tochigi, Japan) (n = 28) was used after that. At the referral hospital, CT images were acquired using a 16‐slice helical system (Brivo CT 385; GE Healthcare, Beijing, China). Image acquisition parameters at the teaching hospital, prior to December 2019, consisted of tube rotation time 0.5 s, 250 mA, and 120 kV, and from December 2019 on, consisted of pitch 0.650, tube rotation time 0.75 s, 150 mA, and 120 kV. Images were obtained using both soft tissue (1.5 mm slice thickness before 2019, 1 mm after 2019) and bone (0.8 mm slice thickness prior to 2019, 0.5 mm after 2019) reconstruction algorithms. CT images were reviewed using commercially available DICOM medical image processing software (Osirix MD, Pixmeo SARL, Geneva, Switzerland).

2.3. Normal CT Anatomy of the Ears

As a detailed CT description is currently lacking in veterinary literature, guinea pigs without any clinical complaint related to otitis and with unequivocally normal external and middle ears on CT were used to establish an anatomical description of the external and middle ears, by consensus between an ECVDI board‐certified radiologist (HH) and an ECVDI resident (MB), both experienced in guinea pig CT interpretation.

2.4. Ear Disease CT Grading

The CT anatomical description of the ears served as a preamble and reference point for the development of a CT grading scale to assess ear disease. This scale was established following consensus between two observers (HH and MB), with clinical relevance confirmed through consultation with a board‐certified specialist in small mammal medicine and surgery (CP) (Tables 1 and 2). For OM, a grade of I–V was established (I = mild, V = severe disease), and for OE, a grade of I–II was established.

TABLE 1.

Computed tomography (CT) grading system for external ear canal disease, descriptive reference.

Grade	Description
I	Material filling the external ear canal, without obliteration of the tympanic membrane.
II	Material filling the external ear canal, with obliteration of the tympanic membrane.

Note: This grading system categorizes the extent of material accumulation in the external ear canal, with distinction on the basis of whether the tympanic membrane remains intact or is obliterated.

TABLE 2.

Computed tomography (CT) grading system for otitis media, descriptive reference.

Grade	Description
I	Material content a in the RC only, without bone lysis
II	Material content a in the CC. Normal wall thickness and shape of the CC IIa: Material only in the CC; RC is normal/absent b IIb: Material in both compartments
III	Material content a in the CC with hyperostosis IIIa: Material only in the CC; RC is normal/absent b IIIb: Material in both compartments
IV	Material content a in the TB with either osteolysis of the TB or inner ear without intracranial communication or concentric hyperostosis of the inner ear with obliteration of fluid‐filled spaces (without osteolysis)
V	Material content a in the TB. Osteolysis of the calvarium base with intracranial communication ± invasion

Note: Grading is based on the presence and distribution of material content within the tympanic bulla (TB), involvement of the rostral (RC) and caudal compartments (CC), and evidence of structural changes, such as hyperostosis or osteolysis.

^a Partial or complete soft tissue/fluid attenuating material occupation.

^b Wall thickness/degree of rostral pneumatization not considered.

The proposed grading scale was evaluated by different observers with different qualifications to assess its level of agreement between users. CT images for all guinea pigs (n = 60) during the study period were retrieved, randomized, and blindly evaluated independently by five observers: two ECVDI board‐certified radiologists (HH and EG), one ECZM (small mammal) specialist (CP), one ECVDI resident (MB), and one ECZM (small mammal) resident (PM). Each observer applied the proposed grading system to each ear (N = 120), and the data were recorded. To verify intra‐observer variability, one reviewer (ECVDI resident) repeated the evaluation at a 1‐month interval.

