Under pressure: compressed arteries in DCM

by Max Stewart

Freddie Mercury immortalising the Queen song 'Under Pressure'

One of the most perplexing things about DCM is the role of spinal cord compression in the disease. It easy to think that the picture is a simple one: as the spine degenerates (joints wearing out, ligaments hardening and bones shifting position) the spinal cord gradually becomes compressed. This compression damages the spinal cord, kills off nerves cells and leads to the development of myelopathy. However, as we’ve mentioned, many people can have compression of the spinal cord but have no sign of myelopathy, while others can have extremely severe symptoms from the same amount of compression. It seems that we need to look at subtler imaging signs to better differentiate between people with and without DCM. 

It appears that the story isn’t quite as simple as we might initially think. Could there be other factors, beyond gross compression of the cord, which determine whether or not someone develops myelopathy? 
A group of researchers at Wenzhou Medical University in China, led by Wu Shiyang [1], thought that one such factor could be reduced spinal blood flow. They reasoned that the same degenerative changes that compress the spinal cord in older people could also compress the blood vessels that supply it, the so called ‘spinal arteries’ (Figure 1). If these arteries became compressed there would be less oxygen reaching the cord, leading to death of nerve cells and inflammation of the surrounding areas. This damage to the spinal cord could lead to the symptoms of myelopathy. 

The cervical spinal cord is supplied by a single anterior and two posterior arteries

What did this study involve?

To test whether spinal artery compression is associated with myelopathy, the Chinese researchers decided to look at the blood flow in one of the biggest arteries supplying the spinal cord, the anterior spinal artery (Figure 2). They compared flow in this artery between healthy people and people with DCM. The researches also compared blood flow before and after CSM surgery, to see if there was any link between improvement in symptoms and restoration of blood flow to the cord.

​To look at blood flow, the researchers used an advanced technique called ‘dual energy computed tomography’ (DECT). Normal CT scans are used widely in hospitals, and some groups have tried to use them image blood flow in DCM. However, these efforts have been unsuccessful [2]. Shiyang’s group are the first to use the more advanced DECT instead, which is able to produce better pictures of blood movement within arteries. DECT is complicated, but here’s a (very) simplified explanation. First, Iodine is injected into one of the patient’s arteries. The patient is then placed in the CT scanner. Two sets of mages are produced; one using a high energy X-ray beam, one using a low energy beam. Body tissues look similar in both types of X-ray, but iodine looks very different. By combining the two sets of images, a computer is able to focus on body structures that contain iodine – in this case, the arteries. The computer can then see how much iodine is flowing through the arteries and use this to work out blood flow.

​The study involved 50 patients with confirmed DCM and 10 health patients. Both groups underwent DECT scans. The DCM group had their level of disability measured using the JOA score. The DCM patients then had surgery – each underwent an anterior cervical discectomy and fusion. The DCM patients then had another DECT scan after surgery (to see if blood flow in the arteries had changed) and then had JOA scores calculated again one month and six months after surgery. 

​Researchers then looked at:
1. The difference in blood flow between pre-surgery DCM patients and healthy patients
2. The difference in blood flow between pre-surgery and post-surgery DCM patients
3. Any link between change in JOA score (disability) and change in blood flow after surgery

Figure 2 – Image produced by the DECT scanner, which shows blood flowing through the anterior spinal artery (white line marked by red arrow). [Taken from reference [1])

​What did this study show?

There were three important results from this study:
1. Blood flow through the anterior spinal artery is significantly lower in pre-surgery DCM patients than healthy patients 
2. Surgery leads to a significant increase in spinal artery blood flow in DCM patients
3. Patients who had bigger improvements in blood flow after surgery had greater recovery in JOA score (i.e. were less disabled) 6 months later 

​What might these results mean for the future?

​This study suggests that reduced blood flow to the spinal cord could be associated with the symptoms of myelopathy and that improvement in blood flow could help predict to recovery from surgery. These links between spinal artery compression and myelopathy could be important because:
1. They could help us better understand the underlying processes which damage the spinal cord in DCM, which in turn would help us develop new treatments. These results could even help explain why spinal cord compression on imaging doesn't always cause DCM – perhaps compression of the spinal arteries is also required for symptoms to develop. 
2. They could help us diagnose myelopathy earlier. Reduced spinal blood flow could now be considered a sign of DCM, which we can combine with our existing diagnostic makers (MRI scans, neck pain, disability etc). The earlier we diagnose myelopathy the sooner we can operate and the less severe lasting disability will be. Remember: time is spine.
3. They could help us assesses how successful surgery has been and predict how much recovery we can expect. A big improvement in blood flow suggests that recovery will be better. This clarity can help us plan your future care and helps you know what to expect.
4. They could help us compare treatments to see which is best. Surgeries that produce the bigger increases in blood flow may lead to better recovery than surgeries which produce smaller improvements in blood flow. Shiyang’s group has already planned studies to see if DECT can be used to compare outcomes of anterior vs posterior decompression operations.

​What are the limitations of this study?

​Obviously, use of DECT and study of blood flow in general is still at a very early stage. Most hospitals don’t have access to DECT yet, so it will be a while before we could think about using it in diagnosing DCM. We’d also need to confirm that there is a consistent difference between healthy and DCM patients that is big enough to detect. Furthermore, we don’t know what processes reduced blood flow is actually triggering to the spinal cord. Finally, it is likely that compression of arteries is only one of many changes and disease processes that drive DCM. Time will tell just how significant the contribution of reduced blood flow is to the disease. 

References

1. Shiyang, W. et al. Is Cervical Anterior Spinal Artery compromised in Cervical spondylotic myelopathy patients? – dual energy Computed tomography analysis of Cervical Anterior Spinal Artery. World Neurosurg. (2018). doi:10.1016/j.wneu.2018.03.217
2. Zhang, Z. & Wang, H. CT angiography of anterior spinal artery in cervical spondylotic myelopathy. Eur. Spine J. 22, 2515–9 (2013).