What is pulmonary alveolar microlithiasis?
Pulmonary alveolar microlithiasis, or PAM, is an ultra-rare inherited lung disease in which countless tiny stones, called microliths, made of calcium phosphate form inside the alveoli (air sacs). Over many years the stones accumulate throughout both lungs, the lung tissue becomes stiff and scarred, and oxygen transfer is reduced.
One of the most striking features of PAM is the gap between the X-ray and the patient. The chest X-ray can look dramatically white, with a fine sand-like pattern often described as a sandstorm, while the person feels well and has few or no symptoms. Symptoms usually appear only after many years.
PAM has been reported worldwide, with a relatively larger number of reported cases from Turkey, China, Japan, India and Italy. It is frequently diagnosed in childhood or early adulthood, often by chance.
Symptoms of PAM
- No symptoms: common for many years, sometimes decades.
- Breathlessness on exertion: slowly progressive as the lungs stiffen.
- Dry cough.
- Coughing up small stones: occasionally reported.
- Chest pain and pneumothorax: uncommon.
- Signs of advanced disease: finger clubbing, low oxygen levels, and swelling of the legs from strain on the right side of the heart (cor pulmonale).
Causes and inheritance
PAM is caused by changes in a gene called SLC34A2. This gene makes a sodium-phosphate transporter in type II alveolar cells, which normally removes phosphate from the fluid lining the air sacs. When the transporter does not work, phosphate builds up and combines with calcium to form microliths.
PAM is inherited in an autosomal recessive pattern. A child is affected when both parents pass on a changed copy of the gene, and parents are usually healthy carriers. It is therefore more common in families where parents are related by blood, and brothers and sisters of an affected person have a substantial chance of also being affected. Blood calcium and phosphate levels are usually normal, and PAM is not caused by diet or by any problem with calcium metabolism elsewhere in the body.
How PAM is diagnosed
In India the commonest pitfall is mistaking PAM for miliary tuberculosis, and many patients receive anti-tuberculosis treatment before the correct diagnosis is made. The well-preserved health of the patient, the lack of fever, and the very dense, sand-like shadowing are important clues.
| Test | What it shows |
|---|---|
| Chest X-ray | Dense, fine sand-like shadowing throughout both lungs, most marked at the bases |
| High-resolution CT | Widespread calcified micronodules, calcification along the lung borders and a dark line beneath the ribs known as the black pleura sign |
| Lung function tests | Normal early on, later a restrictive pattern with reduced gas transfer |
| Bronchoscopy with lavage or biopsy | Microliths in washings or tissue, when confirmation is needed |
| Genetic testing | Changes in the SLC34A2 gene, confirming the diagnosis and enabling family screening |
| Echocardiogram | Signs of pulmonary hypertension in advanced disease |
In many cases the CT appearance is typical enough to make the diagnosis without a lung biopsy. Bronchoscopy is used when there is doubt.
Treatment
There is currently no medical treatment proven to dissolve the microliths or stop the disease. Management focuses on preserving lung health, treating complications and planning ahead.
Supportive care
- Complete avoidance of smoking and dusty environments.
- Influenza and pneumococcal vaccination, and prompt treatment of chest infections.
- Pulmonary rehabilitation to maintain fitness.
- Oxygen therapy when oxygen levels fall.
- Assessment and treatment of pulmonary hypertension in advanced disease, with cardiology input.
Treatments that have been tried
Bisphosphonates such as etidronate have been tried with inconsistent results and are not routinely recommended. Whole-lung lavage, which is effective in pulmonary alveolar proteinosis, does not remove the microliths in PAM. Approaches that lower phosphate in the lung are being studied but are not yet established.
Lung transplantation
For patients with advanced respiratory failure, lung transplantation is the only definitive treatment, and recurrence of the disease in the transplanted lungs has not been reported. Timely referral to a transplant centre is important.
Outlook and family screening
PAM progresses slowly, and many people remain well for decades, although the rate varies and some develop respiratory failure in middle age. Follow-up includes regular lung function tests, oxygen saturation checks and echocardiography. Brothers and sisters of an affected person should be offered a chest X-ray or genetic testing, and genetic counselling is recommended for families. Children with PAM are followed jointly with paediatric pulmonology.
When to seek urgent care
Seek urgent medical attention for sudden chest pain or breathlessness, breathlessness at rest, blue lips, increasing leg swelling, fainting, or fever with worsening cough.
Frequently asked questions
Is PAM the same as tuberculosis?
No. PAM is an inherited condition, not an infection, and it is not contagious. Its X-ray appearance can resemble miliary tuberculosis, which is why it is sometimes misdiagnosed. Anti-tuberculosis drugs do not help PAM.
Why do I feel well when my X-ray looks so abnormal?
In PAM the stones build up slowly, and the lungs adapt over many years. A large difference between a very abnormal X-ray and few symptoms is typical of the condition, particularly in its early stages.
Should my brothers and sisters be tested?
Yes. PAM is inherited in an autosomal recessive pattern, so siblings have a substantial chance of being affected. A chest X-ray or genetic test is a simple way to screen them.
Will reducing calcium in my diet help?
No. PAM is caused by a phosphate transport problem in the lung, not by too much calcium in the diet. Dietary calcium restriction is not recommended and may harm bone health.
Can whole-lung lavage remove the stones?
No. Lavage is effective in pulmonary alveolar proteinosis but does not remove the solid microliths of PAM.
Does PAM come back after lung transplantation?
Recurrence in the transplanted lungs has not been reported, because the new lungs have a normally working phosphate transporter.
Related conditions and procedures
Consult Dr. Gogineni Ratnakara Rao
Apollo Hospitals, Financial District: Monday to Saturday, 8:30 AM to 5:00 PM. Anything in Lung Clinic, Chanda Nagar: Monday to Saturday (Thursday closed), 6:00 PM to 9:00 PM.