Use of magnetic resonance imaging in the diagnosis of upper respiratory obstruction in a calf

Abstract

In a calf with dyspnea, a mass located dorsal to the pharynx was visualized by ultrasonography. Magnetic resonance imaging (MRI) revealed a severe enlargement of the left medial retropharyngeal lymph node, compatible with an abscess. This is the first reported case of MRI use in bovine upper respiratory disease.

A 2 month-old, male, Charolais calf with high pedigree merit, weighing 100 kg, was referred to the Centre Hospitalier Universitaire Vétérinaire (CHUV) for assessment of acute dyspnea and stridor. The calf and dam, originating from a herd of 50 cow-calf pairs, had been kept on pasture. The calf had been treated on the farm with florfenicol (Nuflor; Schering-Plough, Pointe-Claire, Quebec) 10 g/kg bodyweight (BW), SC, 48h prior to referral. No clinical response to therapy had been observed.

Case description

Upon arrival, the calf was tachypneic (80 breaths/min) with increased inspiratory effort, stertorous inspiration, and cyanosis of the mucous membranes. Tachycardia (130 beats/min) was also present. Rectal temperature (39.1°C) was normal. Auscultation of the trachea and the larynx revealed inspiratory stridor. Thoracic auscultation revealed increased bilateral lung sounds and referred laryngeal noises.

Due to the severe dyspnea and cyanosis, a standard tracheotomy was performed immediately after arrival (1). A tracheotomy tube (10 mm ID) was left in place and the whole tracheostomy tube was removed, cleaned and replaced regularly (every 10 to 14 h). The respiratory noises and dyspnea disappeared immediately after the tracheotomy. Jugular venous blood samples were submitted for a complete blood (cell) count and serum biochemical profile. The results were considered normal, except for mild hyperfibrinogenemia (6 g/L; reference range, 3 to 5 g/L). A catheter was placed in the right jugular vein, and antimicrobial treatment was instituted: sodium ampicillin (Ampicillin sodium for injection, USP; Novopharm, Toronto, Ontario), 10 mg/kg BW, IV, q8h, and trimethoprime/sulfadoxine (Trivetrin; Schering-Plough, Pointe-Claire, Quebec), 40/100 mg/kg BW, IV, q12h, for 5d. Antiinflammatory drugs were also given: prednisolone (Soludeltacortef; Pfizer, London, Ontario), 1000 mg, IV, followed by a single dose of ketoprofen (Anafen; Merial, Baie d’Urfé, Quebec) 3 mg/kg BW, IV, 24h later.

The small size of the patient precluded palpation of the pharynx via the oral cavity. Nasopharyngeal videoendoscopy was performed after topical lidocaine spray had been applied to the nasal mucosa. Inflammation of the pharynx with swelling and redness was observed. The laryngeal lumen was narrowed and obscured by the mass effect from the pharynx. The dorsal part of the laryngopharyngal wall collapsed during inspiration and caused severe narrowing of the upper respiratory pathway. No secretions were observed and the laryngeal cartilages appeared normal. The thoracolaryngeal reflex was elicited as described for the horse, and caused a brisk and complete arytenoid abduction on both sides.

Laterolateral digital radiographs of the pharynx confirmed severe narrowing of the nasopharynx caused by an ovoid, soft tissue opacity, retropharyngeal mass (9.5 cm long × 6.1 cm high) (Figure 1). Abscess, lymphadenopathy, granuloma, hematoma, cyst, and neoplasia were all possible etiologies. Severe dorsal pharyngeal wall and retropharyngeal swelling (inflammation, cellulitis) were considered. Thoracic radiographs showed no abnormalities. The calf was then prepared for standing ultrasonography (Aloka 5000; Tokyo, Japan) of the larynx and the pharynx, as described for small animals (2), by using a 7.5 MHz linear-array transducer. The calf was restrained without sedation with the neck slightly extended. A cavitary mass with heterogeneous disposition of hypoechoic and hyperechoic material was detected in the left caudolateral pharyngeal area. The surface of its contour was regular. Due to the deep position of the mass, its size could not be determined. Doppler color flow did not show any blood flow in the mass. Ultrasonography-guided needle aspirations were not possible. The mass was not exfoliating during aspirates. Based on these findings, a retro-pharyngeal abscess, possible hematoma, or lymphadenopathy, was suspected.

Figure 1.

Laterolateral radiograph centered on the pharynx. The nasopharynx is almost completely attenuated by an enlarged retropharyngeal soft tissue structure (white arrowheads).

