However, in the following years it was demonstrated that the appearance of loud, systolic clicking noises over the precordium could also be present in patients with a small spontaneous left-sided pneumothorax.5-9 It was assumed the pneumothorax caused a small amount of air to accumulate in the pleural space in the major fissure inferiorly, which shifted with the cardiac contraction. This results in the noise being present while in the supine position. In the sitting position, the air moves cranially above the heart, meaning it is not influenced by the cardiac contractions and the noise disappears.5-8 The Hamman sign is absent in right-sided pneumothorax, presumably because of the smaller contact surface between the lung pleura and the mediastinal pleura overlying the heart in comparison to the left side. Also, the contractions of the right side of the heart are much weaker and generate less pressure in comparison to the left atrium and ventricle.8,11Only a small amount of air, approximately 25 mL, is enough to produce the typical sound. In larger pneumothorax’s, with more than 125 mL of intrapleural air, these sounds are absent, because the contractions of the heart cannot create enough pressure to cause the accumulated air to shift in the pleural space.5,8-9Sound analysis in left-sided pneumothorax by Roelandt et al7 showed multiple murmurs which can be present in both systole and diastole. In contrast to pulmonic noises, the Hamman sign persists when the patient is holding their breath and disappears with sitting or standing.3,6-10 Furthermore, it must not be confused with extra heart sounds, which present as a “gallop rhythm”, with a strong resemblance in quality to the first normal heart sound (S1). In addition, extra heart sounds are uncommon in healthy patients and do not appear suddenly or temporarily.5,8
The Hamman sign is a rare physical examination finding, only identified in less than 1% of all patients with a pneumothorax.9 However, its presence is so specific that it is strong evidence for an underlying pneumothorax or pneumomediastinum, even if radiographic imaging is normal.10 As previously stated, since the Hamman sign is mostly commonly associated with a pneumothorax consisting of less than 125 mL of air, these can usually be treated conservatively, without the necessity of placing a chest tube or aspiration. However, when a patient experiences significant shortness of breath, the emergency physician should consider ordering additional imaging, in the form of an ultrasound or a computed tomography scan to identify the underlying cause of the Hamman sign and place a chest tube when clinically indicated.
Conclusion
The Hamman sign is a rare clinical examination finding in left-sided pneumothorax or pneumomediastinum, in which a ticking or crackling noise is heard over the thorax. This is mostly synchronous with the heartbeat and not related to respiration. It is caused by a small amount of accumulated air in the pleural space, which is being displaced by cardiac contractions during the cardiac cycle. Although typically small, pneumothoraces have a good prognosis. Recognition of the Hamman sign is important, and physicians must realize that even a normal chest X-ray does not rule out the diagnosis.