Sarcoidosis is a systemic inflammatory disease of unknown origin involving the formation of tiny, abnormal clumps of immune cells called granulomas. These granulomas can develop in almost any organ, leading to a wide variety of symptoms that are sometimes difficult to trace back to the primary disease. Although it is a relatively rare occurrence, sarcoidosis can cause back pain either through direct infiltration of the spine or through indirect effects on the surrounding nerves and muscles.
What Sarcoidosis Is and Where It Usually Strikes
Sarcoidosis is characterized by an exaggerated immune response that results in the formation of non-caseating granulomas. These small nodules, composed of aggregated immune cells, disrupt the normal structure and function of the tissue where they form. When the disease is active, granulomas cause inflammation, and in chronic cases, they may lead to scarring or fibrosis of the affected organ.
The most common sites for sarcoidosis are the lungs and the lymphatic system, with approximately 90% of patients showing some form of lung involvement. Granulomas frequently appear in the lymph nodes, especially those in the chest and neck, and are also commonly observed in the skin and eyes. Because the disease can affect any organ, its presentation is highly varied, ranging from asymptomatic to severe, life-threatening organ dysfunction.
Osseous Sarcoidosis: The Direct Link to Back Pain
The most direct cause of sarcoidosis-related back pain is osseous sarcoidosis, which involves the bone structure. This form of the disease is uncommon, affecting between 1% and 13% of sarcoidosis patients, and vertebral involvement in the spine is considered very rare. When granulomas infiltrate the vertebrae, they cause lesions that directly compromise the integrity of the spinal column.
These lesions can appear as lytic areas, involving bone destruction, or blastic areas, where there is an abnormal increase in bone density. The presence of these lesions leads to severe, localized back pain that does not respond to typical conservative treatments. In some cases, the destructive nature of the granulomas can weaken the bone enough to cause compression fractures of the vertebral bodies.
The lumbar and thoracic regions of the spine are the most common sites for this direct bony involvement. Damage to the vertebrae can also result in spinal instability or the formation of soft tissue masses, which can press on the spinal cord or nerve roots. This mechanical pressure causes intense, persistent pain that is often difficult to distinguish from pain caused by metastatic tumors or severe infections.
Systemic and Neurological Causes of Pain
Sarcoidosis can also cause back discomfort through mechanisms that do not involve the direct destruction of the vertebral bone. One such pathway is neurosarcoidosis, which is the involvement of the nervous system and occurs in 4% to 9% of cases. Granulomas can form in the spinal cord, nerve roots, or peripheral nerves near the back, causing a form of neuropathy.
This nerve involvement can lead to symptoms like radiating pain, numbness, tingling, or weakness in the legs, often mimicking a herniated disc or sciatica. Sarcoidosis can also cause inflammation of the sacroiliac joints, known as sacroiliitis, which is a distinct cause of low back pain. Sacroiliitis is a known source of chronic back pain and complicates the clinical picture for patients with sarcoidosis.
Another indirect cause of back pain is sarcoid myopathy, or inflammation of the muscles, which is symptomatic in a small fraction of patients. Granulomas forming in the back muscles can lead to chronic stiffness, muscle aches, and proximal muscle weakness. Even without direct muscle involvement, the general systemic inflammation associated with active sarcoidosis can contribute to overall fatigue, joint aches, and secondary back discomfort.
Identifying and Diagnosing Spinal Sarcoidosis
Diagnosing spinal sarcoidosis is challenging because its presentation often closely resembles more common or aggressive conditions. The lytic and blastic lesions seen on imaging studies like X-rays, Computed Tomography (CT), and Magnetic Resonance Imaging (MRI) can appear identical to those caused by metastatic cancer or infectious diseases. This overlap requires high clinical suspicion, especially in patients with a known history of sarcoidosis.
Imaging studies are necessary to visualize the extent of bone and soft tissue involvement, including any potential compression of the spinal cord or nerve roots. However, imaging alone is not sufficient to confirm the diagnosis or distinguish it from other serious diseases. The definitive method for confirming spinal sarcoidosis is a tissue biopsy that shows the presence of non-caseating granulomas.
This biopsy may be taken directly from the vertebral lesion, which is an invasive procedure, or from a more accessible site like an enlarged lymph node or a skin lesion. Confirming the presence of granulomas in any organ, alongside characteristic spinal imaging findings, allows clinicians to establish a diagnosis of probable spinal sarcoidosis. The rare nature of this condition means that a thorough exclusion of other potential causes is a necessary step in the diagnostic process.