Diagnostic Tests for Plantar Heel Pain
Diagnostic tests come in many forms, with each possessing strengths and limitations in their usefulness for diagnosing chronic heel pain that has failed to respond to treatment. This article will review the main diagnostic tests used to help diagnose heel pain.
X-rays have been, and are still, the most commonly used imaging tool used to help diagnose heel pain. The most common cause of heel pain by far is plantar fasciitis. X-rays, however, assess for bony abnormality and do not provide information on the soft tissues. As plantar fasciitis is a soft tissue condition an x-ray cannot be used to confirm a diagnosis of plantar fasciitis which begs the question why are x-rays the most common imaging tool for heel pain? The answer is that plantar fasciitis is diagnosed clinically through a detailed patient history and physical examination. Patients should not be referred for x-rays to confirm plantar fasciitis but instead to investigate for other causes of heel pain such as bony abnormalities. However, more times than not the x-ray findings are reported as normal. Occasionally the x-ray finding may identify a heel spur but currently, there is no evidence to prove a causal relationship between heel spurs and plantar fasciitis. Furthermore, there is evidence against a causal link between plantar heel spurs and plantar fasciitis which comes from anatomical and MRI studies (for more information on this check out another article in my blog here). Despite this x-rays do have their use and can also confirm later-stage stress fractures, bone tumours, bone cysts, periostitis, and erosions due to infection or rheumatologic causes.
Diagnostic ultrasound is considered the imaging modality of choice. Unlike x-ray which looks for other causes of heel pain, ultrasound can be used to confirm the diagnosis of plantar fasciitis as it investigates the health and appearance of the plantar fascia. In patients with clinical symptoms of plantar fasciitis, the proximal end of the fascia (the part that attaches to the heel bone is hypoechoic, which can be seen clearly when compared with the surrounding soft tissue. The plantar fascia is also significantly thicker in patients with plantar fasciitis. The consensus is that patients who have a plantar fascia measuring more than 4mm in thickness are classified as having plantar fasciitis, whereas people whose plantar fascia thickness measures 4 mm or less is considered normal. A potential limitation with diagnostic ultrasound is that it is operator-dependent meaning the accuracy of the test relies heavily on the skills and experience of the person operating the machine.
Ultrasound has been useful in the diagnosis of other causes of heel pain. For example, a retrocalcaneal bursa is seen as fluid between the Achilles tendon and the calcaneus (heel bone). A retro-Achilles bursa is fluid lying deep to the tendon. Fat pad atrophy (thinning of the fat pad) is demonstrated by discrete fluid between fat lobules in the plantar aspect of the foot with thinning of the fat. Upon compression, this gets even thinner and is much more compliant than the normal plantar fat. Colour Doppler is useful in diagnosing plantar vein thrombosis. Doppler flow is not generally seen, except for in association with perifascial fluid, in discrete distal nodules and at the calcaneal origin when there are erosions. This latter appearance is very rare and is seen as an extension of Achilles insertional change and erosions, presumably secondary to inflammatory disease (Ieong et al 2013). Colour Doppler can also be used to assess for the presence of neovascularisation which is an influx of intra-fascia arterial blood supply. This is believed to contribute to the pathological process and increased hypersensitivity of the tissue, however, there are conflicting views on the significance of this finding among health professionals.
Similar to x-rays, MRI is more useful to rule out other causes of heel pain than to confirm a diagnosis of plantar fasciitis, such as a stress fracture, tarsal tunnel syndrome, ganglion, infection or tumour. Usual MRI findings include thickening of the plantar fascia, swelling around the plantar fascia, bone marrow oedema of the heel, stress fracture, and tearing of the plantar fascia. A benefit from MRI over x-rays is that it can identify both soft tissue and bony tissue abnormalities and provides richer information which is more useful when investigating the cause of chronic heel pain. MRI may also be useful in establishing the presence of a space-occupying lesion beneath the flexor retinaculum at the ankle joint resulting in tarsal tunnel syndrome, which can mimic symptoms of plantar fasciitis. However, the incidence of such lesions is rare, and it would be hard to justify the expense of this test unless the examiner's index of suspicion was extremely high for such pathology.
Computerised Tomography (CT) Scan
Computerized tomography (CT) scans are generally only considered in cases of nonrespondent heel pain and if other causes of heel pain are suspected such as calcaneal stress fracture.
Technetium 99 Bone Scan
A bone scan is often positive with chronic heel pain and plantar fasciitis due to chronic periostitis and inflammation, with increased activity in the periosteum. A technetium 99 bone scan can also diagnose fatigue or a stress fracture. There will be diffuse, intense increased activity with a stress fracture, whereas plantar fasciitis will show focal uptake in the area of the medial calcaneal tuberosity. This imaging modality seems to be mentioned in textbooks however I have never personally referred a patient for this type of scan and I don't know a single colleague that has either. I think the reason for this is that other options have demonstrated superiority over this method and it has now become outdated.
Electrodiagnostic study - Nerve Conduction Velocity and Electromyography Test
A complete electrodiagnostic study involves nerve conduction studies (NCS) and electromyography (EMG). The two complement each other, in that both give information about peripheral nerves and muscles. Each in isolation does not yield a complete assessment of the cause of the symptoms being assessed in a limb or limbs.
In the first portion of the electrodiagnostic study, nerve conduction velocity test is used to measure the function of the nerves in the body. This test is performed if tarsal tunnel nerve entrapment is suspected. Normal scores are 4.1 and 4.7 milliseconds for the medial and lateral plantar nerves, respectively. Patients with tarsal tunnel syndrome show delays in the nerve conduction velocity studies. However, in some cases of tarsal tunnel syndrome, these studies may be normal. In the second portion of the test electrodiagnostic study, needle electromyography, a narrow needle electrode is inserted into the belly of the muscle being investigated while the patient voluntarily contracts the muscle. Although some literature suggests that only the presence of delayed nerve conduction velocity can establish the diagnosis of neurogenic heel pain, other authors believe that the diagnosis is a clinical one to make.
Electrodiagnostic studies have traditionally been the gold standard for confirming and evaluating the clinical diagnosis of tarsal tunnel syndrome. However false negative NCS and EMG are not uncommon, and so, unfortunately, do not rule out tarsal tunnel syndrome.
Pressure-specific sensory testing
A lesser known diagnostic tool is computer-assisted quantitative sensory testing, also known as a pressure-specified sensory device or PSSD (Dellon 1996). This neurosensory testing device has been shown to provide a more sensitive appreciation of peripheral nerve compromise, which can confirm a clinical diagnosis earlier in the progression of tarsal tunnel syndrome and/or other heel pain nerve abnormalities. The PSSD machine is designed to address the subtle changes that occur in peripheral nerves as nerve damage increases. PSSD may also be of benefit in evaluating the effects of surgical releases, but it should not be regarded as a replacement for a thorough history and physical examination.
Choosing the right test
There is no one perfect test for diagnosing chronic heel pain that is superior to other tests. Each test comes with its unique benefits and limitations. The traditional practice of referring patients for an x-ray as a first line diagnostic test should be stopped as this rarely provides further insight to the diagnosis and exposes patients to radiation. Instead, the diagnostic modality choice should be guided by a thorough patient history and physical examination by a competent clinician . This will allow the clinician to choose the most appropriate test(s) relevant to the individual patient.
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