Pneumomediastinum (from Greek pneuma – "air", also known as mediastinal emphysema) is pneumatosis (abnormal presence of air or other gas) in the mediastinum. First described in 1819 by René Laennec, the condition can result from physical trauma or other situations that lead to air escaping from the lungs, airways, or bowel into the chest cavity. In underwater divers it is usually the result of pulmonary barotrauma. Signs and symptoms
The main symptom is usually severe central chest pain. Other symptoms include laboured breathing, voice distortion (as with helium) and subcutaneous emphysema, specifically affecting the face, neck, and chest. Pneumomediastinum can also be characterized by the shortness of breath that is typical of a respiratory system problem. It is often recognized on auscultation by a "crunching" sound timed with the cardiac cycle (Hammans crunch). Pneumomediastinum may also present with symptoms mimicking cardiac tamponade as a result of the increased intrapulmonary pressure on venous flow to the heart. Cause
It is most commonly caused by:
Esophageal rupture, for example in Boerhaave syndrome
Asthma or other conditions leading to alveolar rupture
Bowel rupture, where air in the abdominal cavity tracts up into the chest.It has also been associated with:

Mycoplasma pneumoniae pneumonia
obesityIt can be induced to assist thoracoscopic surgery.It can be caused by a pulmonary barotrauma induced by a person moving from a higher to a lower pressure environment, such as when a scuba or surface-supplied diver, a free-diver after lung-packing,  or an airplane passenger ascends.
== Symptoms and signs ==
Hemiplegia (Greek hemi = Half), is condition that affects one side of the body. Signs of a hemiplegic migraine attack are similar to what would be presented in a stroke that typically includes sudden severe headache on one side of the brain, weakness of half the body, ataxia and aphasia which can last for hours, days or weeks. Cause
Diagnosis
Classification
The ICHD classification and diagnosis of migraine distinguish 6 subtypes of hemiplegic migraine. Familial hemiplegic migraine (FHM) can be loosely divided into two categories: with and without cerebellar signs. Cerebellar signs refer to ataxia, sometimes episodic and other times progressive, that can accompany FHM1 mutations and is caused by degeneration of the cerebellum. These cerebellar signs result in a phenotypic overlap between FHM and both episodic ataxia and spinocerebellar ataxia. This is unsurprising as subtypes of these disorders (FHM1, EA2 and SCA6) are allelic, i.e., they result from mutations in the same gene. The other forms of FHM seem to be distinguishable only on the basis of their genetic cause. Familial hemiplegic migraine
Familial hemiplegic migraine is a form of hemiplegic migraine headache that runs in families. Hemiplegic migraine is inherited via autosomal dominant manner. Sporadic hemiplegic migraine
There are also non-familial cases of hemiplegic migraine, termed sporadic hemiplegic migraine. These cases seem to have the same causes as the familial cases and represent de novo mutations. Sporadic cases are also clinically identical to familial cases with the exception of a lack of family history of attacks.