Peripheral Neuropathy: Diagnosis and Management

Introduction

Peripheral neuropathy describes dysfunction of one or more peripheral nerves, and covers an enormous range of causes and patterns. The practical starting point for any case is not memorising every cause, but working out which pattern is present - a single nerve, several individual nerves, or a symmetric, distal, 'length-dependent' process - since the pattern itself narrows the differential dramatically.¹

The commonest presentation by far is a distal symmetric sensorimotor polyneuropathy, most often caused by diabetes, and this article focuses on that pattern while covering the other important classifications and their implications.

One further distinction is worth making early because it changes both investigation and prognosis: whether the process is axonal or demyelinating. In axonal neuropathy the nerve fibre itself degenerates, typically dying back from the most distal point first - which is why toxic and metabolic causes produce the length-dependent glove-and-stocking pattern. Recovery depends on axonal regrowth at roughly a millimetre per day and is therefore slow and often incomplete.

In demyelinating neuropathy the axon is preserved but its myelin sheath is damaged, slowing or blocking conduction. Because myelin can be remade relatively quickly, recovery is potentially faster and more complete - and, importantly, most treatable neuropathies (GBS, CIDP, multifocal motor neuropathy) fall into this group. This is why nerve conduction studies are so valuable: identifying a demyelinating pattern immediately raises the possibility of an immune-mediated, treatable cause rather than an irreversible metabolic one.

Peripheral neuropathy is common, affecting perhaps 2-3% of the general population and rising to around 8% in those over 55. Among people with diabetes, roughly half will develop a neuropathy over the course of their disease, which makes diabetic peripheral neuropathy one of the commonest neurological diagnoses a general clinician will encounter - and one where the main clinical work lies in preventing its complications rather than reversing the nerve damage itself.

Classification

Neuropathies are also classified by which fibre types and structures are affected: sensory, motor, autonomic or mixed, and demyelinating versus axonal on nerve conduction studies - a distinction that narrows the cause considerably, since most metabolic and toxic neuropathies are axonal while most acquired immune-mediated neuropathies (GBS, CIDP) are demyelinating.³

Aetiology

Diabetes mellitus is by far the commonest cause of peripheral neuropathy in the UK and worldwide, and every patient with a length-dependent sensory neuropathy should be screened for it if not already known to be diabetic.

An important and easily missed point is that impaired glucose tolerance, short of frank diabetes, is itself associated with a small-fibre predominant neuropathy. A patient with burning feet, normal nerve conduction studies and a normal fasting glucose may still have a glucose-related neuropathy, and an HbA1c or oral glucose tolerance test is worth pursuing rather than labelling the neuropathy idiopathic.

Similarly, alcohol and B12 deficiency frequently coexist with each other and with diabetes, so finding one cause does not exclude the others. Where a neuropathy is more severe or progresses faster than the identified cause would explain, it is worth looking for a second contributing factor rather than assuming the first accounts for everything.

• Metabolic: diabetes mellitus (the commonest cause), chronic kidney disease, hypothyroidism
• Nutritional: vitamin B12 deficiency, thiamine deficiency, chronic alcohol excess (both direct toxicity and associated nutritional deficiency)
• Toxic/drug-induced: chemotherapy agents (vincristine, platinum-based drugs, taxanes), isoniazid (preventable with co-prescribed pyridoxine), amiodarone, nitrofurantoin, metronidazole (with prolonged use)
• Immune-mediated: Guillain-Barre syndrome (acute), chronic inflammatory demyelinating polyneuropathy (CIDP, the chronic counterpart), paraproteinaemic neuropathies
• Infective: HIV, leprosy (the commonest cause of treatable peripheral neuropathy worldwide), Lyme disease
• Vasculitic: polyarteritis nodosa, eosinophilic granulomatosis with polyangiitis, rheumatoid arthritis-associated vasculitis
• Hereditary: Charcot-Marie-Tooth disease - typically presents in childhood or early adulthood with pes cavus, distal wasting ('inverted champagne bottle legs') and a family history
• Malignancy-related: paraneoplastic sensory neuronopathy, direct infiltration, or paraproteinaemia
• Idiopathic - no cause found despite thorough investigation in a meaningful proportion of cases, particularly in older patients

Clinical features

• Sensory symptoms: numbness, tingling, burning or a 'pins and needles' sensation, classically starting in the toes and ascending ('glove and stocking' distribution) before affecting the fingers once the process reaches roughly mid-calf, reflecting the length-dependent nature of most metabolic/toxic neuropathies
• Neuropathic pain - burning, shooting or electric-shock-like, often worse at night, and can be present even with reduced sensation to touch
• Motor symptoms - distal weakness (for example, difficulty with fine hand movements or foot drop) in more severe or mixed neuropathies
• Loss of proprioception - unsteady gait, worse in the dark or with eyes closed (a positive Romberg's sign reflects sensory ataxia)
• Autonomic symptoms - postural hypotension, gastroparesis, erectile dysfunction, abnormal sweating, seen especially in diabetic and some hereditary/amyloid neuropathies
• Reduced or absent ankle reflexes, often the earliest reflex change, progressing proximally in more severe disease

Clinical examination

• Inspection: foot deformity (pes cavus in hereditary neuropathies), ulceration, Charcot joint changes, muscle wasting
• Sensory testing: light touch, pinprick, vibration (128 Hz tuning fork) and proprioception, mapping the distribution carefully - length-dependent versus focal versus patchy
• Motor testing: distal power, particularly ankle dorsiflexion and toe extension
• Reflexes: ankle reflexes especially, which are often the first to be lost
• Romberg's test - for sensory ataxia from proprioceptive loss
• Gait assessment - high-stepping gait with foot drop, or broad-based ataxic gait with proprioceptive loss
• Autonomic screen - lying and standing blood pressure
• Full general examination for an underlying systemic cause - signs of chronic liver disease, thyroid status, rash or other features of vasculitis, lymphadenopathy

Differential diagnosis by pattern

Once the distribution has been established, the differential is generated primarily from the classification table above; a few conditions deserve particular mention because they mimic peripheral neuropathy but are managed very differently.

