How To Diagnose And Treat Interdigital Neuritis
In 1876, Morton described a peculiar and painful affliction of the foot in the area of the fourth and fifth metatarsals as the metatarsals compress the plantar interdigital nerve. Describing this as a neuroma is bit of a misnomer as the condition is more likely an entrapment rather than a true neuroma. The effects of compression on nerve fibers are extensive. Nerve compression can be from external edema surrounding the nerve, edema within the nerve or external compression from other anatomical structures in the area.
It may be more appropriate to refer to this entity as an “interdigital neuritis.” Researchers have investigated many theories, including inflammation, ischemia, muscle imbalances, direct trauma, soft tissue tumors and close proximity of the metatarsal heads.
Biomechanically, we know there is increased stress on the interdigital nerves in late stance and early heel off. As the digits dorsiflex against the weightbearing surface, the nerve becomes compressed and entrapped between the transverse metatarsal ligament and the plantar aspect of the foot. Furthermore, the nerve becomes elongated to its maximum length. There is variability in the severity of injury depending on acute or chronic compression of the nerve.
Histological findings will show a loss of myelinated fibers, thickening and fibrosis of the epineurium and perineurium, and thickening and hyalinization of the wall of the epineural and endoneural vessels. The A-beta fibers (large diameter) are the most susceptible to compression and ischemia. One will see a loss of circumferential A-beta fibers.
Mechanical and ischemic factors can also impair the function of the nerve. Other factors include inhibited microcirculation and axoplasmic transport, intraneural scar formation and edema. Eventually, fiber degeneration and death will occur.
Static touch on the skin is translated by Merkle cells with slow-adapting A-beta nerve fibers. Moving touch is translated by Meissner’s corpuscles, which have quickly adapting A-beta nerve fibers. Diminished oxygen slows axonal transport, leading to disruption of the A-beta fibers and loss of light touch. With compression, there is a circumferential drop out of the large diameter A-beta fibers.
It has been thought that the branches of the medial and lateral plantar nerves in the third interspace, which comprise the fourth plantar interdigital nerve, promote a thicker nerve, potentially leading to more chance of entrapment. Others have argued there is only a 26.8 percent incidence in communication between the medial and lateral plantar nerves in this area.
Other researchers have found the reactive scar tissue from repeated trauma to the surrounding tissue can disrupt the nerves in the surrounding area and lead to sensitivity. Other investigators have found increased evidence and signs of entrapment while examining the nerve microscopically. Some have implicated the intermetatarsophalangeal bursa as a cause of the neuritis.
A Guide To Key Signs And Helpful Diagnostic Modalities
Until recently, the diagnosis of an interdigital neuritis has been mainly subjective. The use of diagnostic tools can be helpful in solidifying a more definite diagnosis of neuroma. These patients will initially present, complaining of pain in the ball of the foot upon ambulation. Other symptoms have included burning and tingling or numbness in the adjacent digits. Many patients say they feel as though they are walking on a pebble. Some say they feel that tissue is rolling in and out through the ball of the foot. Rest usually relieves the symptoms. Patients rarely complain of waking up at night due to pain unless the pathology has been present for an extended period of time.
A positive Mulder sign will often lead toward a positive diagnosis by reproducing these symptoms. Most studies have found that interdigital neuritis is most common in the third interspace with the second interspace occasionally involved as well. There have also been reports of neuromas in more than one location but this is rare.