In order to further specify the nature of the mass and its relationship with other anatomical structures, magnetic resonance imaging (MRI) of the pharynx and larynx was performed 2 d later by using a 1.5 T MR unit (Signa EchoSpeed LX; GE Healthcare Canada, Mississauga, Ontario). The calf was placed in left lateral recumbency under general anesthesia. A surface torso coil (4-channel Torso Array; GE Healthcare Canada) was used to acquire images, using the following parameters: sagittal fast spin-echo T1WI (WI: weighted image) [repetition time (TR) = 550 ms, echo time (TE) = 17 ms]; dorsal, tranverse, and sagittal fast spin-echo T2WI (TR = 4000 to 5000 ms, TE = 90 ms); sagittal T2WI Fluid Attenuation Inversion Recovery (FLAIR) [TR = 8000 ms, TE = 125 ms, inversion time (TI) = 2000 ms], sagittal T2 Gradient Recall Echo (GRE) (TR = 550 ms, TE = 15 ms, flip angle = 20 degrees). In addition, transverse and sagittal fast spin-echo T1-weighted images were obtained immediately after a bolus injection of the contrast agent gadobenate dimeglumine (MultiHance; Bracco Diagnostic Canada, Vaughan, Ontario), 0.1 mmol/kg BW, IV. The sequences were performed with a slice thickness of 3–5 mm with no interslice gap.

The MRI revealed enlargement of the left medial retropharyngeal lymph node (6.7 cm long × 3.5 cm high × 3.2 cm wide). Lymph node contour was mildly lobulated, well-defined cranially, but irregular and poorly defined caudally. A well-defined cavity (1.9 cm long × 1.1 cm high × 1.3 cm wide) was within the center of the lymph node. This cavity was very hypointense compared with the remainder of the lymph node on T2W and FLAIR images (Figure 2, Figure 3), and mildly hypointense on precontrast T1W and GRE images (Figure 4). No contrast enhancement was present within this cavity after gabedonate dimeglumine injection. A 5-mm wide, well-defined halo immediately surrounded the cavity that was mildly hyperintense on T2WI images and moderately enhanced with contrast agent. On T2WI images, a 2nd, more peripheral, halo was seen that was poorly defined and mildly hypointense compared with images of the contralateral medial retropharyngeal lymph node (Figure 2). The remainder of the tissue in the abnormal lymph node was of the same intensity as that of the contralateral normal lymph node. The contralateral lymph node was 3.9 cm long × 3.6 cm high × 2.1 cm wide. The midline cervical fascia was displaced to the right by the enlarged lymph node and the dorsal nasopharyngeal wall was displaced ventrally and to the right. Generalized thickening of the nasopharyngeal mucosa was also present, which further reduced the nasopharyngeal diameter circumferentially.

Figure 2.

Sagittal (a) and transverse (b) T2WI images centered on the abnormal left medial retropharyngeal lymph node (white arrowheads). The low signal intensity central cavity is seen, surrounded by a high signal intensity halo. The normal right medial retropharyngeal lymph node is seen on the transverse image (gray arrows). C: cerebellum, C2: 2nd cervical vertebra, E: epiglottis, P: right parotid salivary gland, NP: nasopharynx (severely attenuated), SH: right stylohyoid bone, T: trachea.

Figure 3.

Sagittal T2* GRE (a) and FLAIR (b) images centered on the abnormal left medial retropharyngeal lymph node. The cavity (gray arrowheads) is mildly hypointense on T2* GRE images and very hypointense on FLAIR images when compared with surrounding tissue.

Figure 4.

Sagittal T1w pre-contrast (a) and post-contrast T1WI (b) images centered on the abnormal left medial retropharyngeal lymph node (white arrowheads). An intensely contrast-enhancing halo is seen surrounding the central cavity (gray arrowhead).

The MRI findings were suggestive of medial retropharyngeal lymph node abscessation. The persistent hypointensity of the material within the cavity indicated that this material was unlike fluid. A thick caseous purulent material was suspected. The contrast enhancing halo surrounding the cavity suggested the presence of vascularized tissue at this level.

The position of the mass and the thickness of its contents, as suggested by ultrasonographs and MR images, led us to believe that surgical drainage was not a rational option. The risks that would be encountered by the procedure were judged to be higher than the potential benefits. Five days after arrival, the calf was otherwise healthy with normal vital parameters. At this time, the antibiotic therapy was changed to procaine penicillin (Depocillin; Intervet, Whitby, Ontario), 22 000 IU/kg, BW, IM, q12h for 1 mo. The tracheostomy tube was removed on day 12 to allow closure of the tracheostomy wound and the calf was discharged from the hospital on day 15. The owner was contacted 6 mo later; he reported that the animal was healthy and performing well in a bull-testing station. The owner did not notice recurrence of respiratory abnormalities after the calf had been discharged. Unfortunately, no clinical examination was performed at this time to observe the pharyngeal area.

Discussion

Retropharyngeal abcessation has rarely been reported as a cause of respiratory distress in calves (3). Upper respiratory tract diseases leading to respiratory distress are a diagnostic challenge and frequently require diagnostic imaging. Medial retropharyngeal lymph node anomalies have rarely been described in cattle (3). Lymphadenopathy can occur secondary to lymphoma (4). Lymphadenitis secondary to green algae infection has also occur in a slaughterhouse study, but no information was available on the clinical signs in these cattle (5). Unusual manifestations of actinobacillosis, sialolithiasis, necrotic laryngitis, and trauma should also be considered in the differential diagnoses of retro-pharyngeal masses (3). In this case, the patient’s age and mild hyperfibrinogenemia, with the absence of secondary systemic repercussions, were in favor of a localized inflammatory or infectious problem rather than neoplasia.

