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Diagnostic Dilemmas

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.    It is important to distinguish this entity from metatarsalgia and other pathologies involving the MTP, especially that of a plantar plate tear, a pre-dislocation syndrome or capsulitis. It is not uncommon to diagnose a neuroma when, in fact, the patient has a torn plantar fascial plate. One can identify metatarsalgia when the patient experiences direct pain upon palpation over the metatarsal head plantarly or on direct distal to proximal palpation of the head with the digit dorsiflexed. Employing diagnostic ultrasound imaging of the MTP can also rule out a plantar plate tear. Making a diagnostic injection into the joint can help rule out metatarsalgia as well. However, this is somewhat questionable as injection leakage into the interspace in cases of a torn plantar plate may also relieve neuroma pain.    A better technique to rule out a plantar plate defect/tear is to strap or splint the associated toe at the base of the proximal phalanx in a plantarflexed position for two to three weeks. If this relieves the plantar MTP pain, a capsulitis or plantar plate tear is suggestive. If there is continued tingling or burning, even with slight relief, a neuroma is highly suspect as the cause of the pain.    With the advent of more sensitive ultrasound technology, diagnostic imaging of the neuroma can be helpful in identifying the nerve as well. Some studies suggest this method of identifying neuromas is limited but others have found they can predict the presence, size and exact location of the neuroma. Some prefer the dorsal approach while others will apply the transducer to the plantar aspect of the foot.    Neuromas are typically well-defined, hyperechoic and oval or round as one would see on ultrasound. Most symptomatic neuromas are greater than or equal to 5 mm in transverse diameter. When using ultrasound, keep in mind that you may observe intermetatarsal bursal fluid and one should not mistake this for a neuroma. Conventional MRI imaging is of little value in identifying neuromas.    Neurosensory testing using the Pressure Specified Sensory Device (PSSD) can also be very useful in identifying the neuroma. This examination can determine the amount of sensory loss in the associated digits. The PSSD will identify the amount of nerve damage from the compression by assessing a patient’s ability to feel a certain amount of pressure being applied by the probe. The pressure reading is fed into a computer and one can compare the results to normal values.    When it comes to interdigital neuritis, the areas one would test are the adjacent sides of the involved digits. One should test other dermatomes on the affected foot as well as the contralateral foot in order to rule out any global neuropathy or more proximal involvement. Clinicians can examine both one- and two-point discrimination. If there is a loss of sensation to the adjacent sides of the digits, which one can identify by elevated one-point discrimination values and or elevated two-point discrimination values, then the nerve is damaged and one can suspect a neuroma.

A Closer Look At Conservative Treatment Options

   As with many podiatric pathologies, there are many treatments for interdigital neuritis. Conservative therapy is about 40 percent successful, especially when clinicians identify the compression early. A good rule for the amount of time to spend using conservative therapies for neuromas is about three to four months. The goal of conservative therapy is to reduce and alleviate the pressure upon the nerve by decreasing the tension of the intermetatarsal ligament. One can achieve this primarily by reducing the inflammation within the nerve tissue or reducing the stress on the nerve by the surrounding structures.    Initially, one should emphasize shoe gear and functional foot orthotics. The orthotic can control the excessive motion of the subtalar joint, which could be contributing to the uneven weight distribution through the metatarsals. In addition, clinicians can add a metatarsal pad to the device to spread the head of the metatarsals in order to allow the nerve to glide more freely in the interspace. Advise the patient not to walk barefoot and to avoid high-heeled shoes.    One can address nerve inflammation in a number of ways. Emphasizing a regimen of ice therapy for upward of two weeks along with antiinflammatory medications can at least reduce symptoms in the acute cases. Traditionally, clinicians have employed local cortisone injections but one should reserve this treatment for the acute and recalcitrant presentations. The number of treatments depend on the amount of resolution from the previous injection. If the patient admits there is no appreciable resolution in the symptoms after the first injection, this indicates another injection would not be helpful.    However, if the patient feels there is a percentage of improvement, a series of three injections can be very useful in conjunction with other therapies. One may use cortisone later as a secondary conservative treatment. In our offices, we give cortisone injections under ultrasound guidance. We find more predictable results when we deliver the medication as close to the nerve as possible.    The effectiveness of physical therapy is often overlooked as a critical component to restoring functionality in our patients. When it comes to patients who have a neuroma, it is recommended to refer them to a physical therapist early in the conservative treatment phase. The therapist can use ultrasound and laser therapy to reduce the inflammation locally. In addition, employing iontophoresis with dexamethasone in the intermetatarsal space can further aid in reducing the inflammatory response. If there is any scar tissue in the area, the physical therapist can break this down by transverse tissue friction massage and other modalities.    Sclerosing agents have also been used essentially to destroy the nerve. This would essentially stop the nerve from transmitting the symptomatic signals. This technique is not as predictable. The injections are painful. It is easy to sclerose other vital structures in the area. If a neurectomy needs to be performed after the sclerosis, it becomes very difficult to visualize the nerve. Be aware that decompression surgery has a higher failure rate when the patient has had sclerosing therapy in the past due to internal damage to the nerve fibers and architecture.

When One Should Explore Surgical Treatment

   If conservative therapies fail and the podiatrist is still confident in the initial diagnosis, surgical treatments may be indicated. Neurectomy used to be the most commonly used procedure for treating interdigital neuritis. Increasingly, surgeons have reserved this for a secondary treatment or when the nerve is grossly thickened. A dorsal incision is preferable over a plantar incision for a primary surgery. Once one identifies the nerve, surgeons should transect it as far proximally and its branches as far distally as possible in order to avoid stump formation. While it is more technically challenging to do so, one may implant the proximal end of the nerve into adjacent muscle belly in order to avoid complications from a stump neuroma.    Patients with recurrent neuromas may benefit from revision surgery. Most podiatrists will use a plantar incision for this revision. Employing the plantar approach can help one better identify the proximal stump and more easily address scar tissue in the area. Remove any and all scar tissue as a result of the primary procedure.

Final Notes

   Interdigital neuritis is most often a compression of the nerve by surrounding structures. The transverse metatarsal ligament is the most deforming force in the area. As the nerve becomes inflamed and it passes under the ligament, it can become even more irritated and entrapped. Many podiatrists have advocated the release or division of the intermetatarsal ligament without neurolysis. Again, the rationale for this technique is that the neuritis is an entrapment syndrome. Therefore, release of the offending intermetatarsal ligament can provide lasting relief. The nerve will then be able to glide more freely. Advantages of this treatment are no loss of sensation or possibility of stump neuroma formation. If there are recalcitrant symptoms following this technique, a neurectomy or sclerosis is still available as a valued treatment. Dr. Baravarian (shown at the right) is Co-Director of the Foot and Ankle Institute of Santa Monica. He is an Associate Professor at UCLA Medical Center and is Chief of Podiatric Surgery at Santa Monica/UCLA Medical Center. Dr. Baravarian may be reached via e-mail at: Dr. Soomekh is an Associate at the Foot and Ankle Institute of Santa Monica, and is an Attending Staff Member of the UCLA Medical Center.

Diagnostic Dilemmas
By David Soomekh, DPM, and Babak Baravarian, DPM
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