2.5. Statistical Analysis

In this study, the reliability of OM and OE grading was assessed through intra‐observer and inter‐observer agreement using kappa coefficients. Intra‐observer reliability was measured using Cohen's weighted kappa (quadratic weighting for ordinal variables) for a single observer's consistency across repeated measurements at a 1‐month interval. Inter‐observer reliability was assessed using Cohen's weighted kappa, calculated as the average of pairwise kappa coefficients (weighted when the variable was ordinal), to evaluate the level of agreement among multiple observers. The kappa values were calculated for various parameters, including material content (MC) in the tympanic bulla (TB), compartment affected, hyperostosis of the caudal compartment (CC), lysis of the TB, lysis or concentric hyperostosis of cochlea/vestibulum, lysis of calvarium base, and the presence or absence of intracranial invasion. The reliability of each observer was assessed using MedCalc Statistical Software version 19.6.3 (https://www.medcalc.org 2021) for Cohen's kappa.

3. Results

3.1. Case Selection

A total of 922 guinea pigs were seen at the teaching hospital, with 44 having a head CT performed. At the referral hospital, 555 were seen, with 16 having a CT performed. All studies included a bone and soft tissue algorithm; 30 cases had a post‐contrast study available. On the basis of CT findings, 24 guinea pigs were presumed healthy (Grade 0 for both OE and OM).

The 36 remaining guinea pigs (25 from the teaching hospital and 11 from the private practice) were diagnosed on CT with presumed otitis. Out of the 36 cases with otitis, 25 (71%) were intact males and 11 (29%) were intact female guinea pigs. Their ages ranged from 5 to 60 months, with a median of 36 months. Their body weight ranged from 350 to 1245 g with a mean of 870 g. Out of the 36 cases with otitis, 30 cases had medical management alone, and 6 had combined surgical and medical management, with 4 having a myringotomy and 2 having a total ear canal ablation with lateral bulla osteotomy (TECALBO). Six bacterial culture and sensitivity tests were performed. The samples were collected during surgery (two during TECA‐LBO and four during myringotomy). A monoculture of bacteria was identified for each sample and included Streptococcus pneumoniae (n = 2), Staphylococcus pseudointermedius (n = 1), Pseudomonas sp. (n = 1), and gram‐positive aero‐anaerobic bacilli (n = 1). One sample was negative. No cases had histology performed.

3.2. Normal CT Anatomy of the Ears

The external ear canal of guinea pigs comprises a tortuous canal. Although most of the canal is cartilaginous, the portion closest to the TB is ossified, formed by the tympanic ring of the temporal bone and fused to the lateral aspect of the TB [21, 22, 23]. Mineralization of the pre‐tympanic segment of the canal was consistently observed in our study and corresponds to small semicircular accessory bones, known as the pars tympanica accessoria [4, 21]. These accessory bones border the bony external auditory canal ventrorostrally. Their number can vary slightly; in our study, we generally observed 3–4 (Figure 1).

FIGURE 1.

Normal anatomy of the external ear canal of healthy guinea pigs. High‐frequency algorithm (bone window) in the transverse plane viewing the normal CT appearance of the right and left external ear canal. The external ear canal (dotted arrow), bony portion of the external canal (arrow), and accessory bones of the pre‐tympanic segment (arrowheads).

The TB is a prominent, air‐filled cavity formed by an extension of the temporal bone at the ventrolateral base of the skull [3, 12, 24, 25]. It is further divided into two main internal compartments, designated rostral and caudal in this article, by a thin bony septum (Figure 2). The rostral compartment (RC) is smaller in size, variably developed/pneumatized, and situated rostro dorsally in relation to the CC. The CC is more voluminous, making up the larger portion of the bulla, and is located caudoventrally to the RC; it presents a smaller caudal recess that extends dorsally and which is also variably pneumatized (Figure 2). These compartments have also been referred to previously as the dorsal and ventral compartments, respectively; however, this appeared somewhat confusing to the authors, as the ventral compartment has a dorsal recess [24]. The TBs and their compartments show variability in volume among guinea pigs. Amongst healthy patients, the degree of pneumatization can differ significantly, especially for the RC, with some animals showing symmetrical lack of pneumatization and others demonstrating asymmetrical pneumatization. When hypopneumatization was identified, the lumen was filled by varying degrees of trabecular bone, with attenuation values exceeding 200–300 HU. Lastly, as previously described, communication between the rostral and CCs of the TB is generally observed, although the number and size of the communications are variable [26].

FIGURE 2.

