Applied Radiology

Prepared by Indu Tara Agarwal, MD and Daniel E. Long, MD of the Department of Radiology, The Ohio State University, Columbus, OH.

CASE SUMMARY

A 20-year-old man diagnosed with oto-brachial renal syndrome and resultant end-stage renal disease presented to The Ohio State University Hospitals for kidney transplant. After general anesthesia and intraoperative placement of a left internal jugular central venous catheter, the renal transplant proce-dure occurred without complication. In prepar-ation for discharge on postoperative day 11, the internal jugular central venous catheter was removed, and the patient subsequently developed chest crepitus with complaints of shortness of breath over the ensuing 24 hours.

DIAGNOSIS

The diagnosis was pneumomediastinum secondary to removal of the internal jugular central venous catheter. Subcutaneous emphysema in the neck upon removal of the central venous catheter dissected to the mediastinum, which resulted clinically in crepitus and difficulty breathing. Over the ensuing days, the pneumomediastinum decompressed itself via subsequent dissection of air along fascial planes to the retroperitoneum, peritoneum, and into loops of bowel.

IMAGING FINDINGS

Prior to discharge on postoperative day 11, the left internal jugular central venous catheter was removed at the patient's bedside, which proceeded without incident. Six hours after the procedure, a nurse noted anterior chest crepitus near the patient's clavicle on physical examination. At this time, the patient felt well and was without complaint. An emergent chest x-ray revealed bilateral subcutan-eous emphysema in the soft tissues of the neck and a small amount of mediastinal emphysema in the upper part of the mediastinum.

There was no pneumothorax. Two liters of oxygen by nasal cannula was ordered, and an occlusive dressing was applied to the spot where the internal jugular catheter had exited the skin. Despite oxygen saturation of 99%, the patient experienced difficulty breathing on postoperative day 12, so the oxygen level was increased to 4 L by nasal cannula. The patient continued to complain of increased swelling around his neck, a choking feeling, and difficulty breathing throughout the early afternoon despite consistent oxygen saturation of 100%. On physical examination, crepitus around the clavicle and down the center of the back were noted. The patient was immediately transported for neck and chest CT. The neck CT scan revealed pneumomediastinum and extensive subcutaneous emphysema throughout the soft tissues of the neck (figure 1A). The chest CT scan demonstrated pneumomediastinum and a small left-sided pneumothorax (figure 1B), as well as pneumoretroperitoneum (figure IC) and air in the anterior chest wall (figure 1B).

Esophagram was negative for leak. Finally, abdominal radiographs showed pneumatosis intestinalis at the hepatic flexure of the colon (figure 2).

Over the ensuing days, the patient improved clinically with improvement of the pneumothorax and subcutaneous emphysema. The patient was discharged on postoperative day 15. On postoperative day 17, outpatient abdominal films were obtained due to complaints of back pain. The films revealed more extensive pneumatosis intestinalis along several bowel loops, indicating dissection of the pneumomediastinum into the abdomen.

DISCUSSION

Pneumomediastinum, defined as free air in the mediastinum, can be caused by trauma, ¹ iatrogenic trauma to the thoracic ^1-3 or abdominal structures, ^2,3 and acts associated with the Valsalva maneuver (such as childbirth, emesis, and cough), ⁴ or via spontaneous alveolar rupture. ^1,3-5 The pathogenesis of pneumomediastinum involves the introduction and subsequent dissection of air along the perivascular, ^2-5 peribronchial, ⁶ or retroperitoneal fascial planes, ⁷ which are continuous with the visceral space in the thorax. Since these facial sheaths are continuous, air in the mediastinum can then dissect to the cervical, retroperitoneal, or abdominal area resulting in subcutaneous emphysema in the neck ^3,7 pneumoretroperitoneum, ³ or pneumatosis intestinalis ³ respectively.