• Spinal cord or cauda equina pathology - can mimic a symmetric sensory disturbance in the legs but usually has a sensory level, sphincter involvement, or upper motor neurone signs, distinguishing it from peripheral neuropathy
• Radiculopathy - dermatomal, not length-dependent, and usually with associated back/neck pain and a specific myotomal weakness
• Motor neurone disease - combined UMN and LMN signs with normal sensation, unlike neuropathy where sensory involvement is expected
• Myopathy - proximal weakness without sensory involvement, in contrast to the distal, sensory-predominant pattern of most neuropathies

Investigations

First-line screen

• HbA1c/fasting glucose - to identify or confirm diabetes
• Full blood count and B12/folate - B12 deficiency is a common and fully reversible cause if caught early
• U&Es - chronic kidney disease
• LFTs and gamma-GT - alcohol-related liver disease
• TFTs - hypothyroidism
• Serum protein electrophoresis and free light chains - screening for a paraproteinaemic cause, particularly in older patients or where the pattern is demyelinating
• Inflammatory markers and autoimmune/vasculitis screen (ANA, ANCA) - if mononeuritis multiplex or another atypical pattern is present

Further investigation

• Nerve conduction studies and electromyography - confirm the presence and pattern of neuropathy, distinguish demyelinating from axonal, and identify whether it is predominantly sensory, motor, or mixed
• Nerve biopsy - reserved for cases where vasculitis, amyloidosis or another specific inflammatory/infiltrative process is suspected and cannot be confirmed by less invasive means
• Genetic testing - for suspected hereditary neuropathies such as Charcot-Marie-Tooth disease, guided by the specific pattern and family history
• Screening for malignancy - where a paraneoplastic cause is suspected, guided by clinical context rather than routinely in every patient

Management

Treat the underlying cause

• Optimise glycaemic control in diabetic neuropathy - the single most effective way to slow progression, though established neuropathy does not reverse with glucose control alone
• B12 replacement if deficient
• Alcohol cessation and thiamine/vitamin replacement in alcohol-related neuropathy
• Review and stop causative drugs where possible (for example, switching a chemotherapy regimen if feasible, or ensuring pyridoxine co-prescription with isoniazid)
• Immunosuppression for CIDP and vasculitic neuropathy, managed by specialist services
• Treat hypothyroidism, chronic kidney disease and other identified metabolic causes

Neuropathic pain management

First-line agents for neuropathic pain are amitriptyline, duloxetine, gabapentin or pregabalin - NICE guidance recommends starting with one of these and switching to an alternative if the first is not effective or not tolerated, rather than combining multiple agents from the outset.² Tramadol may be used for short-term rescue therapy but is not recommended for long-term management.

Foot and skin care

• Diabetic foot care - regular podiatry review, daily foot inspection, appropriate footwear, and prompt treatment of any skin breakdown, since loss of protective sensation is what allows minor injuries to progress unnoticed to ulceration and, eventually, amputation
• Annual diabetic foot screening as part of routine diabetes care
• Education on injury prevention, particularly regarding heat (burns from hot water bottles or baths) and pressure

Complications

• Foot ulceration and, if unrecognised or poorly managed, amputation - the most feared complication of diabetic sensory neuropathy
• Charcot neuroarthropathy - progressive joint destruction from loss of protective sensation and proprioception
• Falls and injury from sensory ataxia and motor weakness
• Chronic neuropathic pain significantly affecting quality of life and sleep
• Autonomic complications - postural hypotension with falls, gastroparesis, erectile dysfunction, bladder dysfunction
• Progression to significant disability in severe or unrecognised cases

Red flags

Prognosis

Prognosis depends entirely on the underlying cause. Reversible causes (B12 deficiency, drug-induced, hypothyroidism) can improve substantially, though recovery may be incomplete if damage was longstanding before treatment. Diabetic and other chronic metabolic neuropathies typically progress slowly despite optimal risk factor control, though good glycaemic control slows the rate of progression. Immune-mediated neuropathies (CIDP) often respond to immunosuppression but may relapse and require long-term treatment. Neuropathic pain, once established, can be persistent and requires ongoing, sometimes multimodal, management.

Setting expectations about neuropathic pain treatment is an important part of management. The realistic target is a 30-50% reduction in pain rather than its abolition, and only a minority of patients achieve even that from any single agent. Drugs should be titrated slowly to an adequate dose and given an adequate trial before being abandoned, and switching within the first-line group is preferred to combining agents early. Explaining at the outset that pain relief will be partial, and that the aim is improved sleep and function rather than a pain-free state, prevents the cycle of escalating and abandoning treatments that many patients experience.

It is worth acknowledging that despite thorough investigation, no cause is identified in a significant proportion of patients, particularly older people with a chronic, slowly progressive, sensory-predominant neuropathy. Provided the pattern is typical, progression is slow and there are no atypical features, this chronic idiopathic axonal polyneuropathy carries a good prognosis and does not warrant repeated or escalating investigation. Saying so explicitly is more helpful than leaving the patient with the impression that something has been missed.