Normal anatomy of the tympanic bulla of healthy guinea pigs. High‐frequency algorithm (bone window) in (A) dorsal and (B) sagittal planes, viewing the normal CT appearance of the right and left tympanic bullae. Rostral compartment (arrows) and caudal compartment (arrowheads). Communication between the rostral and caudal compartments (dotted arrow).

3.3. Establishment of a CT Grading Scale

To determine if the external or middle ears were diseased, the authors based their decision on the presence or absence of MC in the external ear canal and within the TBs. External ear canal wall thickness and enhancement were not assessed due to inconsistent availability of post‐contrast images. The grading scale was inspired by the one currently used in rabbits and tailored to guinea pigs [20]. For the external ear canal, Grade I corresponds to the presence of content within the external canal without obliteration of the tympanic membrane (Figure 3A). Grade II reflects the accumulation of soft tissue material within the external ear canal with obliteration of the tympanic membrane (Figure 3B).

FIGURE 3.

CT imaging of external otitis severity grades in guinea pigs. High‐frequency algorithm (bone window) in the transverse plane, viewing the abnormal CT appearance of otitis externa. (A) Grade 1 otitis externa with material content in the external ear canal without tympanic membrane obliteration (white arrow). (B) Grade 2 otitis externa with material content in the external ear canal obliterating the tympanic membrane (arrowheads).

Regarding the middle ear, the different parameters contributing to the grading system were as follows: the presence or absence of MC with soft tissue/fluid attenuating material within the TB, with further distinction as to which compartment was affected, and evidence of hyperostosis and osteolysis of the different compartments. Given the variations in pneumatization of the RC, only hyperostosis of the CC was considered.

The different grades were defined as follows, and a summary is found in Table 2:

In Grade I, MC is confined to the RC of the TB, with no evidence of osteolysis (Figure 4A). Grade II involves material within the CC of the TB, without any osseous remodeling. This grade is further subdivided: In Grade IIa, the material is restricted to the CC, and the RC appears normal or is absent (Figure 4B); in Grade IIb, material is present in both the RC and CC (Figure 4C). Grade III is characterized by the presence of material in the TB along with hyperostosis of the CC. This grade also has two subtypes: Grade IIIa, where material is limited to the CC, and the RC is normal or absent (Figure 4D); and Grade IIIb, where both compartments contain material (Figure 4E). Grade IV describes material in the TB accompanied either by osteolysis (Figure 4G) of the TB or inner ear without intracranial communication or by concentric hyperostosis of the inner ear that obliterates its fluid‐filled spaces (Figure 4F). Finally, Grade V includes cases where material is present in the TB alongside osteolysis of the base of the calvarium, with evidence of intracranial communication ± invasion (Figure 4H).

FIGURE 4.

CT imaging of otitis media grades in guinea pigs. High‐frequency algorithm (bone window) in the transverse plane, viewing the abnormal CT appearance of otitis media. (A) Grade 1 otitis media with MC in the RC only (arrow). (B) Grade 2a otitis media with MC in the CC only, RC pneumatized (open arrowhead). (C) Grade 2b otitis media with MC in both compartments (RC open arrowhead; CC arrow). (D) Grade 3a otitis media with MC in the CC only (arrow), hyperostosis (arrowhead) and hypo‐pneumatized RC (open arrowhead). (E) Grade 3b otitis media with MC in both compartments (RC open arrowhead; CC arrow) with hyperostosis (arrowhead). (F) Grade 4 otitis media with concentric hyperostosis of the inner ear with obliteration of fluid‐filled spaces (arrowhead). (G) Grade 4 otitis media with osteolysis of the inner ear (dotted arrow). (H) Grade V otitis media with severe osteolysis of the inner ear and intracranial communication (dotted arrow). CC, caudal compartment; MC, material content; RC, rostral compartment.

Given inter‐individual variations complicating the interpretation of certain factors, additional consensual guidelines have been provided: In cases where one TB appeared thin‐walled and contained MC, whereas the contralateral bulla was thickened but empty, the thickened, empty bulla was considered “normal.” In this case, the presence of MC in the thin‐walled bulla determined disease classification, whereas the empty thickened bulla was considered an incidental/conformational finding or hyperostotic secondary to previous otitis.