Tracheotomy immediately alleviates respiratory distress, and is easy to perform in a standing animal. This procedure allows the clinician to perform ancillary tests in a more stable patient. Radiographs of the pharynx and larynx are of limited informative value when investigating dyspnea and stridor. Despite the fact that radiographs are helpful in identifying a mass causing the dyspnea, they have limited informative value as to the cause or exact location of such a mass. In large animals with retropharyngeal lymph node infection, soft tissue opacity in the retropharyngeal area causing a mild to severe reduction of the pharyngeal lumen can be seen radiographically (3,6), but superposition of structures limits precise localization of the anomaly. Positive contrast pharyngography, following PO administration of barium with a syringe, can reveal ventral displacement of the laryngopharynx; however, this is rarely necessary (7). Radiographs will also reveal the presence of a metallic foreign body, but foreign bodies are infrequent.

Ultrasonography is useful to characterize laryngeal and pharyngeal disorders in small animals (2,8,9) and horses (6,10). However, in cattle, images of the ventral neck area have been described only once (11). In a previous report of medial retropharyngeal lymph node abscess in 3 adult cattle, ultrasonographic findings were described in 1 case, where the abscess appeared as an anechoic cavitary mass, contrary to what was observed in this case, where the deep position, the heterogenicity of the mass, and the impossibility of obtaining a specimen for cytologic examination justified further diagnostic imaging.

Magnetic resonance imaging has been used in dogs and horses to evaluate both the normal (12,13) and abnormal (14,15) upper respiratory tract. Despite the high costs of equipment and anesthetic, the information obtained is accurate and useful. Tissue composition and vascularization can be determined by using different sequences. The exact anatomic localization of the lesion and the involvement of surrounding structures can be observed (14), so that the best surgical approach can be determined, particularly if the lesion is deeply located (14,15).

There are few reports in veterinary medicine describing MRI findings in cases of abscessation and most of these have involved the cerebrum (16–22). Magnetic resonance imaging of an abnormal retropharyngeal lymph node in a pony with a cerebral abscess has been reported, but the specific appearance was not described (19).

Purulent content within the central cavity of an abscess typically has an appearance that is characteristic of fluid: high T2WI signal intensity, T1WI medium or low signal intensity, and no contrast enhancement (16,17,21,22). Occasionally, less marked high T2WI signal intensity has been observed (18,19,23) and a bovine cerebral abscess was described as being of low signal intensity on T2W images (20). Signal intensity of non-moving tissue depends on 3 factors: proton density, T1 relaxation time, and T2 relaxation time. In the present case, the T2WI and FLAIR low signal intensities, in combination with the T1WI low signal intensity, was unexpected. This is indicative of a very short T2 relaxation time and either an extremely short or an intermediate T1 relaxation time (24). Tissues exhibiting these types of signal characteristics have 1 or more of the following: magnetic susceptibility effects (deoxyhemoglobin) (25), abundant collagen and low cellularity (tendon, muscle, ligament or fibrosis) (26), a paucity of mobile protons (air or cortical bone), low water content (26), or a solid high protein content (26). High fibrinogen content can cause markedly low T2WI signal intensity with mild T1WI low signal intensity (27,28). A high production of fibrinogen is usually associated with an inflammatory response in cattle (29). It would be reasonable to consider that an elevated fibrinogen content, possibly in combination with a low water content and low cellularity, could explain the signal intensity seen within this lesion. Unfortunately, it was not possible to compare MRI findings with macroscopic and histopathologic findings, since no surgery was performed.

On T1W images, the fibrotic capsule of an abscess appears as a medium signal intensity ring, whereas on T2W images, the rim appears as a lower signal intensity structure (16,18,23). Although marked T2W low signal intensity of the abscess capsule can allow differentiation between infection and tumor (30), this low signal intensity rim is not seen in all abscesses and the rim may even have a high signal intensity (19). Typically, following IV administration of the radiocontrast agent gadolinium, the rim enhances (17,21–23,30). In cases of late brain abscessation in which inflammatory response in the capsule of the abscess is decreased, there is less intense ring enhancement after gadolinium injection (25). In our case, the intermediate intensity enhancement characteristics of the ring suggested chronicity of the abscess. When an abscess is suspected on conventional MR images, diffusion-weighted imaging may be useful to differentiate a pyogenic lesion from a necrotic or cystic tumor (31).

This case is the 1st report of pharyngeal and laryngeal MRI examination as a helpful diagnostic procedure to manage an upper respiratory disorder in a calf. The MRI examination can be a valuable diagnostic aid, especially when endoscopic, ultrasonographic, and radiographic interpretations provide limited information. This imaging modality is useful to localize the lesion, visualize extension of the pathologic process, and serve as a tool to help the clinician select the best therapeutic approach.