The diaphragm can become fatigued if the force of contraction during inspiration exceeds 40% of the force it can develop in a maximal static effort. When this occurs acutely in patients with exacerbations of chronic airflow limitation or CF or in quadriplegics, positive-pressure ventilation is required followed by attempts to increase the strength and endurance of the diaphragm by breathing against a resistance for 30 min a day.
UNILATERAL DIAPHRAGMATIC PARALYSIS is common and symptomless. The affected diaphragm is usually elevated and moves paradoxically on inspiration. A sniff causes the paralysed diaphragm to rise, the unaffected diaphragm to descend. Causes include:
• Carcinoma of the bronchus with involvement of the phrenic nerve
• Neurological, including poliomyelitis, herpes zoster
• Trauma to cervical spine, birth injury, subclavian vein puncture
• Infection: tuberculosis, syphilis, pneumonia
BILATERAL DIAPHRAGMATIC WEAKNESS or paralysis causes breathlessness in the supine position and is a cause of sleep apnoea leading to daytime headaches and somnolence. Tidal volume is decreased and respiratory rate increased. Vital capacity is substantially reduced when lying down, and sniffing causes a paradoxical inward movement of the abdominal wall best seen in the supine position. Causes include viral infections, multiple sclerosis, motor neurone disease, poliomyelitis, Guillain-Barre syndrome, quadriplegia after trauma and rare muscle diseases. Treatment is either diaphragmatic pacing or nighttime assisted ventilation.
COMPLETE EVENTRATION OF THE DIAPHRAGM
invariably left-sided) is a congenital condition in which muscle is replaced by fibrous tissue. It presents as marked elevation of the left hemidiaphragm, sometimes associated with gastrointestinal symptoms. Partial eventration, usually on the right, causes a hump (often anteriorly) on the diaphragmatic shadow on X-ray.
HERNIAS occur through the diaphragm, the commonest being through the oesophageal hiatus, but occasionally anteriorly, through the foramen of Morgani, posterolaterally through the foramen of Bochdalek or at any site following traumatic tears.
The mediastinum is defined as the region between the pleural sacs. It is additionally divided. Tumours affecting the mediastinum are rare. Masses are detected very accurately on CT scan.
Retrosternal or intrathoracic thyroid
The commonest mediastinal tumour is a retrostemal or intrathoracic thyroid, which is nearly always an extension of the thyroid present in the neck. Enlargement of the thyroid by a colloid goitre or malignant disease and, rarely, in thyrotoxicosis causes displacement of the trachea and oesophagus to the opposite side. Symptoms of compression develop insidiously before producing the cardinal feature of dyspnoea. Very occasionally an intrathoracic thyroid may be the cause of dysphagia and, rarely, of hoarseness of the voice and vocal cord paralysis from stretching of the recurrent laryngeal nerve. The treatment is surgical removal.
The thymus is large in childhood and occupies thesuperior and anterior mediastinum. It involutes with age but may be enlarged both by cysts, which are rarely symptomatic, or turnours, which may cause the symptoms of myasthenia gravis or may lead to compression of the trachea or, rarely, the oesophagus. Surgery is the treatment of choice. Approximately half of the patients resenting with a thymic turnour have myasthenia gravis.
These cysts, which may be up to 10 ern in diameter, are filled with dear fluid and are usually situated anteriorly in the cardiophrenic angle on the right in 70% of cases. Infection only rarely occurs; malignant change does not occur. The diagnosis is usually made by needle aspiration. No treatment is required, but these patients should be followed up as an increase in cyst size suggests an alternative pathology; surgical excision is then advisable.
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