How is a Morton’s neuroma best diagnosed, clinical examination, x-ray, CT scan, MRI, Ultrasound?
The taking of a good medical history, followed by a thorough physical examination and appropriate medical imaging are the 3 pillars of Morton’s neuroma diagnosis.
So, if you are planning on seeing Mr Weaver, or any other medical professional, about the pain in your forefoot, you might be wondering, how will my problem be assessed and investigated? You will have to answer the ‘Socrates’ questions.
Don’t worry you are not going to be quizzed about your knowledge of one of the forefathers of western philosophy, instead, you are going to be asked questions about the nature of your pain. This is because characteristics of forefoot pain are extremely important, as they give so many important clues about what might be causing your pain.
By using the mnemonic ‘Socrates’ a good clinician will mentally work through the following questions and will be able to make a good working diagnosis, based on the answers you give to the following questions:
Site of pain – where does it hurt?
Onset – when did it first occur …what were you doing? Etc.
Character -is it a burning pain is it a dull ache is it a sharp ache? Etc.
Radiation – does the pain radiate into your toes or up the foot? Etc.
Alleviating factors – what makes it more manageable or better? Etc.
Timing – how long does the pain last how often? Etc.
Exacerbation factors – what make it worse? Etc.
Severity – how bad is the pain at its worse on a scale of 1-10? 1-10 – (10 being childbirth).
After a medical history has been taken your clinician is most likely going to deploy what ‘many a patient’ calls the ‘poking around method’ otherwise known as clinical examination.
Simply pinching or gently squeezing the metatarsal interspace of the suspected nerve, between the thumb and index finger is thought by most professionals to be the definitive test for the presence of Morton’s neuroma.
This simple test has been shown to ‘pick-up’ the presence of Morton’s neuroma or bursitis in about 95% of cases. The main limitation of the test is that it can’t distinguish between Morton’s neuroma and bursitis.
The Mulder’s Click Test.
A ‘Mulder’s click’, named after Jacob Mulder, who first came up with the test, in 1951, was regarded by many, for decades, as the definitive test for the presence of Morton’s neuroma. In essence, if a click was felt by the examiner, the result was regarded as being positive for the presence of either Morton’s neuroma or bursitis. However, the click is only present in approximately 62-65% of people with Morton’s neuroma or bursa, meaning that 35% of people with a bursa or neuroma will have a negative click test.
Thus, the Mulder’s test is highly sensitive for the presence of Morton’s neuroma or bursitis, but unfortunately, is not very ‘specific’ in that, it misses at least 35% of people that have either Morton’s neuroma or bursitis.
A positive click test occurs when the metatarsal bones are squeezed together whilst simultaneously applying an upward force with the thumb to the underside the forefoot, as seen above. The clicking sensation is generated by the ‘subluxation’ or displacement of the enlarged nerve against surrounding structures.
The Mulder’s manoeuvre is performed at the site of the ‘gap’ between the metatarsal bones in the forefoot, firstly between the 2-3rdinterspaces, followed by the 3-4thinterspaces. After the gaps between the metatarsals have been assessed, the examiner will turn their attention to the metatarsal heads.
Normally, the metatarsal heads are not particularly tender or painful when palpated, pain on palpation, at the metatarsal heads can indicate that the source of the pain is not a neuroma, rather a separate condition, often relating to the joint between the long metatarsal bone, and the short first short bone of the toe.
After having taken a detailed history, and completed a ‘hands on’ examination, that might also involve tapping on the nerve below the inner ankle, many podiatrists at this stage may say they have enough information to make, or exclude a diagnosis of Morton’s neuroma or bursitis.
If an x-ray is requested, it generally means that problems with bones and arthritis need ‘ruling out’ as the cause of your pain.
Importantly, x-rays are not in themselves able to detect the presence of Morton’s neuroma. This is because a Morton’s neuroma is a soft tissue condition, and as such, does not show up on an x-ray (x-ray imaging is well suited to detecting issues with bones and not soft tissue structures such as nerves.).
On rare occasions, Morton’s neuroma can be ‘picked-up ‘on x-ray indirectly, because very large neuroma can force the metatarsal bones to pull away from each other, a process known as diastasis. However, diastasis also referred to as a Sullivan’s sign can often be seen with the naked eye, because diastasis causes a ‘v’ splaying of the two toes either side of the neuroma.
Computed Tomography for the detection of Morton’s neuroma was first described in 1991, at a time when MRI scanners were still in their infancy.
C.T. scanning is a mode of imaging that takes multiple x-ray slices, taken at various angles and are ‘stitches’ together by computer, to generate 3 dimensional images. Since 1991, there have only been a handful of published studies describing the use of C.T. scans for the detection of Morton’s neuroma. This is because (where ultrasound imaging is unavailable) MRI has overtaken C.T. scanning for the detection of Morton’s neuroma.
MRI has largely outside of the US, rendered CT scanning for Morton’s neuroma obsolete. This is because MRI provides more detailed images and unlike CT does not use radiation or ‘irradiate’ the foot.
C.T. scanners however, can be used as an alternative to MRI, for those who are unsuitable for MRI, those who have metal clips in their brain, those with recent internal metal work, those with pacemakers or internal defibrillators fitted.
C.T. scanning is once more becoming a little bit more common in the United states, this is because C.T. scans for Morton’s neuroma are half the price of MRI scans, combined with the fact that modern C.T. scanners generate far less radiation now than they did in 1991, and are considered much safer.
Magnetic resonance imaging (MRI) verses ultrasound imaging
During the last two decades the detection of Morton’s neuroma via imaging has mainly, relied on two types of imaging. MRI scanning, a form of imaging that utilises very powerful magnets that generate magnetic fields 60,000 times stronger than the earth’s magnetic field, and ultrasound, a form of imaging that utilises very high frequency sound waves.
