Morton neuroma, or interdigital neuroma, is a common condition that involves enlargement of the interdigital nerve of the foot. Morton neuroma most commonly affects the third intermetatarsal space, but it can also affect the second intermetatarsal space, although it does so less often. Lesions in the fourth and first interspaces are unusual.
See the images of Morton Neuroma below.
The etiology remains controversial, and treatment varies among practitioners. Diagnosing the condition involves clinical judgment, and in difficult cases, imaging may be useful in determining the precise diagnosis.
The differential diagnosis of Morton neuroma includes a stress fracture, a tendon sheath ganglion, a foreign-body reaction, a nerve-sheath tumor, strain of the plantar capsule, and capsulitis or bursitis at the level of the plantar metatarsal-phalangeal joint (MPJ). [1, 2] Occasionally, the diagnosis can be challenging because, in addition to inflammation of the capsule or bursa, inflammation of the adjacent nerve also may be present, causing the neuritic sensation of a Morton neuroma.
Peripheral neuropathy also can cause symptoms such as a burning or neuritic sensation, but these tend to occur throughout the toes and on both feet. Additional differential diagnoses with similar symptoms involve neoplasms and fibromas. Imaging studies, particularly MRIs, are a valuable tool in differentiating these conditions.
Neuromas are a common ailment seen by foot and ankle specialists. The diagnosis is often straightforward because of the description of a sharp, shooting pain or numbness in the affected digits. However, because the patients' descriptions of the pain can vary or because the pain can be related to another ailment in the differential diagnosis, MRI has become a useful imaging tool in obtaining a precise diagnosis. Ultrasonography also is commonly used. It is more widely available and less costly than MRI. Despite these advantages, the operator-intensive aspects of sonography, as well as its steep learning curve, are limiting factors. Whether ultrasonography will eventually replace MRI as the primary imaging modality for this condition remains to be determined.
In a study of 37 patients (43 intermetatarsal spaces) with confirmed Morton neuroma, MRI or ultrasound was performed to complement clinical diagnosis. An MRI was performed in 41 cases and ultrasonography in 23 cases. Of the 41 MRIs performed, 34 were positive for Morton neuroma and 7 were negative. MRI sensitivity was 82.9% [95% confidence interval (CI): 0.679-0.929]. Of 23 ultrasounds performed, 13 were positive and 10 were negative. US sensitivity was 56.5% (95% CI: 0.345-0.768). 
Although a Morton neuroma can often be confidently diagnosed on the basis of the clinical findings alone, imaging is useful in the preoperative setting or in cases with atypical symptoms.  Morton neuromas have been imaged successfully by using MRI, CT scanning, and ultrasonography. [2, 4] MRI has been the preferred imaging modality for demonstrating Morton neuroma largely because of MRI's superior contrast resolution and reproducibility. [5, 6]
Ultrasonography has the advantage of being less expensive and more portable than MRI, but because of its high learning curve and dependence on the operator's skill, whether ultrasonography will replace MRI as the preferred imaging modality for this condition remains to be seen. Relatively few radiologists and technologists in the United States are experienced in using ultrasonography to diagnose Morton neuroma. The employment of ultrasonography in this diagnosis, although described in the literature, most commonly occurs in subspecialty academic settings and outside the United States. 
Morton neuroma was confirmed on ultrasonography at the site of clinical diagnosis in 98% of patients (53 out of 54 feet) in one study. In the study, the diagnostic accuracy of 7 clinical tests for Morton neuroma were compared with ultrasonography. The 7 tests and their results were as follows: thumb index finger squeeze (96% sensitivity, 96% accuracy), Mulder's click (61% sensitivity, 62% accuracy), foot squeeze (41% sensitivity, 41% accuracy), plantar percussion (37% sensitivity, 36% accuracy), dorsal percussion (33% sensitivity, 26% accuracy), and light touch and pin prick (26% sensitivity, 25% accuracy). The thumb index finger squeeze test was the most sensitive screening test for the clinical diagnosis of Morton neuroma. 
Not uncommonly, Morton neuromas are incidental findings on MRIs; they are detected best on short-axis (transverse) T1-weighted MRIs through the metatarsal heads. Morton neuroma is typically seen as a bulbous mass arising between the metatarsal heads. Because neuromas have a highly cellular and/or fibrous nature, lesions demonstrate low signal intensity on both T1- and T2-weighted images (see the images below). The lesions are highly vascular and are typically enhancing after the intravenous administration of a gadolinium-based contrast agent.  Contrast enhancement is best visualized on T1-weighted fat-saturated images.
Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis or nephrogenic fibrosing dermopathy (NSF/NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. NSF/NFD has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or magnetic resonance angiography scans.
NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape.
Reliably differentiating Morton neuroma from a normal plantar nerve is difficult unless the lesion is 5 mm or larger.  An intermetatarsal space lesion that is bright on T2-weighted images is unlikely to represent a Morton neuroma. Lesions with hyperintense signal on T2-weighted images are more likely to represent a true neuroma, a synovial or rheumatoid nodule, or bursal fluid. The administration of contrast material assists in differentiating these entities because true neuromas are intensely enhancing, and bursal fluid is not enhancing. 
MRI findings in a suggested Morton neuroma effectively influence therapy. In one study, imaging findings changed the treatment plan in 57% of patients, with the withdrawal of the diagnosis in 28% and with changes in the location or number of detected neuromas in more than 33% of the feet of the remaining patients.  Interspace lesions with low intensity appear on imaging studies in asymptomatic patients.  Therefore, careful correlation with the clinical findings is essential prior to initiating therapy.