History
A 5-year-old 2.9-kg (6.4-lb) castrated male domestic lop-eared rabbit (Oryctolagus cuniculus) was evaluated because of lethargy, bruxism, and decreased appetite, water intake, defecation, and urination for 4 days. On physical examination, the rabbit was quiet, lethargic, and responsive. The rabbit had moderately increased respiratory effort, tachypnea (100 breaths/min; reference range, 30 to 60 breaths/min1), less than anticipated gastrointestinal sounds on auscultation, and a body condition score of 1.5 (on a scale of 1 to 5). In addition, cecotropes were present in the perianal area, and the rabbit showed signs of pain on manipulation of the limbs, particularly both hip joints.
The rabbit was premedicated with midazolam hydrochloride (0.5 mg/kg [0.2 mg/lb], IM) and placed in an oxygen cage set at 40% oxygen saturation. After the rabbit was sedated, general anesthesia was induced with isoflurane in oxygen (5 L/min) delivered through a face mask with the anesthetic vaporizer set at 5%. General anesthesia was maintained with isoflurane in oxygen (2 L/min) with the vaporizer set at 2%. A blood sample was obtained from a lateral saphenous vein, and results for a CBC and plasma biochemical analyses were unremarkable, with the exception of leukopenia (2,500 WBCs/μL; reference range, 4,100 to 10,800 WBCs/μL1). Radiography was performed on the anesthetized rabbit (Figure 1).
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Radiographic Findings and Interpretation
There was a large amount of free gas in the pleural space of the left hemithorax (Figure 2). The left lung lobes were severely retracted, small, and soft tissue opaque. The mediastinum was severely shifted rightward, and the diaphragm was caudally displaced. Degenerative changes in the thoracic region of the vertebral column were also evident. On the basis of radiographic findings, a severe left-sided tension pneumothorax was diagnosed. There was no radiographic evidence or history of trauma or iatrogenic causes; therefore, the pneumothorax was suspected to have been spontaneous.
A right lateral thoracic radiographic image was obtained, and findings included an approximately 1.8-cm-diameter circular gas opacity structure with a thin soft tissue opacity rim, consistent with a pulmonary bulla, in the caudoventral region of the left hemithorax near the level of the fifth and sixth intercostal spaces (Figure 3). Thoracocentesis was performed on the left side in the sixth intercostal space with a 25-gauge needle attached to an extension set, 3-way stopcock, and 60-mL syringe, and 120 mL of air was removed. Thoracic radiography was repeated and revealed improvement of the pneumothorax, including resolution of the rightward mediastinal shift and reduced gas opacity in the left pleural space (Figure 4). Given the radiographic finding of a pulmonary bulla, we suspected that rupture of a bulla caused the tension pneumothorax.
Treatment and Outcome
The rabbit recovered from anesthesia without complication and was placed in an oxygen cage set at 40% oxygen saturation, for observation. The rabbit's respiratory rate and effort appeared clinically normal, and its oxygen saturation of arterial hemoglobin measured by pulse oximetry was > 98% following thoracocentesis and throughout recovery. After recovery, the rabbit was bright, alert, and responsive. Free choice hay, commercial rabbit pellets, and water were offered, and the rabbit showed signs of having a good appetite.
Approximately 4 hours after recovery, the rabbit developed increased respiratory rate and effort and became lethargic and anorexic. The rabbit's condition continued to decline over the next 2 hours. Because of the recurrence of clinical signs and guarded prognosis without advanced treatment, the owner elected euthanasia for the rabbit.
On necropsy, the rabbit's tracheal and bronchial mucosae were congested (Figure 5). Approximately 60% of the peripheral lung tissue was expanded by emphysema and multiple bullae that were up to 2 cm in diameter. The cranial and middle lung lobes were the most severely affected bilaterally; however, the lungs were otherwise completely collapsed (atelectasis). The right middle lung lobe was adhered to the thoracic aspect of the diaphragm by a band of fibrous connective tissue. The pericardial sac was distended with approximately 6 mL of clear, pale yellow fluid. Bilaterally, the acetabula and hip joint capsules were thickened, and the femoral heads were flattened and rough, consistent with chronic osteoarthritis.
Histologic evaluation of tissue samples from the cranial and middle lung lobes bilaterally revealed severe distention of the alveolar spaces and occasional rupture of the alveolar septae that resulted in cavernous spaces in distal portions of the airway, consistent with emphysema (Figure 6). In some areas across the surface of the lungs, there were small areas of discontinuity of the pulmonary visceral pleura. Sections from the right middle lung lobe contained a thick area of fibrosis that replaced and compressed adjacent pulmonary parenchyma. Minimal inflammation that consisted of local lymphoplasmacytic interstitial pneumonia was present near the area of fibrosis.
