Mononeuritis (Mononeuropathy)

What mononeuritis means

The word combines mono (one), neur (nerve) and -itis (inflammation). Strictly, "mononeuritis" implies inflammation, while mononeuropathy is the broader term for any damage to a single nerve. In practice doctors use the two interchangeably.

A peripheral nerve is a cable that carries movement signals out from the spinal cord and sensory signals back. Each nerve serves a fixed patch of skin and set of muscles. That is why mononeuritis produces a sharply defined pattern: for example numbness only in the thumb side of the hand, or weakness only when lifting the front of the foot. This pattern sets it apart from peripheral neuropathy (polyneuropathy), which affects many nerves evenly, usually starting in both feet.

Mononeuritis multiplex

In mononeuritis multiplex, two or more separate nerves in different parts of the body are damaged, often in quick succession. For example, a foot drop on one side followed weeks later by numbness in the opposite hand. The asymmetric, patchy pattern points to damage to the small blood vessels that feed the nerves, usually from vasculitis. It is a warning sign of a systemic illness and needs prompt investigation.

What mononeuritis is usually caused by

Compression and entrapment (most common)

Nerves passing through tight spaces or over bone are easily squeezed:

• Median nerve at the wrist: carpal tunnel syndrome, the most common mononeuropathy.
• Ulnar nerve at the elbow: cubital tunnel syndrome, affecting the ring and little fingers.
• Radial nerve in the upper arm: "Saturday night palsy" from sleeping with the arm over a chair, causing wrist drop.
• Common peroneal (fibular) nerve at the knee: from crossing the legs, weight loss, prolonged squatting or a tight cast, causing foot drop.
• Lateral femoral cutaneous nerve at the hip: meralgia paresthetica, burning numbness on the outer thigh, linked to tight belts, obesity and pregnancy.
• Tibial nerve at the ankle: tarsal tunnel syndrome.

Injury

Fractures, dislocations, cuts, stretch injuries and injections near a nerve can damage it directly.

Diabetes

Diabetes makes nerves more vulnerable to compression and can cause sudden single-nerve palsies, for example a third cranial nerve palsy with a drooping eyelid and double vision, or painful thigh weakness (diabetic amyotrophy).

Inflammation and infection

• Bell's palsy, a mononeuritis of the facial nerve, often linked to viral infection
• Shingles, Lyme disease, leprosy (a major cause worldwide), HIV and hepatitis C
• Sarcoidosis

Causes of mononeuritis multiplex

• Vasculitis: polyarteritis nodosa, granulomatosis with polyangiitis, eosinophilic granulomatosis with polyangiitis
• Connective tissue diseases: rheumatoid arthritis, lupus, Sjögren's syndrome
• Diabetes
• Infections: hepatitis B and C, HIV, leprosy, Lyme disease
• Cryoglobulinemia, amyloidosis, sarcoidosis and, rarely, cancer
• Hereditary neuropathy with liability to pressure palsies, an inherited tendency to repeated compression palsies

Risk factors

Diabetes, hypothyroidism, rheumatoid arthritis, pregnancy, obesity, alcohol misuse, vitamin B12 deficiency, kidney disease and repetitive manual work all raise the risk.

Symptoms

Symptoms follow the territory of the affected nerve:

• numbness or reduced feeling
• tingling, pins and needles or burning pain
• weakness or clumsiness of particular muscles, such as weak grip or a slapping foot
• muscle wasting over months, for example at the base of the thumb
• pain that may spread up or down the limb from the point of compression

Compression symptoms are often worse at night or in certain positions. Vasculitic mononeuritis typically starts suddenly with deep, aching pain followed by numbness and weakness over hours to days.

Diagnosis

1. History and examination: the doctor maps which areas are numb and which muscles are weak to identify the nerve and the likely site of damage.
2. Nerve conduction studies and EMG: these measure how fast and how strongly nerves carry signals and show whether damage is mild, moderate or severe.
3. Blood tests: glucose and HbA1c, thyroid function, vitamin B12, kidney function, and for multiplex patterns, inflammatory markers (ESR, CRP), ANCA, ANA, rheumatoid factor, hepatitis serology and cryoglobulins.
4. Imaging: ultrasound or MRI of the nerve or surrounding area to look for a cyst, tumor or anatomical squeeze.
5. Nerve biopsy: sometimes needed to confirm vasculitis, usually of the sural nerve at the ankle.

Treatment

Treating the cause

• Compression: avoid the position or pressure, change work habits, pad the area, and lose weight if that is a factor.
• Diabetes: better blood sugar control.
• Vasculitis: steroids and other immune-suppressing medicines, started promptly to prevent further nerve damage.
• Infection: specific antiviral or antibiotic treatment where relevant.

Supporting the nerve and function

• Splints: a wrist splint at night for carpal tunnel, an elbow pad or night splint for ulnar neuropathy, an ankle-foot orthosis for foot drop.
• Physical and occupational therapy to maintain strength and joint movement.
• Nerve pain medicines such as amitriptyline, duloxetine, gabapentin or pregabalin, which work better than standard pain relievers for burning nerve pain.
• Steroid injections for some entrapments, such as carpal tunnel syndrome.

Surgery

Decompression surgery, such as carpal tunnel release or ulnar nerve release, is offered when symptoms are severe, persistent or causing weakness and wasting. Damaged nerves after trauma may need repair or grafting.

Outlook

Recovery depends on how badly the nerve is damaged. Mild compression often recovers fully within weeks once the pressure is removed. When the nerve's insulation (myelin) is damaged, recovery usually takes 6 to 12 weeks. When the nerve fibers themselves are damaged, they regrow at about 1 mm a day, roughly an inch a month, so recovery can take many months and may be incomplete. Mononeuritis multiplex from vasculitis can stabilize with treatment, but some deficits often remain.

Frequently asked questions

What is mononeuritis?

Mononeuritis is damage or inflammation of a single peripheral nerve. It causes numbness, tingling, pain or weakness confined to the area that nerve supplies, such as part of the hand or foot. Carpal tunnel syndrome and Bell's palsy are common examples.

What is mononeuritis usually caused by?

Most cases are caused by pressure on a nerve where it passes through a tight space or over bone, such as the wrist, elbow or knee. Injury and diabetes are other common causes. When several nerves are affected, vasculitis and connective tissue diseases are the usual culprits.

What is the difference between mononeuritis and polyneuropathy?

Mononeuritis affects one nerve, so symptoms are patchy and confined to that nerve's territory. Polyneuropathy affects many nerves evenly, usually starting as numbness and tingling in both feet and later the hands, in a "stocking and glove" pattern. Diabetes is the commonest cause of both.

Is mononeuritis multiplex serious?

It can be. Damage to several separate nerves often signals vasculitis or another systemic disease that can also affect the kidneys, lungs or skin. Prompt testing and treatment, often with steroids, limit further nerve damage, so see a doctor quickly if new nerve symptoms appear in different limbs.

Can mononeuritis heal on its own?

Often, yes. Mild compression usually improves once the pressure is removed, sometimes within days to weeks. More severe damage heals slowly as the nerve regrows, which can take months. Persistent weakness, muscle wasting or symptoms that keep coming back are reasons to consider surgery.