Lower Motor Neuron Disorders

Also written as LMN lesions.

Lower motor neuron (LMN) lesions cause muscle wasting, fasciculations, reduced tone and absent reflexes. In children, common causes are infections, metabolic disorders and trauma. Rapid assessment and targeted investigations guide treatment.

Definition and diagnosis

LMN refers to the nerve cells in the spinal cord or cranial nerve nuclei that directly innervate skeletal muscle. Damage leads to weakness, atrophy, fasciculations and loss of reflexes.

• Clinical exam: Identify weakness, atrophy, fasciculations, reduced tone and absent reflexes

• Clinical exam: weakness, atrophy, fasciculations, reduced tone, absent reflexes

Severity and staging

Severity is staged by the extent of weakness and functional loss. The Medical Research Council (MRC) grading (0–5) is commonly used.

• MRC 0: no contraction
• MRC 1: flicker
• MRC 2: active movement
• MRC 3: active against gravity
• MRC 4: active against resistance
• MRC 5: normal

Causes, most likely first

In low‑resource settings the most common LMN causes are infections, metabolic derangements and trauma.

• Infections: polio, HIV‑associated neuropathy, TB meningitis, malaria‑related neuropathy
• Metabolic: hypoglycaemia, vitamin B12 deficiency, thiamine deficiency
• Trauma: birth injury, penetrating wounds, spinal cord injury
• Congenital: spinal muscular atrophy, congenital myopathies

How it presents

Children present with sudden or progressive weakness, often asymmetric. Facial weakness includes forehead involvement, leading to inability to close the eye. Reflex loss is a key sign.

• Weakness, atrophy, fasciculations
• Absent or reduced deep tendon reflexes
• Facial weakness with forehead involvement

Investigations

Investigations are guided by the suspected cause and available resources.

• Nerve conduction study (NCS): Reduced amplitude, slowed conduction
• Serum electrolytes: Low glucose or electrolytes
• HIV test: Positive
• Cerebrospinal fluid (CSF) analysis: Elevated protein, lymphocytic pleocytosis
• Plain radiograph: Normal or fracture visible

Red flags and complications

Urgent red flags include respiratory muscle weakness, severe pain, rapid progression, or signs of systemic infection.

• Respiratory distress
• Severe pain
• Rapid progression
• Fever, rigors

Management

Treat the underlying cause, provide supportive care and monitor for complications. Pharmacologic choices are limited by availability and safety.

• Antitubercular therapy: Isoniazid, rifampicin, pyrazinamide, ethambutol
• Antiretroviral therapy: Tenofovir, lamivudine, efavirenz
• Vitamin B12: Cyanocobalamin
• Analgesia: Gabapentin or amitriptyline

• Treat infection: antitubercular therapy, antiretroviral therapy, antimalarials

Follow-up and prevention

Regular follow‑up to assess recovery, prevent complications and monitor for recurrence. Prevention focuses on infection control and nutritional support.

• Vaccination against polio
• Early treatment of infections
• Nutritional supplementation

Often confused with

Upper motor neuron lesions, myasthenia gravis and metabolic myopathies can mimic LMN signs.

• Metabolic myopathies: exercise intolerance, normal reflexes

Where results can mislead

Normal reflexes can be seen in early LMN disease; CSF protein may be normal in early polio.

• Polio: CSF protein normal early

Questions an examiner may ask

• A 6‑year‑old presents with sudden weakness of the left arm and facial droop. What is the first investigation? — Look for polio; order a nerve conduction study or CSF analysis if available.
• A child with HIV has progressive limb weakness. What drug should be avoided due to neurotoxicity? — Trimethoprim‑sulfamethoxazole (can worsen neuropathy).
• A 4‑year‑old shows muscle wasting but reflexes are normal. What is the next step? — Repeat examination after 1–2 weeks; early LMN may have preserved reflexes.
• A child with suspected TB meningitis has normal CSF protein. What explains this? — Early disease may have normal protein; repeat CSF after 1 week.
• A child with facial weakness cannot close the eye. What is the most likely diagnosis? — Lower motor neuron facial palsy (Bell's palsy or infectious).
• A child with severe pain in the limbs and absent reflexes is found to have hypokalaemia. What is the management? — Correct potassium and monitor for respiratory compromise.

When to seek help

• Respiratory muscle weakness
• Severe pain
• Rapid progression
• Fever or rigors

Quick questions

How do you differentiate LMN facial palsy from upper motor neuron palsy?

LMN palsy involves the forehead; the patient cannot close the eye. UMN palsy spares the forehead.

What is the most common infectious cause of LMN palsy in East Africa?

Poliomyelitis remains a leading cause where vaccination coverage is low.

Why is thiamine given before glucose in suspected Wernicke's?

Glucose can precipitate thiamine deficiency, worsening neurological status.