Neurology is a jungle, and for the unwary, a minefield. It perhaps has the most diverse number of complex diseases than any other medical specialty. Patients are often bewildered by neurological processes and procedures, from the searching questions to the bizarre examination ritual. They are more confused by the variety of tests required to assess one symptom. The […]
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!
They will use any natural orifice to gain access to our bodies, or they will create their own. They will burrow and nibble their way to the most accommodating organ they can find, and then latch on with hooks, tentacles, or just sheer determination. They will permanently ingratiate themselves to their unwary and unwelcoming hosts. They will […]
Pseudo-subarachnoid hemorrhage: a potential imaging pitfall Lin CY, Lai PH, Fu JH, Wang PC, Pan HB. Can Assoc Radiol J 2014; 65:225-231. Abstract Background: Increased density of the basal cisterns and subarachnoid spaces on computed tomographies (CT) is a characteristic finding of acute subarachnoid hemorrhage (SAH). Excluding head injury, SAH leads to the performance of […]
Clinical phenotype and outcome of hepatitis E virus-associated neuralgic amyotrophy van Eijk JJJ, Dalton HR, Ripellino P, et al. Neurology 2017; 89:909-917. Abstract OBJECTIVE: To determine the clinical phenotype and outcome in hepatitis E virus-associated neuralgic amyotrophy (HEV-NA). METHODS: Cases of NA were identified in 11 centers from 7 European countries, with retrospective analysis of demographics, clinical/laboratory findings, and treatment and outcome. Cases of HEV-NA were […]
Neurologists are often slightly nervous when their patients start planning a family. It’s even worse when the patients fall pregnant–unexpectedly. This is because neurologists need super thinking hats not only to anticipate the potential impact of pregnancy on their patients neurology, but also to preempt the adverse effects of neurological treatments on the developing baby. The nervousness […]
Myasthenia gravis (MG) is an iconic neurological disorder. It is classical in its presentation, weakness setting in with exertion and improving with rest. This fatigability is demonstrable in the laboratory when repetitive nerve stimulation (RNS) of the muscles results in a progressively decremental response. Clinically, myasthenia gravis is often a benign disorder which restricts itself to the muscles of the eyes: this ocular MG manifests just with droopy eyelids (ptosis) and double vision (diplopia). At the extreme however is generalised MG, a severe and life-threatening condition that justifies its grave appellation.
Myasthenia gravis depletes the energy reserve of muscles, something which is entirely dependent on acetylcholine (ACh), a chemical released at nerve endings. After release, ACh traverses the neuromuscular junction (NMJ) to attach itself to the acetylcholine receptor (AChR), which is comfortably nestled on the surface of the muscle. This binding of chemical to receptor is a significant event, setting sparks flying, and muscles contracting. In myasthenia gravis, this fundamental process is rudely disrupted by the onslaught of acetylcholine receptor antibodies. These aggressive AChR antibodies, produced by the thymus gland in the chest, vent their rage by competitively binding to the receptor, leaving acetylcholine high and dry. Eventually, the rampaging antibodies destroy the receptor in an act of unjustified savagery.
In tackling myasthenia gravis, it is no wonder that neurologists first have to hunt down the ferocious AChR antibodies. They whisk off an aliquot of serum to a specialist laboratory, but waste no time in planning a counteroffensive, confident that the test will return as positive. The strategy is to boost the level of acetylcholine in the NMJ, tilting the balance in favour of ACh against the antibodies. The tactic is to zealously despatch a prescription for a drug that will block acetylcholine esterase inhibitor, the enzyme which breaks down acetylcholine. The neurologist then closely observes the often dramatic response, one of the most gratifying in clinical medicine; one minute as weak as a kitten, the next minute as strong as an ox. MG is therefore one disorder which debunks the wicked jibe that neurologists know so much…but do so little to make their patients better!
Unfortunately for the neurologist, every now and then, the AChR antibody test result comes back as negative. In the past, the dumbfounded and befuddled, but nevertheless undaunted neurologist, will march on, battling a diagnosis of antibody-negative MG. Nowadays however, this not a comfortable diagnosis to make because AChR antibody is no longer the only game in town. We now know that there are many other antibodies that are jostling for commanding positions in the anti-myasthenia coalition. These include anti LRP4, cotarctin, titin, agrin, netrin 1, VGKC, and ryanodine. However, the clear frontrunner in this melee is anti-MUSK antibody, responsible for 30-50% of MG in which there are no AChR antibodies.
Anti MUSK syndrome has many distinguishing features that set it apart from the run-of-the-mill myasthenia gravis. Below are five distinctive markers of anti-MUSK syndrome:
- Subjects with anti-MUSK syndrome are typically middle-aged women in their 3rd or 4th decades. This is younger than the usual age of people with AChR MG. Indeed neurologists now recognise typical myasthenia as a disease of older people.
- People with anti-MUSK syndrome present with acute and prominent involvement of head and neck muscles. This results in marked swallowing and breathing difficulties. They are therefore at a higher risk of myasthenia crisis.
- Single fiber electromyogram (sfEMG), a specific and reliable neurophysiological test of MG, is often normal in anti-MUSK syndrome. This is partly because the limb muscles are usually spared in anti MUSK syndrome.
- People with anti-MUSK myasthenia often do not benefit from, nor do they tolerate, the acetylcholinesterase inhibitors which are used to treat MG. Indeed, these drugs may worsen anti-MUSK syndrome.
- Thymectomy, removal of the thymus gland, is not beneficial in people with anti-MUSK syndrome, unlike its usefulness in AChR MG.
All this is just the tip of the evolving myasthenia gravis iceberg. You may explore more of myasthenia in our previous blog posts:
You may also explore anti-MUSK, and all the other myasthenia gravis subtypes, in neurochecklists. Go on…you know you want to know more!