Equine Pneumothorax Study Guide

Overview and Clinical Importance

Pneumothorax refers to the presence of air within the pleural space, disrupting the normal negative pressure required for lung expansion. In horses, this condition carries unique clinical significance due to the fenestrated nature of the equine mediastinum, which allows unilateral pneumothorax to potentially become bilateral and life-threatening.

Pneumothorax in horses most commonly occurs secondary to pleuropneumonia (42.5%), open thoracic wounds (22.5%), and closed thoracic trauma (17.5%). In neonatal foals, rib fractures sustained during parturition are a significant cause of pneumothorax and can result in hemothorax, cardiac laceration, and sudden death.

Relevant Anatomy

Equine Thoracic Anatomy

Horses have 18 pairs of ribs, with 8 pairs classified as true (sternal) ribs that attach directly to the sternum via costal cartilage, and 10 pairs of false (asternal) ribs that form the costal arch. The equine thorax is notably longer and larger than in ruminants, providing greater respiratory capacity but also creating unique clinical considerations.

Key Anatomical Features

• Fenestrated Mediastinum: In adult horses, tiny openings (fenestrations) develop postnatally in the caudal mediastinum where the two mediastinal pleural walls are adherent. This allows air and fluid to potentially pass between pleural cavities.
• Pleural Space: A potential space between visceral (pulmonary) and parietal pleura, normally containing only a small amount of serous fluid for lubrication.
• Line of Pleural Reflection: Follows the costochondral junctions to approximately the ninth rib, then travels dorsally paralleling the costal arch.
• Cardiac Notch: Larger on the left side (3rd to 6th rib) than the right (3rd intercostal space), providing windows for echocardiography and making left-sided rib fractures potentially more dangerous.

Classification of Pneumothorax

Etiology

Causes in Adult Horses

Rib Fractures in Neonatal Foals

Rib fractures are a common but under-recognized finding in neonatal foals. Studies indicate that approximately 21% of foals less than 3 days of age have evidence of thoracic trauma, and up to 65% of critically ill neonatal foals at referral centers have fractured ribs. In one study, 14 of 56 foals (25%) with rib fractures died as a direct result of their injuries.

Risk Factors

• Dystocia - Excessive traction during assisted delivery
• Primiparous mares - Narrow pelvic canal
• Large birthweight foals
• Pressure on thorax during passage through pelvic canal

Anatomical Considerations

• Most commonly affected ribs: Ribs 3-8 (cranioventral thorax)
• Most common location: Costochondral junction and immediately proximal area
• Critical ribs: Left ribs 3-6 overlie the heart - fractures here carry highest risk of cardiac laceration
• Equal frequency on left and right sides

Potential Complications of Rib Fractures

Clinical Signs

Pneumothorax in Adult Horses

Rib Fractures in Neonatal Foals

Clinical signs in foals may range from subtle to severe:

• Lethargy, spending excessive time in sternal recumbency
• Groaning or moaning during respiration or when rising to nurse
• Stiff, reluctant gait (walks as though painful)
• Asymmetry of thoracic cavity on visual inspection
• Crepitus and pain on rib palpation
• Subcutaneous emphysema overlying ribs or ventral thorax
• Tachypnea and respiratory distress
• Sudden death (if cardiac laceration occurs)

Exam Focus: Physical examination technique: Palpate ribs bilaterally and synchronously with both hands while the foal is STANDING. Compare symmetry while feeling for crepitus, swelling, or pain response.

Diagnostic Approach

Thoracic Ultrasonography

Ultrasonography is the diagnostic modality of choice for evaluating pneumothorax in horses due to portability, availability, and superior sensitivity compared to radiography for small-volume pneumothorax. Studies have shown that M-mode (84%) and 2D-mode (80%) ultrasound have significantly higher sensitivity than radiography (48%) for detecting small pneumothorax.

Key Ultrasonographic Findings

Thoracic Radiography

Radiography remains useful for evaluating pneumothorax, especially for assessing underlying pulmonary disease and documenting pneumothorax extent.