Over recent decades both forms of imaging have proved accurate and reliable methods of diagnosing Morton’s neuroma. However, neither methods are 100% reliable, both forms of imaging have limitations and advantages over each other.
Both forms of imaging are also likely to miss the presence of Morton’s neuroma in 9-10 people out of every 100 who have a neuroma. The effectiveness or reliability of scans are determined by two key factors, sensitivity and specificity.
Sensitivity and specificity
With any medical test, the test should to be able to detect the condition that is being tested for, this is known as sensitivity. The test also needs to be able to correctly identify those without the condition being tested for, known as specificity. Put another way, a test with a 90% sensitivity rate will return a positive result in 90% of those with the condition, but will also return a negative result (a false-negative) for 10% of the people who have condition and should have tested positive.
How sensitive are M.R.I. and ultrasound scans for the detection of Morton’s neuroma?
A paper published in 2015 pooled the data from 14 previously published studies. The analysis revealed a pooled overall sensitivity rate of 90 % for MRI, with the least amount of sensitivity being reported being 82%, and the maximum 96%.
Similarly, for ultrasound, the results showed a pooled overall sensitivity of 91%, ranging from 83% of least sensitivity to a 96%. Of maximum sensitivity.
When it came to the question of specificity, i.e. ‘you definitely haven’t got a Morton’s neuroma’, pooled MRI data showed it was pretty much 100% reliable. Compared to ultrasound that had a pooled specificity of 85%, meaning that ultrasound was only accurately able to say you don’t have a Morton’s neuroma in 85% of cases.
A big weakness of this paper however was that it did not adequately discuss important differences between types of MRI scanners and scanning procedures. For instance, it is well known, that subtle ‘hard to see’ Morton’s neuroma can be missed if a contrast agent isn’t used. Contrast agents such as gadolinium (not suitable for those with kidney disease) are given generally via a drip or sometimes an injection.
When the contrast agent is the circulation it helps ‘light up’ a neuroma that might otherwise have gone unnoticed. Similarly, the study did not adequately account for differences in scanning techniques, for instance there is published evidence that the MRI images for Morton’s neuroma are better if the person being scanned is laid on their stomach, as this helps keep the foot still and neuroma in a better position to be seen.
Similarly, ultrasound scans are often performed in different ways. Sometimes ultrasound scans are performed with the ultrasound probe being placed on the top of the foot, (the opposite end to where neuroma are found), and some scans are performed with the probe on the bottom of the foot closer to the site of the neuroma.
The conclusion of the 2015 paper was Ultrasound has proved to be overall the most sensitive and cost-effective way of diagnosing Morton’s neuroma via imaging.
What advantages do MRI scans have over ultrasound?
Unlike ultrasound, they are not dependent on the skills of the operator, and they also generate good images of surrounding structures, so if the source of the pain is not a Morton’s neuroma an MRI scan is probably likely to show the more obscure alternative causes of forefoot pain.
What are the advantages of ultrasound over MRI?
- They are quick, and cost a lot less than MRI. On occasions, clinicians like to compare the ‘bad’ foot with the ‘good’ foot, this is because we are all individual and as such people frequently demonstrate ‘anatomical variation’ i.e. differing sizes and dimensions of nerves and muscles and fat tissue. It can be very useful to look at the good foot and see what is ‘normal for the patient’. Ultrasound lends itself very well to being able to quickly look at the opposite foot. Due to time and cost constraints of MRI, comparison of the ‘good foot’ is almost unheard of.
- Bursitis (a sack of fluid above and sometimes around the nerve) hand often go ‘hand in hand’ with Morton’s neuroma, and sometimes bursitis is mistakenly misdiagnosed as Morton’s neuroma. Ultrasound has a distinct advantage over MRI in that the bursae can be manipulated during an ultrasound scan. With a small amount of gentle pressure, the bursa can be compressed, revealing the full extent for the remaining nerve or neuroma.
- An emerging form of ultrasound called elastography enables detailed assessment of the density of the nerve, which is often a ‘tell-tale’ sign of subtle neuroma. For more information about our use of elastography see our July 2019 blog.
- A skilled operator can very quickly assess the forefoot for the presence of Morton’s neuroma, and the majority of the alternative causes of forefoot pain if a neuroma is not identified.
Are there any disadvantages to ultrasound scanning? Yes, there is a big one, ultrasound scans require a lot of technical skill and knowledge to perform and are ‘operator dependent’, meaning the scan is only as good as the person performing the scan.
In summary the benefits of imaging include:
Confirmation of diagnosis, this is especially important if you are contemplating any form treatment such as injections or surgery etc. Nerves are delicate structures, and you really don’t want to disturb a nerve unless you are convinced that is the source of your pain.
As those that read our February blog know it is also important to distinguish bursitis from Morton’s neuroma because bursa can more often than not be resolved with fairly basic straight forward conservative treatments whereas Morton’s neuroma tend to be more complex, especially when they have exceeded 6.5 mm in diameter.
Here at www.mortonsneuroma.co.uk we have a long commitment to medical ultrasound. Mr Weaver was one of the very first UK Podiatrists to adopt ultrasound imaging into everyday routine practice. Mr Weaver also hold a Masters degree with distinction in Medical ultrasound.
At our UK treatment centres in London and Sheffield ultrasound imaging is performed on every single patient with a suspected Morton’s neuroma or bursitis. This is because, firstly accurate diagnosis is vital for maintaining excellent patient outcomes. And secondly, cryosurgery for Morton’s neuroma is tailored to each individual needs and requirements and these needs can only be assessed and planned for with a pre-treatment scan.
Keep an eye on our website, as we will be announcing some great new offers for cryosurgery when it is safe for us to resume practice.
In the meantime stay home and stay safe.