Mauder et al ³ state that deep cervical fascia divides the neck into middle, and posterior compartments. The anterior compartment, the previsceral space, and the posterior compartment, the prevertebral space, surround the middle compartment, the visceral space. The visceral space invests the trachea and esophagus in the neck, and extends posterolaterally to surround the carotid sheaths. It descends inferiorly and extends past the pulmonary hilar vessels and airways to the distal bronchovascular sheaths. Continuing inferiorly, the visceral space also invests the retroperitoneal soft tissues, and extends to the extraperitoneal fat which lies deep to the transversalis fascia. Thus, the visceral space extends superiorly from the neck to the mediastinum, and finally to the retroperitoneum and abdomen. ³ If air is introduced into any of these areas, it can track along the fascial planes, ^3,4 eventually collecting in an area that may be as much as a meter from the original site of air leak. Finally, unlike the visceral space, the previsceral and the prevertebral spaces terminate inferiorly at the upper thoracic spine, so they do not communicate with the mediastinum. ³

Pneumomediastinum is considered a benign process, with spontaneous resolution in the majority of cases, yet progressive pneumo-mediastinum can be fatal if left untreated. ^1,5 As air pressure in the mediastinum increases, the trapped air simply flows out of the mediastinum along continuous fascial planes that line the visceral space. ³ This escape mechanism allows air to track superiorly to the neck or inferiorly to the abdomen. ⁵ In the neck, the air collects and presents as subcutaneous emphysema. In the abdomen, the air can aggregate retroperitoneally, resulting in pneumoretroperitoneum, or it can collect in the walls of the hollow viscera resulting in pneumatosis intestinalis. Incomplete decompression of the air superiorly or inferiorly may result in rupture of the mediastinal pleura, leading to pneumothorax. ^1,3,5 Additionally, if the escape mechanism does not open at all, then pressure in the mediastinum can be high enough to cause tension pneumomediastinum, which occurs when accumulated mediastinal air compresses the heart and causes cardiac tamponade ^1,3,5 and impaired normal circulation from accumulated air impinging upon blood vessels. ^1,3

The signs and symptoms of pneumo-mediastinum and subcutaneous emphysema include pain radiating to the neck, ^4,8 back, ⁴ and shoulders; ⁴ dyspnea with ¹ or without cyanosis; ⁴ and neck crepitus. ⁸ Additionally, air is seen in the mediastinum on chest x-ray. ^1,3,4

The first-line treatment for pneumo-mediastinum is to relieve the inciting factor. ⁴ Otherwise, no specific therapy is recommended for uncomplicated cases, except palliative measures such as bed rest, reassurance, and analgesics since symptoms usually subside spontaneously. ^4,8 Bodey ¹ suggests that the most effective treatment is 95% oxygen by mask. After just a few hours of breathing 95% oxygen, the partial pressure of nitrogen in the blood decreases markedly due to gas exchange, yet the air bubbles in the tissues retain their nitrogen partial pressure. As the partial pressure of nitrogen in the blood is reduced, the partial pressure gradient between the blood and the bubbles widens. This allows more rapid absorption of the air into the blood, thereby reducing the emphysema. ¹ Additionally, treatment of contributing causes allows an increase in relative oxygen intake, further widening the partial pressure gradient. Such adjuvant treatments include antibiotics for infection, bronchodilators for bronchial spasm, steroids to reduce airway inflammation, and humidified air to loosen secretions. ^3,5 Rarely, surgical opening of the fascial planes might be considered ⁸ if the situation is serious enough to warrant this intervention.

In this case, removal of the left internal jugular central venous catheter presumably introduced air into the carotid sheath, resulting in perivascular air dissection to the media-stinum. Within hours, the mediastinal air was decompressed by dissection along fascial planes to the neck and abdomen, presenting with subcutaneous emphysema and pneumatosis intestinalis. Additionally, the mediastinal air decompressed itself by forming a small, left-sided pneumothorax. Further air was prevented from entering the carotid sheath by an occlusive dressing placed over the catheter site. The patient was placed on oxygen by nasal cannula once he became symptomatic, which likely aided resorption of the subcutaneous emphysema and pneumothorax, leading to clinical improvement.