LMNL Lower Motor Neuro Lesion

Question 1

What is it

Answer 1

A lesion affecting the lower motor neurons (LMNs), which originate in the spinal cord or brainstem and directly innervate muscles. Leads to flaccid weakness, muscle atrophy, and loss of reflexes.

Question 2

Clinical Features

Answer 2

✅ Key Signs of LMN Lesion:

Muscle weakness (paresis or paralysis) WITH atrophy 💪❌
Flaccidity (decreased muscle tone, hypotonia) 🛌
Hyporeflexia (reduced or absent deep tendon reflexes) 🦵❌
Fasciculations (involuntary muscle twitches) 🌀
Normal or absent Babinski sign (toes curl downward)
🚫 NO spasticity, hyperreflexia, or clonus (UMN signs).

Question 3

Epidemiology

Answer 3

Common in nerve injuries, peripheral neuropathies, spinal cord diseases, and motor neuron diseases.
Can occur at any age depending on cause.

Question 4

Age Groups Affected

Answer 4

• Infants & children: Spinal muscular atrophy (SMA), polio
• Young adults: Peripheral nerve injuries (trauma, Guillain-Barré Syndrome)
• Middle-aged to older adults: ALS, diabetic neuropathy, radiculopathies

Question 5

Risk Factors

Answer 5

✅ Modifiable:
Diabetes (diabetic neuropathy) 🩸
Vitamin B12 deficiency (affects myelin, leading to neuropathy)
Toxins (alcohol, heavy metals, chemotherapy drugs)
Infections (poliovirus, syphilis, Lyme disease)
Compression (herniated discs, spinal stenosis)

🚫 Non-Modifiable:
Genetic disorders (SMA, ALS, Charcot-Marie-Tooth disease)
Aging-related nerve degeneration
Autoimmune conditions (Guillain-Barré Syndrome, myasthenia gravis)

Question 6

Clinical Presentation

Answer 6

🔹 If Peripheral Nerve Lesion (e.g., Guillain-Barré, nerve trauma):
Flaccid paralysis/weakness in affected limb
Absent reflexes
Sensory loss may be present
Fasciculations in chronic cases

🔹 If Spinal Cord LMN Lesion (e.g., Polio, ALS, radiculopathy):
Weakness & atrophy in specific myotomes
Loss of reflexes at affected level

No upper limb involvement if lower cord affected
🔹 If Motor Neuron Disease (e.g., ALS – both UMN & LMN signs):
Muscle weakness with atrophy
Fasciculations
Normal cognition (no sensory loss)

Question 7

Prognosis

Answer 7

🔹 Depends on the cause:
Guillain-Barré Syndrome: Can recover with treatment but may have residual weakness
Peripheral nerve injuries: May recover if axon regenerates (slow process)
ALS (both UMN & LMN affected): Progressive & fatal
Diabetic neuropathy: Can be managed but not reversed
Polio survivors: Can develop post-polio syndrome with worsening weakness later in life

🔹 Treatment & Management:
Physical therapy & rehabilitation 🏋️‍♂️
Vitamin supplementation (if nutritional cause) 💊
Immunotherapy for autoimmune causes (e.g., IVIG for GBS)
Surgical decompression for nerve compression syndromes

Question 8

Tests

Answer 8

1. Muscle strength & tone - checking for flaccidity & Atrophy
2. Deep tendon reflexes - checking for HYPOreflexia
3. Fasciculation test - involuntary muscle twitches
4. Babinski sign (plantar reflex)