Morton neuromas, also known as interdigital or intermetatarsal neuromas, are focal areas of symptomatic perineural fibrosis around a plantar digital nerve of the foot. The condition is thought to be due to chronic entrapment of the nerve by the intermetatarsal ligament.
The term neuroma is a misnomer because the abnormality is non-neoplastic and does not represent a true neuroma. It may more correctly be known as Morton metatarsalgia.
It most often occurs in middle-aged individuals and is many times more common in women than men. Approximately 30% of asymptomatic middle-aged persons have the radiologic-pathologic findings of a Morton neuroma.
Patients typically present with forefoot pain which radiates from midfoot to toes . Symptoms are often progressive and worsened by activity. The Mulder sign is a physical sign associated with Morton neuroma, which may be elicited while the patient is in a supine position. The pain associated with the neuroma, as well as a click, can be reproduced by squeezing the two metatarsal heads together with one hand, while concomitantly putting pressure on the interdigital space with the other hand.
A number of other clinical tests for Morton neuroma have been described, all of which have high specificity and aim to either reproduce symptoms of pain or paresthesia. These include :
- thumb-index finger squeeze: squeeze the symptomatic intermetatarsal space between the index (dorsal) and thumb (plantar) surfaces
- foot squeeze test: compression of metatarsal heads between fingers and thumb
- plantar and dorsal percussion tests: percussion of the dorsal and plantar intermetatarsal spaces with a finger
- light touch sensory test: stroking the tip of the affected toe produces sensation different to the adjacent toes
It is characterized by neural degeneration with epineural and endoneural vascular hyalinisation, and perineural fibrosis around a plantar digital nerve .
The 3 web-space (between 3 and 4metatarsal heads) is the most commonly affected site. The 2 web-space is less often involved while the remaining web-spaces are rarely involved. 10% of lesions are bilateral.
Occasionally specific terms are used when occurring certain spaces
- 1 intermetatarsal space: Heuter neuroma
- 4intermetatarsal space: Iselin neuroma
Symptomatic lesions tend to be slightly larger (mean 5.3 mm vs. 4.1 mm in one large series ). Lesions >5 mm are very likely to be symptomatic.
Typically seen as a round to ovoid, well-defined, hypoechoic lesion in the intermetatarsal space proximal to the metatarsal head . A Morton neuroma is not compressible. A small proportion can have mixed echotexture . A sonographic Mulder sign may be elicited with the probe .
Dumbbell/ovoid-shaped lesion at a similar position to that described on ultrasound :
- T1: typically low-to-iso signal
- T2: typically low signal but can sometimes be intermediate in signal
- T1 C+ (Gd): variable enhancement
Treatment and prognosis
Ultrasound-guided interdigital injection of steroid and local anesthetic has been demonstrated to have a relatively high success rate .
Surgical excision can also be performed, also with a relatively high success rate (~80% ).
History and etymology
It is named after Thomas George Morton (1835-1903), an American surgeon, who described a case series in 1876 . However, it was first described by Civinini in 1835 .
Other causes of metatarsalgia:
- intermetatarsal bursa/bursitis
- extruding out in between the metatarsal bones on the plantar aspect of the foot
- metatarsophalangeal joint synovitis/capsulitis
- changes secondary to a plantar plate tear/disruption
- soft tissues tumors, e.g. schwannoma
- metatarsal head insufficiency fracture / Freiberg disease
- tenosynovial giant cell tumor