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What
happens in AS?
In early
disease, bone is removed from the skeleton. Thus, most patients have
mild osteoporosis, that is decreased bone mineral density in the spine.
As the disease progresses bone is laid down in specific areas and there
is thicker bone than normal in these areas (i.e. increased bone mineral
density). However, the areas where the extra bone has not been laid
down are still thinner than normal. Therefore, in advanced disease there
are areas of thinner bone and areas of bone thicker than normal. Bone
loss occurs mainly in the center of the vertebrae, and bone grows (syndesmophytes)
on the rim or corners of the vertebrae.
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Why
does this happen in AS?
The inflammation
of AS is caused by chemicals called cytokines. These chemicals also
have effects on bone mineral absorption. There is increased bone turnover
with more bone being absorbed than laid down. Therefore, in early disease
the osteoporosis could be due to the inflammation.
The laying
down of bone in later disease may be protective. Osteoporosis can lead
to fractures. However, when the body lays down bone (in later disease)
this causes the spine to thicken and protects it against fractures.
(However, the bone is laid down between the vertebrae and so makes the
spine more ridge and stiff). Therefore, the laying down of bone may
not be all bad but may actually protect the spine from some types of
fracture.
What
does this mean?
The inflammation
causes more bone to be removed from the spine than to be laid down.
Therefore, the spine becomes thinner and has lower density. If this
continued there would be the risk that it could not support the weight
of the body and would develop ‘crush’ fractures. However,
the body lays down bone between the vertebra. This thickens the spine
and allows it to support the weight of the body (even though some part
are still thin). However, accidents like falls will still mean that
a very stiff and ridge spine can not bend and can still fracture in
the areas which are low bone density or thin.
In a study
of 66 men with AS, 11 (17%) had a fracture of the vertebra. However,
this rate would be influenced by how long you have had disease, how
active you are, and your age.
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How
serious is a fracture in the spine?
That depends
on where exactly the fracture occurs. Very severe fractures can obviously
kill, or cause paralysis but this is extremely rare. Very mild fractures
may go undiagnosed and heal themselves. More often a fracture is a crack
in the vertebra which is painful but with proper treatment should not
cause long term problems. However, it is a warning that there is a risk
that a a severe fracture could occur in the future.
Can
AS be treated with drugs designed for osteoporosis?
Perhaps.
Trials are currently underway to investigate if bisphosphonates, drugs
normally used in osteoporosis can be used in ankylosing spondylitis
. These are drugs which are absorbed into the bone and suppress bone
reabsorption. Thus, current treatment is not targeted at removing the
bone from between the vertebrae but is aimed at stopping the loss of
bone in the first place. Even though the stiffness and change in posture
appears the greatest problem with ankylosing spondylitis, the osteoporosis
(which is often never felt) actually poses a greater risk. These drugs
may also have an effect on cytokine (ie the chemicals causing inflammation)
levels when injected (instead of being taken in tablet form). People
have reported less pain, fatigue, stiffness and tenderness, better function
(or movement) and fewer swollen joints when given an injected form of
the drug. However, these effects are only seen 6 months after receiving
treatment so the benefits take time.
Recent
studies have shown that people the bone mineral density of patients
with AS increases when treated with anti-TNF.
Therefore, osteoporosis associated with AS can perhaps be treated with
anti-TNF.
In summary,
the inflammation of AS causes an imbalance in the absorption and replacement
of calcium in bone. This means people with AS have some osteoporosis.
As the disease progresses there is a risk of fracture of the vertebrae
in the spine and neck. Current studies are investigating if drugs used
in osteoporosis and anti-TNF can be effective in ankylosing spondylitis.
References:
Haibel
H, Braun J, Maksymowych W. Bisphosphates -Targeting bone in the treatment
of spondyloarthritis . Clin. Exp Rheumatol 2002; 20 (Suppl. 28): S162-S166.
Allali
F, Breban M, Porcher R, Maillefert J, Dougados M, Roux C. Increase in
bone mineral density of patients with spondyloarthropathy treated with
anti-tumor necrosis factor. Ann Rheum Dis 2003; 62: 347-349.
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