When it comes to imaging the nervous system, nothing but an MRI will do for the fastidious neurologist. CT has its uses, such as in detecting acute intracranial bleeding, but it lacks the sophistication to detect or differentiate between less glaring abnormalities. It also comes with a hefty radiation dose. MRI on the other hand, relying on powerful magnetic fields, is a ‘cleaner’ technology.
MRI scans on their own are however often insufficient to sate the craving of the neurologist for precision. A plain MRI scan, for example, will not tell if a multiple sclerosis lesion is old or new, and it may fail to detect subtle but significant lesions such as low grade brain tumours or lymphoma. Many lesions on routine MRI scan are also ill-defined and non-specific, and could pass for abscesses, vasculitis, inflammation or just small vessel disease (wear and tear) changes.
To silence the niggling doubts, the neurologist often requests an MRI scan with contrast. The idea is to use a dye to separate the wheat from the chaff, the active lesions from the silent ones. This works because sinister lesions have a bad and dangerous habit of disrupting the blood brain barrier. All such insurgencies across the hallowed BBB is sacrilege, a sign that something serious is afoot, (or is it underfoot?). Contrast dyes, on the other hand, are adept at detecting these breaches, traversing them, and staining the sinister lesion in the process. This stain appears on the MRI scan as contrast enhancement. MRI with contrast is therefore invaluable, and a positive study is a call to arms.
Without any doubt, gadolinium is the favoured dye for contrast MRI scans. Gadolinium (Gd) is a lanthanide rare earth metal and it is one of the heavier elements of the periodic table with atomic number 64. It is named after the thrice-knighted Finnish chemist Johan Gadolin, who also discovered the first rare earth metal, yttrium.
We know a lot about some of the risks of injecting gadolinium into the body, such as its tendency to accumulate in people with kidney impairment (who cannot excrete it efficiently). We also know that it may cross the placenta to damage the developing baby. These are however hazards with simple and straight-forward solutions: avoid gadolinium in pregnancy, and don’t use it in people with poor renal function.
Much more challenging is the problem of gadolinium deposition in the brain of people with normal renal function. This is concerning because it is unpredictable, and because it has the potential to compromise brain structure and function. This blog has previously asked the question, “Is gadolinium toxic?“. The question remains unanswered, and regulatory agencies are still studying the data to provide guidance to doctors. Patient groups on the other hand have been up in arms, as one would expect, impatiently waiting for answers. What then is the state of play with gadolinium? Should neurologists and their patients really be worried? Below are 8 things we now know about gadolinium and its potential brain toxicity.
1. Gadolinium deposition is related to its insolubility at physiological pH
The toxic potential of gadolinium is thought to be the result of its insolubility at physiological pH. Furthermore, gadolinium competes against calcium, an element fundamental to cellular existence. This competition is obviously detrimental to the body.
2. The less stable gadolinium agents are the most toxic
There are two forms of gadolinium based contrast agents (GBCAs): the less stable linear GBCAs, and the more stable macrocyclic GBCAs. The linear GBCAs are more toxic, of which Gadodiamide (Omniscan) stands out. Other linear agents are gadobenate dimeglumine (MultiHance), gadopentetate dimeglumine (Magnevist), gadoversetamide (OptiMARK), gadoxetate (Eovist), and gadofosveset (Ablavar). The macrocyclic GBCAs, even though safer, are not entirely blameless. They include gadobuterol (Gadavist), gadoterate meglumine (Dotarem), and gadoteridol (ProHance). Therefore, choose your ‘gad’ wisely.
3. Gadolinium deposits in favoured sites in the brain
It is now established that gadolinium deposits in three main brain areas. The most favoured site is the dentate nucleus of the cerebellum. Other popular regions are the globus pallidus and the pulvinar. This deposition is, paradoxically, visible on plain T1-weighted MRI scans where it shows as high signal intensity.
4. The risk of deposition depends on the number of injections
The risk of gadolinium deposition in the brain is higher with multiple administrations. Stated another way, and to stretch this paragraph out a bit longer, the more frequently contrast injections are given, the higher the chances gadolinium will stick to the brain. The possible risk threshold is 4 injections of gadolinium. The fewer the better…obviously!
5. Gadolinium also deposits outside the brain
The favoured site of gadolinium deposition outside the brain is the kidney, where it causes nephrogenic systemic fibrosis, a scleroderma-like disorder. This however occurs mostly in people with renal impairment. Gadolinium also deposits in other organs outside the brain including bone, skin, and liver. (Strictly speaking, this item has nothing to do with the brain, but it helped to tot up the number to 8 in the title of this blog post, avoiding the use of the more sinister se7en).
6. Harm from gadolinium brain deposition has not been established
Whilst we know for sure that gadolinium deposits in the nervous system, harm from deposition has not been definitively established. There are, however, reports that gadolinium deposition may produce muscle and eye symptoms, and chronic pain. There are also reports of cognitive impairment manifesting as reduced verbal fluency.
7. Precautions may reduce the risk of gadolinium brain deposition
The current recommendation is not to withhold the appropriate use of gadolinium, but to observe simple precautions. Sensibly, use GBCAs only when absolutely necessary. Also consider preferentially using macrocyclic GBCAs and evaluate the necessity for giving repeated GBCA administrations.
8. There are emerging ways to avoid gadolinium toxicity
The safest use of gadolinium is not to use it at all. There are some developments in the pipeline to achieve this, although probably not in the very near future. Such developments include manganese based contrast agents such as Mn-PyC3A. A less definitive option is to mitigate the effects of gadolinium by using chelating agents; two such potential agents are nanoparticles and 3,4,3-LI(1,2-HOPO).
Why not get the snapshot view of gadolinium toxicity in the neurochecklist:
…and leave a comment!