Radiographic Findings

• Visible visceral pleural line: Thin, sharp white line separated from chest wall
• Absent pulmonary vasculature: No lung markings peripheral to pleural line
• Enhanced visualization: Aorta and mediastinal structures more clearly visible
• Lung margin: Dorsal lung border visible as distinct line
• Rib fractures: May be visible, particularly in foals

Thoracocentesis

Thoracocentesis serves both diagnostic and therapeutic purposes. Aspiration of air from the dorsal thorax confirms pneumothorax.

Technique

• Site: 7th-8th intercostal space on left, 6th-7th on right; DORSAL third for air removal
• Clip and aseptically prepare skin
• Local anesthesia (optional but recommended)
• Insert needle CRANIAL to rib to avoid intercostal vessels and nerves
• Use teat cannula or bitch catheter with attached syringe/stopcock
• Apply negative pressure to aspirate air

Additional Diagnostics

• Arterial Blood Gas: Hypoxemia (PaO2 less than 80 mmHg), possible hypercapnia
• Pulse Oximetry: SpO2 less than 90% indicates significant hypoxemia
• CT (Foals): Most sensitive for detecting rib fractures; increasingly used preoperatively
• CBC/Chemistry: Evaluate for anemia (hemothorax), infection (pleuropneumonia)

Treatment

Emergency Management

Tension pneumothorax is a LIFE-THREATENING EMERGENCY requiring immediate intervention.

Immediate Stabilization

• Needle thoracocentesis: Immediate decompression if tension pneumothorax suspected
• Supplemental oxygen: Intranasal insufflation at 15 L/min if hypoxemic
• Wound occlusion: Cover open chest wounds immediately (suturing, packing, or occlusive dressing)
• Transport consideration: Cling film wrap or equine compression suit can minimize air entry during transport

Definitive Treatment

Chest Tube Placement

• Site: Dorsal thorax, 12th-14th intercostal space for air removal
• Technique: Blunt dissection through muscle layers; tube advanced cranially along pleural surface
• Heimlich valve: One-way valve allows air to exit but not enter; preferred for ambulatory patients
• Monitoring: Daily ultrasound to assess resolution; remove when air production ceases

Treatment of Rib Fractures in Foals

Conservative Management

Indicated for stable fractures without significant axial displacement or threat to thoracic structures:

• Strict stall confinement: 1-4 weeks minimum; no turnout
• Analgesics: NSAIDs (flunixin meglumine 1.1 mg/kg IV q12-24h) to control pain
• Gastroprotection: Omeprazole (4 mg/kg PO q24h) or sucralfate to prevent gastric ulceration
• Prophylactic antibiotics: If pulmonary contusion or pneumonia risk present
• Positioning (flail chest): Place foal in lateral recumbency with fractured ribs DOWN to stabilize

Surgical Management

Indications for surgery: Significant axial displacement threatening vital thoracic structures, flail chest, ongoing hemorrhage, or fractures overlying the heart.

Surgical Techniques

Prognosis

Memory Aids

PNEUMO-THORAX Mnemonic for Clinical Signs:

P - Pain on breathing, Percussion hyperresonant

N - No lung sounds dorsally

E - Emphysema (subcutaneous)

U - Ultrasound: absent gliding sign

M - Mediastinum fenestrated (bilateral risk)

O - Oxygen therapy needed

T - Tachypnea, Thoracocentesis diagnostic

Foal Rib Fracture Risk: "LEFT = LETHAL"

Left-sided rib fractures (ribs 3-6) overlie the heart - remember that the cardiac notch is larger on the LEFT, leaving the heart more exposed to sharp rib ends.

Thoracocentesis: "CRANIAL to the RIB"

The intercostal NVB (Nerve, Vein, Artery) runs along the CAUDAL border. Insert CRANIAL to avoid hemorrhage. Think: "VAN" runs in the back of the rib bus!