Toxicology – Peripheral Neuropathy

Definition

Peripheral neuropathy is dysfunction or injury involving the peripheral nervous system, producing abnormalities of:

• Sensory function
• Motor function
• Autonomic function

Toxic peripheral neuropathies are commonly symmetric polyneuropathies affecting the distal extremities first.

A typical pattern is:

Distal sensory symptoms → progressive proximal involvement → possible motor weakness and reflex loss

Clinical Pattern

Sensory Symptoms

Sensory abnormalities commonly include:

• Tingling
• Prickling
• Burning
• Numbness
• Stinging sensations
• Neuropathic pain
• Reduced sensation

Symptoms usually begin distally in the feet and toes, although some toxic neuropathies may initially involve the hands.

As the neuropathy progresses, symptoms extend proximally.

The classic distribution is described as:

“Stocking-glove” sensory loss

Lower-extremity involvement usually becomes more prominent than upper-extremity involvement.

Motor Symptoms

Motor neuropathy may produce:

• Distal weakness
• Difficulty walking
• Foot drop
• Wrist drop
• Reduced grip strength
• Muscle wasting in chronic cases

Weakness commonly progresses:

Distal → proximal

Deep tendon reflexes may become reduced and severe polyneuropathy can produce generalized areflexia.

Autonomic Neuropathy

Peripheral autonomic nerve involvement can produce:

• Orthostatic hypotension
• Abnormal sweating
• Gastrointestinal dysmotility
• Urinary dysfunction
• Sexual dysfunction
• Abnormal heart-rate responses

Autonomic abnormalities may accompany sensory or motor neuropathy depending on the underlying toxicant.

Pathophysiologic Classification

Toxic neuropathies can be classified according to the portion of the peripheral nerve that is primarily injured.

Axonopathy

Axonopathy refers to damage primarily involving the nerve axon.

It is one of the most common patterns of toxic peripheral neuropathy.

Long axons are particularly vulnerable, producing a length-dependent neuropathy.

Therefore:

Longest nerves affected first → feet before hands → distal before proximal

Recovery can be slow because damaged axons must regenerate.

Neuronopathy

Neuronopathy involves injury to the nerve-cell body.

Depending on the affected neurons, injury may involve:

• Anterior horn cells → motor dysfunction
• Dorsal root ganglia → sensory dysfunction
• Autonomic neurons → autonomic dysfunction

Sensory neuronopathy may produce a pattern that is less strictly length-dependent than typical distal axonopathy.

Myelinopathy

Myelinopathy results from injury to the myelin sheath surrounding peripheral nerves.

Large myelinated fibers may be particularly affected.

Clinical manifestations can include impaired:

• Vibration sensation
• Proprioception
• Light touch
• Motor conduction
• Reflexes

Electrodiagnostic testing can help distinguish demyelination from primary axonal injury.

Mononeuropathy

Mononeuropathy involves one individual peripheral nerve.

Examples include isolated dysfunction of the:

• Median nerve
• Ulnar nerve
• Radial nerve
• Peroneal nerve

Compression and traumatic injury are common nontoxic causes.

Polyneuropathy

Polyneuropathy involves multiple peripheral nerves.

Most toxic neuropathies are:

• Diffuse
• Bilateral
• Relatively symmetric
• Distal-predominant

This pattern is an important clue to systemic toxic, metabolic, nutritional, or medication-related disease.

Toxic Causes of Predominantly Sensory Neuropathy

Important agents include:

• Cisplatin
• Taxanes
• Pyridoxine excess
• Nitrous oxide
• Colchicine
• Some antiretroviral medications
• Selected antimicrobial agents

Symptoms may include:

• Paresthesias
• Numbness
• Burning pain
• Loss of vibration
• Impaired proprioception

Nitrous Oxide

Repeated or substantial nitrous oxide exposure can produce neurologic dysfunction by functionally inactivating vitamin B12.

This interferes with methionine synthase and normal myelin metabolism.

Clinical manifestations may include:

• Paresthesias
• Sensory loss
• Gait abnormalities
• Weakness
• Impaired proprioception
• Ataxia

Neurologic toxicity can occur even when the measured serum vitamin B12 concentration is not dramatically reduced.

Toxic Causes of Combined Sensory and Motor Neuropathy

Important causes include:

• Arsenic
• Thallium
• Lead
• Chronic ethanol exposure
• Isoniazid
• Metronidazole
• Nitrofurantoin
• Phenytoin
• Vincristine
• Carbon disulfide
• Acrylamide
• n-Hexane
• Certain organophosphates
• Amiodarone

The exact pattern depends on the toxicant and duration of exposure.

Arsenic

Acute or chronic arsenic exposure can produce a painful sensorimotor peripheral neuropathy.

Neurologic findings may include:

• Painful paresthesias
• Numbness
• Weakness
• Reduced reflexes

Severe acute poisoning may initially cause:

• Gastrointestinal symptoms
• Hypotension
• Dysrhythmias
• Encephalopathy

Neuropathy can develop after the acute systemic illness, sometimes with delayed onset.

Chronic exposure may also produce characteristic skin and nail abnormalities.

