A 56-year-old male with known type II diabetes mellitus presents to the pain clinic with pain along the left lateral thigh without evidence of back or hip pain. He states that the pain is lancinating and burning in character and associated with a pins and needles sensation in his left lateral thigh. Diagnostic studies show an HbA1c of 12 and blood glucose of 300, and MRI of the lumbar spine does not indicate any significant abnormalities. A fascicular biopsy of the nerve in the lateral thigh shows small diameter regenerating myelinated fibers
and evidence of diabetic microangiopathy.
Mononeuropathies are a not uncommon source of pain, of which one of the most notable is postherpetic neuralgia, which is covered in Chapter 3. Other notable painful mononeuropathies include diabetic mononeuropathy and amyotrophy, mononeuropathy multiplex, entrapment neuropathies, and mononeuropathies due to peripheral nerve tumors.
TYPES OF MONONEUROPATHIES
Diabetic Mononeuropathy and Amyotrophy
Mononeuropathy is a nerve injury that specifically targets one nerve, and thus the symptoms exhibited correspond to the anatomical function of the nerve that was injured. Compared to the normal population, diabetic patients have an increased predisposition to mononeuropathies, of which mononeuropathies of the motor nerves serving extraocular movements, as well as of the peripheral nerves, are the most common in location. Painful mononeuropathies of the cranial nerves responsible for extraocular movements can manifest days prior to the onset of actual muscle weakness, and are characterized as pain starting behind or around the eye. Mononeuropathies can also extend to the periphery as well as the thoracic nerves specifically targeting the median, ulnar, peroneal, femoral, and lateral cutaneous nerves. As with the nerves of extraocular movement, pain is a common symptom of peripheral nerve mononeuropathy.
Another painful diabetic proximal neuropathy is termed diabetic amyotrophy. Diabetic amyotrophy is now more commonly termed either proximal diabetic neuropathy or lumbosacral radiculoplexus neuropathy and described as painful proximal muscle wasting and weakness. Presentation is usually asymmetric, and there is usually a loss of normal patellar reflexes on the side that is affected. EMG can be performed and will show evidence of a mixed axonal and demyelinatingneuropathy affecting the proximal musculature. The pathology is thought to be due to microangiopathic changes causing ischemia to the peripheral nerves.
Mononeuritis multiplex is a disease state characterized by multiple noncontiguous mononeuropathies occurring acutely to chronically over months to many years that presents with loss of sensory or motor function of individual peripheral nerves. This type of neuropathy is often associated with pain that is neuropathic in quality, and is also characterized as deep, aching pain, worse at night, and located mostly in the back, hip, and thigh. EMG can be done to examine the patient and will show signs consistent with multifocal sensory motor axonal neuropathy.
Commonly associated systemic illnesses include diabetes mellitus, vasculitides, autoimmune disease including SLE and sarcoidosis, leprosy, Lyme disease, HIV infection, amyloidosis, cryoglobulinemia, and chemical agents.
Entrapment neuropathies are most commonly due to compression of nerves traversing narrow passages. Most common locations include the carpal tunnel and spinal roots exiting the foramen due to compression by adjacent herniated discs or hypertrophy of the associated facet joint. This usually results in a painful condition that may be associated with paresthesias, muscle weakness, and numbness along the pathway of the nerve affected. The underlying pathophysiology indicates that entrapment leads to damage and reduction of the myelinated fibers with persistence of the C-fibers. This is thought to be due to microangiopathic changes leading to ischemia of the larger myelinated fibers. Patients will describe the pain as burning, pins and needles, and lancinating pain. EMG in early disease may not show evidence of demyelination of the larger nerves, and does not do a particularly good job of describing pathology of the C-fibers.
There are many examples of entrapment neuropathies, including carpal tunnel syndrome, Morton's neuralgia, and radiculopathy. Morton's neuralgia is due to severe entrapment of the plantar digital nerve on the metatarsal heads of the foot. Patients will complain of burning pain at the bottom of their foot and at the bottom of their toe.
Proper history and physical, as well as EMG are the diagnostic tools of choice. MRI of the spine may be helpful for showing structural abnormalities including herniated discs and facet hypertrophy as a cause of spinal nerve root compression. For those with pain associated with thoracic outlet syndrome, MRI is again the diagnostic test of choice to examine structures that may be compressing the exiting brachial plexus causing severe pain in the upper shoulder.
Peripheral Nerve Tumors
Peripheral nerve tumors are a rare source of painful mononeuropathy but must be included in the differential when evaluating a patient with mononeuropathy. Tumors associated are commonly benign schwannomas, neurofibromas, neurinomas, and less commonly, malignant neuromas. Presenting symptoms may include local tenderness, paresthesias in the distribution of the affected nerve, swelling, pain, and palpable masses for those that are more superficially located. The diagnostic modality of choice for those that may be deeper is MRI with contrast. Tissue biopsy should be performed for histologic characterization of the mass.
The best treatment for the aforementioned disease states is careful treatment of the underlying source, which can include tight control of blood glucose, treatment of infectious causes with antibiotics or antivirals, and so forth. Medical therapy utilizing anticonvulsants and antidepressants may be useful in targeting the neuropathic pain components. As well, topical anesthetics may be used for symptoms of allodynia and dysesthesia. As a last resort, if all other therapies do not appear helpful, spinal cord stimulation may prove to be useful in decreasing pain.
Key Points to Remember
• Mononeuropathies are a not uncommon source of pain, of which one of the most notable is postherpetic neuralgia, which is covered in its own section of this book.
• Other notable painful mononeuropathies include diabetic mononeuropathy and amyotrophy, mononeuropathy multiplex, entrapment neuropathies, and mononeuropathies due to peripheral nerve tumors.
• Diagnosis is via EMG, MRI, and tissue biopsy in some cases.
• Treatment is aimed at treating the underlying disorder and palliation with oral anticonvulsants or antidepressants.
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