Two scenarios have been left open to interpretation for bulla hyperostosis: cases of both TB symmetrically full and thickened, and cases of both TB filled and asymmetrically thickened. In the latter case, the thickest bulla was considered to be hyperostotic, whereas the other was left open to interpretation.

3.4. Application of the CT‐Grading Scale

Grading is based on the presence and distribution of MC within the TB, involvement of the RC and CC, and evidence of structural changes, such as hyperostosis or osteolysis. A total of 36/60 animals were diagnosed with otitis, including 3/36 with OE, 16/36 with OM, and 17/36 with both.

3.4.1. Otitis Externa

In the external ear canal, the presence of fluid/soft tissue‐attenuating material within the lumen was evaluated using CT imaging. Of the 36 guinea pigs with otitis, 20/36 (53%) had OE (alone or with OM), and its presence was graded on the basis of CT appearance (Table 3) for each individual ear, as Grade I (n = 17/120) or Grade II (n = 11/120). Only 3/36 cases exclusively had OE, with one presenting related clinical signs.

TABLE 3.

Computed tomography (CT) grading results of otitis externa.

Grade	Number of ears (N = 120)
0	92
1	17
2	11

3.4.2. Otitis Media

Of the 36 guinea pigs diagnosed with otitis, 33 (10 females and 23 males) had OM, accounting for 53 individual diseased ears. The frequency for each assigned grade per ear is summarized in Table 4, and the results are as follows: Grade I (n = 2), Grade IIa (n = 13), Grade IIb (n = 12), Grade IIIa (n = 5), Grade IIIb (n = 6), Grade IV (n = 8), and Grade V (n = 7). Bilateral OM was observed in 20 of the guinea pigs (20/33), whereas 13 were unilateral. In our study population, 11/33 of guinea pigs with OM had vestibular signs, including head tilt, vertiginous crisis, ataxia, and facial paralysis. These signs were observed across various grades of OM (Grade IIa: n = 2; Grade IIb: n = 2; Grade IV: n = 4; and Grade V: n = 3).

TABLE 4.

Distribution of otitis media by grade.

Grade	Number of middle ears (n = 120)	Percentage (%)
Grade 0	67	55.80
Grade 1	2	1.70
Grade 2a	13	10.80
Grade 2b	12	10.00
Grade 3a	5	4.20
Grade 3b	6	5.00
Grade 4	8	6.70
Grade 5	7	5.80
Total	120	100

3.5. Statistical Analysis

The results show varying levels of reliability for intra‐ and inter‐observer agreements for both proposed grading systems (OE and OM).

For OM, the weighted intra‐observer reliability for the final grade score was high (κ = 0.97), demonstrating excellent consistency within individual observers. Similarly, intra‐observer reliability remained strong across the various parameters, with weighted Cohen's kappa values ranging from 0.72 to 1. Inter‐observer reliability was more variable, ranging from fair to excellent, with an excellent average overall kappa value of 0.8 for the final grade score. Inter‐observer agreement varied the most when comparing the different parameters, with the average of Cohen's kappa values ranging from 0.49 to 0.80, indicating moderate to excellent consistency. Notably, parameters like “MC in TB” showed the highest inter‐observer agreement (0.80), whereas others like “lysis of TB” and “hyperostosis of CC” showed lower reliability (around 0.55–0.58). These discrepancies highlight potential challenges in achieving consistent agreement across different raters for certain complex clinical features.

For OE grading, intra‐observer reliability was high (weighted κ = 0.95), whereas inter‐observer reliability was more variable, with an overall average of Cohen kappa of 0.75 and parameter‐specific values ranging from 0.25 to 0.81. The parameter “MC in the external ear canal with obliteration of the tympanic membrane” showed the highest inter‐observer agreement (0.81), whereas “MC without obliteration of the tympanic membrane” had the lowest (0.25), suggesting significant disparity in observer agreement for this parameter.