Thallium

Thallium poisoning classically produces:

• Severe painful peripheral neuropathy
• Paresthesias
• Weakness
• Gastrointestinal symptoms
• Alopecia

A useful diagnostic combination is:

Painful neuropathy + gastrointestinal illness + delayed alopecia → consider thallium

Lead

Chronic lead exposure can produce predominantly motor neuropathy.

Classic findings include:

• Wrist drop
• Foot drop
• Extensor muscle weakness

Other manifestations may include:

• Abdominal pain
• Cognitive or neurologic abnormalities
• Anemia
• Renal abnormalities
• Hypertension

Diagnosis is based primarily on the blood lead concentration and exposure history.

Mercury

Mercury exposure can produce neurologic abnormalities, particularly after chronic exposure.

Possible findings include:

• Tremor
• Paresthesias
• Weakness
• Neuropsychiatric abnormalities

Elemental mercury vapor exposure may additionally produce:

• Gingivostomatitis
• Excessive salivation
• Respiratory injury after substantial inhalation

The clinical pattern varies considerably with the chemical form of mercury.

Isoniazid

Isoniazid can cause peripheral neuropathy by interfering with pyridoxine metabolism.

The neuropathy is typically:

• Symmetric
• Distal
• Sensory or sensorimotor

Risk is increased in patients with nutritional deficiency and other predisposing conditions.

Pyridoxine supplementation is commonly used to prevent neuropathy in patients at increased risk during isoniazid therapy.

n-Hexane

Chronic exposure to n-hexane can cause a progressive sensorimotor axonopathy.

Its neurotoxic metabolite, 2,5-hexanedione, damages peripheral nerves.

Clinical manifestations include:

• Distal paresthesias
• Weakness
• Reduced reflexes
• Progressive motor impairment

Symptoms may continue to worsen temporarily even after exposure stops.

Organophosphates

Most acute organophosphate toxicity produces a cholinergic syndrome, rather than peripheral neuropathy.

However, certain organophosphates can cause organophosphate-induced delayed neuropathy after the acute poisoning.

This may appear after a latent period and cause:

• Distal weakness
• Paresthesias
• Gait abnormalities
• Foot drop
• Progressive motor dysfunction

This delayed neuropathy is mechanistically distinct from the acute cholinergic syndrome.

Vincristine

Vincristine is an important medication-associated cause of peripheral neuropathy.

Possible manifestations include:

• Paresthesias
• Sensory loss
• Weakness
• Reduced reflexes
• Foot drop
• Autonomic dysfunction

Autonomic involvement may contribute to constipation or ileus.

Amiodarone

Chronic amiodarone exposure can occasionally produce peripheral neuropathy.

Associated toxicity may involve other organs, including:

• Thyroid
• Liver
• Lungs
• Eyes

Therefore, neuropathy in a patient receiving long-term amiodarone should be interpreted in the context of other potential medication toxicities.

Colchicine

Colchicine toxicity or chronic colchicine exposure may produce a neuromyopathy rather than an isolated peripheral neuropathy.

Features may include:

• Proximal weakness
• Sensory abnormalities
• Elevated CK
• Reduced reflexes

Risk may increase with renal dysfunction or interacting medications.

Important Nontoxic Causes

Toxic exposure is only one possible cause of peripheral neuropathy.

Common alternatives include:

• Diabetes mellitus
• Vitamin B12 deficiency
• Thiamine deficiency
• Chronic kidney disease/uremia
• Guillain-Barré syndrome
• Nerve compression or trauma
• Hypothyroidism
• Autoimmune disease
• Vasculitis
• Malignancy
• Amyloidosis
• Hereditary neuropathies
• Infections

A toxic cause should therefore not be assumed solely because neuropathy is present.

Guillain-Barré Syndrome

Guillain-Barré syndrome is an important alternative diagnosis, especially when weakness progresses rapidly.

Typical features include:

• Symmetric weakness
• Reduced or absent reflexes
• Progressive ascending involvement
• Possible cranial nerve dysfunction
• Possible respiratory muscle weakness
• Autonomic instability

CSF may demonstrate:

Elevated protein with relatively few cells

However, this finding may be absent early in the disease.

Neuromuscular Disorders That Can Mimic Neuropathy

Several disorders cause weakness without primarily damaging peripheral sensory nerves.

Important examples include:

• Myasthenia gravis
• Lambert-Eaton myasthenic syndrome
• Botulism
• Tick paralysis
• Hypokalemia
• Neuromuscular-blocking drug toxicity
• Myopathy

Careful examination helps determine whether dysfunction is primarily:

Peripheral nerve vs neuromuscular junction vs muscle vs CNS

Clinical Examination

A complete neurologic examination should assess:

Mental Status

Helps identify associated CNS involvement.

Cranial Nerves

Abnormalities may suggest an alternative or specific toxicologic diagnosis.

Motor Function

Assess:

• Strength
• Muscle bulk
• Tone
• Distribution of weakness

Sensory Function

Assess:

• Pain
• Temperature
• Light touch
• Vibration
• Proprioception

Reflexes

Reduced or absent reflexes support peripheral nerve involvement.

