Interdigital nerve compression syndrome: “Morton’s Neuroma”

Morton’s Neuromas are a common cause of metatarsalgia (pain in the forefoot).

Morton’s Neuroma is a compression neuropathy of one of the common digital plantar nerves. In simpler terms, this means that a Morton’s Neuroma occurs when one of the nerves in the sole of your foot that go to the toes is being squashed, leading to pain. It most often occurs in the 3rd intermetatarsal space (66% of cases), sometimes in the 2nd intermetatarsal sapce (30% of cases), and rarely in the 1st or 4th intermetatarsal spaces. Morton’s Neuroma most often affects only one intermetatarsal space, although it can happen that a person has more than one in one foot. This condition is unilateral in 79% of cases (meaning it affects only one foot in most people). It is the second most common compression neuropathy, second only to Carpal Tunnel Syndrome. People are most often diagnosed with Morton’s between the ages of 40 and 60 years, with women being more often affected than men (4:1 ratio), possibly due to footwear choices. People with a BMI over 25 are also at greater risk than those with a BMI in the normal range.

Morton’s Neuroma is not a true neuroma, despite its name. True neuromas show proliferative activity, meaning that true neuromas are a form of benign (non-cancerous) tumour. Morton’s Neuromas do not have this quality, and instead show signs of fibrous tissue forming around the nerve, a process called “perineural fibrosis”, as well as signs of the nerve degenerating and demyelinating (losing its myelin sheath). The lesion lies approximately 5-10mm deep to the plantar skin surface, just proximal to the point where the common digital nerve divides into the plantar digital nerves proper. See the image below for a picture of this location.

Plantar view of the foot with superficial structures removed

There are believed to be four possible causes of Morton’s Neuroma:

  1. Compression or tensile forces acting upon the plantar digital nerves: this could happen because the 3rd common digital nerve is thicker than the others, as it is formed from branches of both the medial and lateral plantar nerves; the 3rd metatarsal is quite immobile in the sagittal plane compared to the 4th, meaning that the nerve can be put under tension as the 4th moves and the 3rd remains in place during gait; hyperextension of the digits at the metatarsal-phalangeal joints, often by footwear such as high heels, forcing the plantar digital nerves up against the transverse intermetatarsal ligament, compressing the nerve.
  2. Distal extension of the intermetatarsal bursa: hypermobility of the forefoot, abnormal subtalar joint pronation, and lateral compression due to restrictive footwear creates shear on the intermetatarsal bursa, leading to inflammation and increased volume of fluid in the bursa. The increase in intrabursal pressure causes it to extrude distally and impinge on the neurovascular bundle, leading to an entrapment neuropathy of the plantar digital nerve and compression of the local artery, leading to ischaemia.
  3. Transient neuroischaemia: changes in blood flow due to intermittent compression of the vasa nervosum, the artery feeding the nerve, either by abnormal biomechanics or by distal distention of the intermetatarsal bursa (see no.2 above), leading to reduced blood flow to the nerve and the forming of fibrous tissues (perineural fibrosis).
  4. Nerve compression or entrapment due to poor lower limb biomechanics – the nerve becomes entrapped between the plantar aspect of the foot (sole), the the ground it is in contact with, and the transverse intermetatarsal ligament during gait. Clinical experience suggests that most people with Morton’s Neuroma have inversion of the forefoot when the foot is non-weightbearing. This tends to lead to a compensating behaviour at toe-off where the lateral column of the foot dorsiflexes and the medial column plantarflexes when the forefoot is loaded at toe off, leading to stretching of the intermetatarsal tissues, chronic inflammation, and the forming of fiberous tissue around the common digital nerve at the site where is divides into the plantar digital nerves.

Risk factors for developing Morton’s Neuroma include:

  1. High-arched feet.
  2. Wearing tight/badly fitting shoes that squeeze the forefoot or increase pressures on the forefoot.
  3. Repetitive high impact activities, such as running or dancing.
  4. Above normal BMI (25+).
  5. Being female.
  6. Being aged 40-60 years.

Prognosis is poor without treatment, with the symptoms being unlikely to resolve by themselves. Some sort of treatment or intervention is therefore recommended.

People with Morton’s Neuroma usually present with sharp, stabbing, burning, shooting pain in the forefoot, which may be associated with cramps or numbness of the toes. The pain is usually brought on by standing, walking or running. Half of people will describe strange or altered sensations, usually something along the lines of a “pebble” in the shoe or feeling as if their socks are ruckled up under their toes or “pins and needles” (parasthesia) or numbness. These are signs of neuropathy (nerve injury). The pain often comes and goes, with episodes lasting from a few minutes to several hours. The pain can be severe, even disabling. These episodes can be frequent or infrequent with weeks or months between attacks. In chronic cases, the pain can become constant. Around 25% of people with Morton’s report pain at night and at rest.

A podiatrist can diagnose Morton’s Neuroma using the history of the condition, presenting symptoms, and several non-invasive “hands on” tests such as Mulder’s Test where the forefoot is compressed laterally and plantar pressure is applied the distal intermetatarsal spaces. If the pain is reproduced, this indicates a possible Morton’s Neuroma. If there is an audible “click” (Mulders Click), this is further evidence supporting a diagnosis of Morton’s Neuroma. Interdigital neurological testing tools, such as a 10g monofilament or a Neurotip, can also be used and if there are clear signs of sensory loss compared to the other interdigital spaces, this is also a clear indictor of Morton’s. If the diagnosis is unclear, X-rays can be used to rule out other pathologies that include fractures, although neuromas cannot be visualised using radiographs. Blood tests could be recommended to exclude osteomyelitis, gout and inflammatory arthritis. Ultrasound imaging can also be used to diagnose this condition if necessary. MRI can also be used but is usually not needed.

Sometimes people come in thinking they have a Morton’s Neuroma when they do not. Differential diagnoses for this condition include:

  1. Biomechanical issues leading to increased tissue stress and pain or discomfort in the area.
  2. Metatarsal-phalangeal joint capsulitis.
  3. Stress fractures.
  4. Gout.
  5. Osteoarthritis.
  6. Tarsal Tunnel Syndrome.
  7. Plantar Plate Tears or Ruptures.
  8. Vascular disease.
  9. Muscle pain.
  10. Peripheral Neuropathy.
  11. Freiberg’s Disease.
  12. Rhuematoid Arthritis and/or nodules.
  13. Plantarfibromatosis.
  14. Plantar fatty pad atrophy.

This isn’t an exhaustive list.

Treatments in podiatry practice include recommending changes to footwear, prescribing in-shoe, functional foot orthoses (usually with a metatarsal dome) following a full biomechanical assessment, and manual- or electro-acupuncture for pain relief. Oral analgesic medication can be prescribed, such as NSAIDs (non-steroidal anti-inflammatory drugs). In some cases, corticosteroid injections or surgery may be necessary to alleviate symptoms, but this can often be avoided with conservative interventions.

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