Amiodarone lung

Amiodarone lung is an interstitial lung diseaseseen in patients being administered amiodarone, and can manifest in a number of histopathologic patterns.

Epidemiology

The reported prevalence of pulmonary toxicity in the patients receiving amiodarone ranges from 2-18% ⁸.

Patients are usually elderly, and have been exposed to amiodarone, usually for at least six months, although there is a poor correlation with dosage or cumulative dose. Risk factors include ⁶:

• treatment longer the 2 months
• age over 60 years
• daily dose > 400mg;400 mg
• antecedent lung disease or surgery
• prior angiographic investigations

Overall pulmonary toxicity occurs in 5-10% of treated patients ^4-6.

Clinical presentation

They typically present with exertional dyspnoea as the dominant symptom. Low grade fever, anorexia, muscle weakness have been reported also ². In approximately a third of patients, presentation may mimic pulmonary infection ⁶.

Respiratory function tests are usually abnormal with a restrictive pattern on spirometry and decreased gas transfer ^2,6. Hypoxaemia is almost always present ⁶.

In some cases tissue diagnosis is required to establish the diagnosis, although usually the combination of appropriate clinical history and radiographic features suffice to guide therapy.

Pathology

Amiodarone hydrochloride  is is a triiodinated antiarrhythmic, comprised of 37% iodine by weight, which accumulates in type II alceocytes ^5,7. As is the case with other drug induced pulmonary toxicity, amiodarone can cause a variety of histopathologic patterns including ^6-7:

• chronic interstitial pneumonia - most: most common
• organising pneumonia (OP)
• diffuse alveolar damage (DAD) - : AIP and even ARDS

A distinctive feature of amiodarone lung is the presence of foamy histiocytes which contain intracytoplasmic osmiophilic lamellar bodies. However, this feature is also seen in patients with amiodarone exposure with no evidence of toxicity. 

Radiographic features

There are two main patterns of involvement, which may co-exist.

1. multiple peripheral areas of dense air-space opacity: most common ⁵
2. interstitial fibrosis

Plain film

Appearances on chest radiography are non-specific typically comprising of :

• peripheral areas of consolidation
• upper lobe predominance
• underlying interstitial disease

HRCT

As with other pulmonary disease with an interstitial component, HRCT is the modality of choice. Changes are usually bilateral, asymmetrical and particularly prominent in the lung bases ⁶. Findings include:

• areas of consolidation
  • peripheral
  • often hyper densehyperdense ^1,6 c.fcf. muscle (on account of the iodine)
• patchy ground-glass opacities
• co-existing interstitial disease
  • reticulo-nodular opacities

In addition, the liver (80% of cases) and sometimes the heart (20%) are high density ⁶. However, high hepatic and splenic attenuation is also seen in patients exposed to amiodarone in the absence of drug toxicity. 

Nuclear medicine

Gallium⁶⁷ scan

• sensitive but non-specific.

Treatment and prognosis

Cessation of amiodarone and treatment with steroids arrests and often results in resolution of imaging findings over time ³.

Overall mortality from amiodarone lung is less than 10% ⁶.

Differential diagnosis

Imaging differential considerations include:

• pulmonary eosinophilia
  • patients with amiodarone lung have normal blood and tissue eosinophil counts ²
  • also peripheral distribution
• pulmonary tuberculosis: cavitation common
• pulmonary fibrosis from other causes
• Churg-Strauss syndrome
• other hyper-attenuatinghyperattenuating pulmonary abnormalities ³