Coordination and Gait

Evaluate:

• Ataxia
• Balance
• Proprioceptive dysfunction
• Foot drop

Respiratory Muscle Assessment

Progressive motor neuropathy can involve respiratory muscles.

Warning findings include:

• Dyspnea
• Weak cough
• Difficulty clearing secretions
• Bulbar weakness
• Rapidly progressive generalized weakness

Serial respiratory measurements such as forced vital capacity and inspiratory pressure can help identify impending ventilatory failure.

Pulse oximetry alone may remain normal until relatively late and does not adequately assess ventilation.

Laboratory Evaluation

Testing should be guided by the suspected cause.

Possible investigations include:

• CBC
• Electrolytes
• Glucose
• Renal function
• Liver function
• Vitamin B12
• Thyroid studies
• CK

Targeted toxicologic testing may include:

• Blood lead concentration
• Arsenic testing
• Mercury testing
• Thallium testing

Testing should be based on a credible exposure history and compatible clinical syndrome rather than indiscriminate heavy-metal screening.

Electrodiagnostic Studies

Nerve conduction studies and electromyography (EMG) are useful for characterizing peripheral neuropathy.

They can help determine whether the process is predominantly:

• Axonal
• Demyelinating
• Motor
• Sensory
• Sensorimotor

They can also help distinguish neuropathy from primary muscle or neuromuscular-junction disorders.

Lumbar Puncture

Lumbar puncture is not routinely required for toxic neuropathy.

It may be useful when an alternative neurologic diagnosis is suspected, particularly Guillain-Barré syndrome.

Imaging

MRI may be appropriate when findings suggest:

• Spinal cord disease
• CNS demyelinating disease
• Tumor
• Structural neurologic disease

Imaging is generally used to investigate alternative diagnoses, rather than to confirm toxic peripheral neuropathy.

Nerve Biopsy

Nerve biopsy is rarely required for suspected toxic neuropathy.

It is invasive and generally provides limited additional information when the exposure history, neurologic examination, and electrodiagnostic studies already establish the pattern.

Management

The most important intervention in toxic peripheral neuropathy is:

Identify and discontinue or eliminate the causative exposure.

Management may include:

• Discontinuing the responsible medication when appropriate
• Removing occupational or environmental exposure
• Treating an identified nutritional deficiency
• Treating neuropathic pain
• Physical therapy
• Occupational therapy
• Mobility assistance
• Prevention of falls and injuries

Specific treatment depends on the underlying toxicant.

Decontamination

Gastrointestinal decontamination usually has no role once peripheral neuropathy has developed.

Toxic neuropathies commonly appear after enough time has passed that the original substance has already been absorbed.

The priority is therefore:

Stop ongoing exposure and prevent additional injury.

Antidotes

There is no universal antidote for toxic peripheral neuropathy.

Some underlying poisonings have specific treatments, but these treatments address the toxic exposure rather than directly reversing established nerve damage.

Examples include:

• Chelation for selected confirmed heavy-metal poisonings
• Pyridoxine for appropriate isoniazid-related problems
• Prussian blue for thallium poisoning

Treatment should be directed toward the specific toxicant.

Prognosis

Recovery depends on:

• Toxicant involved
• Severity of nerve injury
• Axonal versus demyelinating injury
• Duration of exposure
• Speed of exposure removal
• Presence of underlying neurologic disease

Neurologic abnormalities may continue to worsen for days or even weeks after exposure stops.

This phenomenon does not necessarily indicate continued exposure.

Axonal regeneration is slow, so recovery may take:

• Weeks
• Months
• Occasionally longer

Some patients recover completely, while others retain residual sensory or motor deficits.

Key Points

• Peripheral neuropathy causes sensory, motor, and/or autonomic dysfunction of peripheral nerves.
• Most toxic neuropathies are symmetric, distal polyneuropathies.
• Axonopathy is a common mechanism of toxic neuropathy.
• Length-dependent axonal injury typically affects the feet before the hands.
• Sensory symptoms include paresthesias, burning pain, numbness, and impaired sensation.
• Motor involvement causes distal weakness, foot drop, wrist drop, muscle wasting, and reduced reflexes.
• Important toxic causes include arsenic, thallium, lead, mercury, isoniazid, nitrous oxide, n-hexane, vincristine, metronidazole, certain organophosphates, and several medications.
• Painful neuropathy followed by alopecia is an important clue to thallium poisoning.
• Wrist or foot drop is classically associated with lead neuropathy.
• Nitrous oxide can produce neurologic dysfunction through functional vitamin B12 inactivation.
• Some organophosphates can produce a delayed neuropathy after the acute cholinergic syndrome has resolved.
• Electrodiagnostic studies help distinguish axonal from demyelinating neuropathy.
• Rapidly progressive weakness requires assessment for respiratory muscle involvement.
• Always consider common nontoxic causes such as diabetes, vitamin deficiencies, uremia, Guillain-Barré syndrome, and nerve compression.
• Treatment centers on removing the causative exposure, treating the specific poisoning when possible, controlling neuropathic symptoms, and rehabilitation.
• Neuropathy may temporarily continue to worsen after exposure stops, and neurologic recovery can require weeks to months.