4. Discussion

This study aimed to develop a CT‐based grading system for otitis in guinea pigs, which required defining the normal CT anatomy of the external and middle ear in this species. It highlights the challenges of diagnosing and grading otitis in this species, particularly given their complex middle ear anatomy and significant inter‐individual anatomical variations. This complexity is especially evident in cases where thickening of the TBs occurs without definitive signs of active disease.

Our proposed grading system for the external ear is simplified compared to the one proposed by Richardson et al. in rabbits, with only two grades, due to the absence of lateral pouching of the external ear canal in this species or the visualization of tympanic membrane deviation. In rabbits, there is the ability to form a pocket or diverticulum located between the cartilage of the acoustic meatus and the tragus within which pus/cerumen can accumulate, reflecting more severe disease; this was never observed in our population [19, 20]. Despite the simplicity of the OE grading system, Grade I had the lowest inter‐observer agreement (0.25), reflecting variability in radiologists’ sensitivity to minimal or equivocal soft‐tissue content. Although such cases pose interpretative challenges, the clinical context, often available in practice, helps guide imaging interpretation. To improve the interpretation of these discrepancy cases, standardizing contrast studies could aid in the detection of ear canal mucosal enhancement, wall thickening, irregularity, and periauricular soft tissue swelling/inflammation.

Although some may question whether differentiating between two grades of OE is clinically relevant or potentially confusing, it is important to consider the practical implications. Maintaining these two nuanced grades allows clinicians to more efficiently guide treatment decisions; for instance, distinguishing the higher grade, which may require surgical cleaning under anesthesia, from those that are manageable with local topical therapy administered by the owner seems relevant. Moreover, when examining the data more closely, observers generally agree when grading severe otitis as Grade II, further supporting the usefulness of this distinction.

Regarding OM, five grades and subgrades were necessary due to the complexity of the anatomy of the TB in this species, especially the presence of two separate compartments. Among the assessed parameters, TB hyperostosis posed one of the greatest challenges, leading to the most disparity among observers (𝜅 = 0.54) even when comparing radiologists versus exotic specialists. Hyperostosis of TBs was noted in multiple contexts. Although it is commonly associated with chronic OM, it was also observed in healthy guinea pigs and in empty TBs. The reason for this is unclear, and authors suspect that hyperostosis could be a sequela of past otitis or could have a conformational/breed component. There may be conformational abnormalities or dysmorphic features that superficially resemble brachycephalic breeds, even though this was not clearly observed in our population. It has also been established in humans that calvarial hyperostosis can occur in metabolic disorders, such as hypovitaminosis A [27]. Although this is not a typical feature of guinea pigs with hypovitaminosis A, it could be considered in cases with thickened calvarium and symmetrical thickening of the TB [14]. Despite grading guidelines for CC hyperostosis, some cases required subjective interpretation due to the lack of an objective thickness cutoff, particularly when bullae were filled but asymmetrically thickened, making it difficult to assess whether the thinner side was truly normal. To address these ambiguities, the authors deliberately excluded hyperostosis of the RC as a criterion due to the incidental variability in its pneumatization. They suggest that this factor significantly influenced inter‐observer repeatability. Despite this, the interpretation of the hyperostosis of the CC alone still presented a disparity.

Other factors may have influenced the results. The authors determined that when both TBs were symmetrically thickened but only one contained material, the filled bulla was not hyperostotic, and it was more likely a conformational/incidental finding. According to the grading criteria used in this study, a TB without MC was not considered diseased, even if thickening was present. The authors assume that the likelihood of hyperostosis occurring as a permanent sequela of past OM while remaining symmetrical to the other bulla is low.

Overall, these findings emphasize the need for careful interpretation of bulla wall thickness and content in the assessment of OM and externa, respectively, and explain the moderate inter‐observer reliability. Although the presence of MC remains the defining criterion for disease classification, structural changes, such as hyperostosis, may warrant further investigation as potential indicators of past or ongoing pathology. Refinement of the grading system to account for these ambiguous cases could improve the inter‐observer reliability and enhance diagnostic accuracy while improving clinical decision‐making for veterinary practitioners managing guinea pig otitis.