Comments
There are few reports of pneumothorax or pulmonary bullae rupture in rabbits in the literature. One case report2 described iatrogenic tension pneumothorax in a New Zealand white rabbit following accidental overinflation of the lungs during anesthesia, and rabbits have been used as a laboratory model for pneumothorax and pulmonary bullae.3 In humans and dogs, pneumothorax is well described, and the predominant cause of spontaneous pneumothorax in dogs is rupture of pulmonary bullae or blebs.4,5
Although the initial clinical evaluation of the rabbit of the present report did not yield a suspicion of pneumothorax, results of radiography indicated pneumothorax with gas opacity in the pleural space of the left hemithorax and was helpful in ruling out primary causes (eg, trauma). Further, the pneumothorax was classified as a tension pneumothorax on the basis of the radiographic finding of the rightward shift of the mediastinum and caudal displacement of the diaphragm. The left lung lobes of the rabbit had soft tissue opacity because of atelectasis from the pneumothorax, which was confirmed and improved by thoracocentesis and removal of free gas from the thoracic cavity. Although ultrasonography was not performed on the rabbit of the present report, it may be a useful point-of-care screening tool for pneumothorax in small animals, particularly less stable patients, in that the absence of a glide sign (movement at the parietal and visceral pleural interface) is a finding consistent with pneumothorax in small animals.6
Pulmonary bullae, as were visible on the right lateral thoracic radiographic image of the rabbit of the present report before thoracocentesis, are frequently associated with pneumothorax in other species. On necropsy of this rabbit, numerous bullae were noticed throughout the lungs, and a 2-cm-diameter bulla was observed in the left cranial lung lobe, corresponding to the large bulla evident on the right lateral radiographic image. Spontaneous pneumothorax caused by pulmonary bullae can be difficult to diagnose because the source is not evident on the basis of patient history or physical examination findings. Further, pulmonary bullae and blebs are frequently not evident with radiography, which has poor sensitivity for detection of these abnormalities.4 In patients with pneumothorax secondary to bulla rupture, the inciting bulla is collapsed and may not be identified, but the radiographic evidence of other bullae supports the diagnosis. Computed tomography may also be useful in diagnosing these types of lesions. For instance, a study7 shows that CT may have greater sensitivity than radiography in detecting pulmonary bullae or blebs in dogs; however, another study8 shows that CT may be ineffective in accurately detecting bullae in dogs. One explanation for poor sensitivity of CT may be the presence of atelectasis with pneumothorax.7 Other means of diagnosis include video-assisted thoracoscopy or thoracotomy.
In humans, pleural adhesions are associated with increased risk of pneumothorax; however, this association has not been described in rabbits.9 The rabbit of the present report had a fibrous adhesion (consistent with resolved, chronic, localized pneumonia) between the right middle lung lobe and the diaphragm, and it was possible that this adhesion led to tearing of the lung lobe and secondary spontaneous pneumothorax. Treatment of spontaneous pneumothorax in small animals includes therapeutic thoracocentesis for stabilization, followed by addressing any underlying causes. For dogs and cats with spontaneous pneumothorax, cats may have better outcomes with nonsurgical treatment, whereas surgical intervention is considered the treatment of choice for dogs.4,10 After initial stabilization, conservative management of spontaneous pneumothorax includes cage rest and ancillary treatments, such as supplemental oxygen, which when compared with room air, was better for early resolution of experimental pneumothorax in rabbits.11 In dogs, early surgical management may be beneficial in the treatment of spontaneous pneumothorax and is associated with lower mortality rates and recurrences than nonsurgical treatments alone.5
There is limited information regarding cause, diagnosis, and treatment of spontaneous pneumothorax or pulmonary bullae or blebs in rabbits. When performing thoracocentesis, the procedure is similar to that in dogs and cats; however, the fifth to sixth intercostal spaces may be the preferred site in some rabbits because of the relatively small size of the thoracic cavity and location of the heart.12 In the rabbit of the present report, thoracocentesis was performed in the sixth intercostal space based on the radiographic appearance of the pneumothorax. In rabbits, there is no information on whether medical versus surgical management is superior. The owners of the rabbit in the present report initially elected conservative management; however, treatment of pulmonary bullae typically requires partial or complete lung lobectomy, and the preferred method is median sternotomy to allow exploration of all lung lobes.4,5
References
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