Paget’s disease is a rare disorder of the bone affecting the rate of formation and destruction of various bones of the skeleton. It is common in the elderly and those from European descent. The exact causes of this condition are unclear. (1-6)
Physiology of the bone
Normally a bone breaks down regularly with use and aging. As a result of this breakdown in the younger individuals there is normal bone repair and growth that is called remodelling and takes place every day.
Cells that breakdown or absorb and dissolve bone cells are called osteoclasts and those that form new bone cells are called osteoblasts. The rest of the bone is made up of a protein called collagen and a mineral called hydroxyapatite.
Pathophysiology of Paget’s disease
In Paget's disease the osteoclasts become more active than the osteoblasts creating a discrepancy between bone breakdown and formation. This means that there is more bone breakdown than usual.
The osteoblasts try to keep up by making new bone. The whole process becomes chaotic leading to formation of deformed bones that are large, misshapen, and dense, while all the while weak and brittle and easy to fracture or bowing or bending due to pressure. The bone is deformed, and fits together haphazardly.
A normal bone when viewed under the microscope shows a tight overlapping structure that appears as a brick wall. In Paget’s disease there is an irregular mosaic pattern, as though the bricks were just gathered and left together haphazardly.
Cause of Paget’s disease
The cause of Paget's disease is unknown. Genetics play an important role in development of this disease. This is evident because people of the Anglo Saxon descent are more at risk of this condition.
Similarly those of African or Asian origin rarely are affected by Paget’s disease. Thus Paget's disease occurs more commonly in European populations and their descendants.
Genes are said to be activated when exposure to a virus. It may be present in as many as 25 percent to 40 percent of the direct descendants of someone with the disease.
Genetics of Paget’s disease
The inheritance is said to be Autosomal dominant. There are four to seven genes that when faulty cause Paget's disease. Of these a special gene mutation or variation called the sequestosome 1 (SQSTM1) is the most important.
Patients carrying this mutation seem to be severely affected by Paget's disease and there is a high risk of transmitting the infection in offspring. The disease commonly affects elderly and men are slightly more at risk of the disease than women (3 men affected for every 2 women with the condition).
Environmental causes of Paget’s disease
Environmental conditions are also responsible for causation of this disease. This is evident by the fact that those who live in certain geographic areas, like Northern and western Europe, the United States, Australia and New Zealand are more at risk.
Paget’s disease is uncommon in those living in China, Japan, Scandinavia and India.
There is also a marked variation in the number of cases of Paget’s disease in different regions of Britain. For example, the condition is less common in the south of the country, (estimated 1 in 100 people over the age of 55 is affected), and it is more common in the north, (county of Lancashire where 1 in 50 people over 55 may be affected).
The number of people affected in Britain is on the decline since due to immigration of people of African and Asian origin there is an ethnic mix of people who do not get this disease. However, the disease remains the commonest in the UK with over 1 million affected patients.
Other causes of Paget’s disease
Mechanical stress may play a role in causation of Paget’s disease. Some researchers suggest the infections with the following viruses may be possible triggers for Paget’s disease:-
- Paramyxovirus including measles,
- canine distemper virus affecting dogs primarily
- respiratory syncytial virus affecting babies
This however is not proven.
Due to vaccines, both measles and distemper are now uncommon infections. This could also account for the decrease in cases of Paget's disease.