The term “otitis” is used throughout this study to describe the presence of material within the TB, including in asymptomatic animals, although definitive confirmation of inflammation or infection was not obtained in all cases. It is important to acknowledge that the observed bulla content may not always indicate an active infectious or inflammatory process, as it was observed in one case of OM in this study, with a negative bacterial culture. Similar to cases of primary secretory otitis media (PSOM) in dogs, the material could be sterile and represent a non‐inflammatory/mucoid accumulation, particularly in clinically normal individuals [28]. Without cytological, microbiological, or histopathological confirmation, the use of “otitis” remains presumptive and should be interpreted with caution, especially when evaluating subclinical findings.

We deliberately excluded OI from our classification scheme, as CT cannot reliably diagnose inner ear inflammation; it only allows assessment of bony changes within the inner ear structures. Given the close anatomical relationship between the inner ear and TB, any osseous lesions suggestive of inner ear involvement, such as osteolysis, were incorporated into Grades IV and V of our grading scale.

A retrospective study collecting epidemiological and clinical data from guinea pigs diagnosed with otitis has been conducted and submitted for publication [30]. The grading system developed here aims to support clinicians in case management. Following physical examination and CT evaluation, practitioners require guidance on treatment options and prognosis to tailor the most appropriate therapeutic approach for each case. Due to the limited availability of strong retrospective or prospective data on otitis in guinea pigs, several treatment modalities have been proposed in the literature, including medical management, TECALBO, and myringotomy [15, 16, 28]. However, none of these reports consider the anatomical division of the TB into two compartments, which may complicate clinical decision‐making and influence prognosis. Although partial communication between the rostral and CCs may exist, the RC remains surgically inaccessible with current techniques. This anatomical complexity may increase the risk of recurrence when both compartments are involved, compared to cases where only the CC is affected. For this reason, our grading system includes subgrades based on whether one or both compartments are affected. A prospective study correlating clinical data, CT‐based grading, treatment strategies, and outcomes in guinea pigs is necessary.

The main limitation of this study is that OM and OE were based mostly on CT images only. Bacterial culture confirmation of otitis was only available for six out of the 36 cases of otitis, and none had anatomohistopathological confirmation, meaning that CT‐based diagnoses relied solely on imaging findings.

Another limitation is the variability in CT acquisition protocols between individuals and institutions. To correctly assess intracranial invasion and external ear canal mucosal changes, IV post‐contrast studies are needed; in this study, they were not widely available (30/60). This is largely attributed to the challenges associated with establishing intravenous access in these small animals.

Finally, future research is needed to investigate the clinical outcomes associated with various grades of otitis. Incorporating bacterial culture and histopathological evaluation in such studies could improve diagnostic accuracy and provide further validation of the proposed grading system. Additionally, evaluating treatment responses on the basis of CT grading would offer valuable insights into the prognostic relevance of each disease stage and support the development of more targeted therapeutic approaches.

5. Conclusion

We established a grading system that provides a standardized way to evaluate guinea pigs’ ears, facilitating consistent assessment both in longitudinal follow‐up of individual patients and across different individuals. Additionally, this system highlights the unique and previously undescribed anatomical complexity of the guinea pig ear. This study demonstrated that the CT Ear Disease Grading System provides a reasonably consistent assessment of middle and external ear pathology among different observers. By applying this grading scale, middle ear abnormalities on CT in domestic guinea pigs can be systematically evaluated, offering a valuable complement to clinical examination, guiding treatment planning, and facilitating follow‐up CT reports in patients with ear disease. Further research is needed to explore the correlation between CT grade and factors such as clinical signs, prognosis, treatment efficacy, and clinical outcomes.

Disclosure

No EQUATOR Network checklist is available for the present study design. This manuscript has not been published or presented elsewhere in part or its entirety and is not under consideration by another journal.

Conflicts of Interest

The authors declare no conflicts of interest.

Brennan T., Beauvois M., Pignon C., et al. “Computed Tomographic Grading of Ear Diseases in Guinea Pigs (Cavia porcellus).” Veterinary Radiology & Ultrasound 67, no. 2 (2026): e70143. 10.1111/vru.70143

Data Availability Statement

Supporting data are available from the corresponding author upon reasonable request.