The nerve that runs between the third and fourth (usually) toes is often entrapped between the bones. The condition is called Morton's neuralgia. After recurrent episodes of pain, the nerve becomes chronically swollen. The main symptoms of a patient with Morton's is pain. This is usually present on standing or walking. The pain tends to be felt at the base of the toes and can often, radiate up and down the toes with a tingling sensation.
Conduction of impulses from nerve fibres in the skin to muscles or the reverse is studied using nerve conduction studies. When there is a nerve abnormality, this is indicated by a slowing in the nerve conduction.
Motor NCS are performed by electrical stimulation of a peripheral nerve and recording from a muscle supplied by this nerve. The time it takes for the electrical impulse to travel from the stimulation to the recording site is measured. This value is called the latency and is measured in milliseconds (ms). The size of the response - called the amplitude - is measured in millivolts (mV). By stimulating in two or more locations along the same nerve, the NCV across different segments can be determined. Calculations are performed using the distance between the different stimulating electrodes and the difference in latencies.
Sensory NCS are performed by electrical stimulation of a peripheral nerve and recording from a purely sensory portion of the nerve, such as on a toe. Like the motor studies, sensory latencies are on the scale of milliseconds. Sensory recordings are usually in the microvolt (μV) range. The sensory NCV is calculated based upon the latency and the distance between the stimulating and recording electrodes.
An MRI is also useful in diagnosing a Morton's neuroma. It clearly shows the area of thickened nerve, between 3 rd and fourth metatarsals.
A cross-sectional MRI showing the thickened nerve between the 3 & 4 toes
Soft insoles can be worn, so that the whole front of the foot can be off-loaded. This may be comfortably worn inside a shoe or alternatively, soft MCP footwear can be worn.
Steroid and local anaesthetic mixture is injected into the relevant web space, which contains the Morton's neuroma. Injection if given early, works well. It can be repeated.
Injection spot for Morton's neuroma
Often conservative management fails to relieve the symptoms, requiring surgery, to remove the thickened and inflammed nerve. The top of the forefoot between the two toes is opened, under local or regional block or general anaesthesia. A thickened and inflammed nerve is usually found. Patient has to be warned that permanent numbness in the area might occur.