Upper vs Lower Motor Neuron Lesions: USMLE Neurology Review

Upper motor neuron lesions cause hypertonia, hyperreflexia, clonus, positive Babinski sign, and spastic paresis with no atrophy or fasciculations. Lower motor neuron lesions cause hypotonia, areflexia, negative Babinski sign, and flaccid paralysis with atrophy and fasciculations. ALS is a neurodegenerative disease causing degeneration of both upper and lower motor neurons, making it a neuromuscular disease. It affects males and females with a male-to-female ratio of 1.8:1, typically diagnosed in ages 50-70. Patients have both upper and lower motor neuron signs bilaterally. Extraocular muscles, bowel, bladder, and autonomic functions remain intact. Respiratory involvement occurs when the diaphragm is affected, leading to restrictive lung disease.

Amyotrophic lateral sclerosis (ALS) is characterized by the combination of upper motor neuron (UMN) findings (hyperreflexia, Babinski sign) and lower motor neuron (LMN) findings (muscle wasting, fasciculations) without sensory abnormalities, resulting from degenerative changes in the anterior horn cells and corticospinal tracts.

ALS (amyotrophic lateral sclerosis/Lou Gehrig's disease) is a progressive neurodegenerative disease affecting both upper motor neurons in the brain and lower motor neurons in the brainstem and spinal cord. Clinically, patients present with progressive bilateral hand weakness, fine motor decline, gait imbalance, hyperreflexia, positive Hoffman sign, and MRI findings of cervical stenosis with cord compression. Despite technically perfect surgery, patients may worsen due to underlying ALS pathology. The disease causes simultaneous upper motor neuron signs (stiffness, exaggerated reflexes) and lower motor neuron signs (muscle wasting, fasciculations). Bulbar involvement (difficulty speaking and swallowing) indicates brainstem degeneration. Diagnosis requires progressive motor weakness, evidence of both motor neuron types in multiple regions, and no alternative explanation.

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease with an incidence of 3 per 100,000 annually in Germany. Etiology remains unclear, with 5-10% familial cases (identifiable genes) and 90-95% sporadic cases. Onset typically occurs between ages 60-81, though rare early cases exist (e.g., Stephen Hawking at 21). ALS specifically affects first and second motor neurons: first motor neurons degenerate in the primary motor cortex and pyramidal tracts, while second motor neurons degenerate in the spinal cord anterior horn. This asynchronous degeneration explains progressive, uneven muscle involvement. Lower motor neuron damage causes flaccid paralysis, absent reflexes, muscle atrophy, fibrillations, and fasciculations. Upper motor neuron damage causes spastic paralysis with increased tone and reflexes. The disease name derives from Greek 'amyotroph' (muscle atrophy) and 'lateral sclerosis' (pyramidal tract degeneration visible in spinal cord cross-sections).

Amyotrophic lateral sclerosis (ALS) involves both upper and lower motor neurons. This distinguishes ALS from other motor neuron diseases that may involve only one type of motor neuron. The involvement of both motor neuron types is a key diagnostic feature of ALS.

This segment covers fundamental neurological reflex testing. The Babinski reflex is tested by stimulating the sole of the foot while the patient lies supine; a negative finding (no Babinski response) is normal in adults. Hoffman's sign tests upper motor neuron function by flicking the patient's middle fingernail while they extend their fingers. Wrist clonus is tested by dorsiflexing the wrist and releasing, observing for rhythmic contractions. Ankle clonus, similarly tested by dorsiflexing the foot and releasing, indicates upper motor neuron lesion when positive. These reflex tests assess different neurological pathways and help identify central nervous system dysfunction.

Hoffman's sign is elicited by flicking the nail of the middle or ring finger, producing flexion of the index finger toward the thumb. The Babinski reflex is tested by running a sharp instrument along the lateral border of the foot from the calcaneus, which produces extension of the big toe and fanning of the other toes (upgoing toes). Both tests indicate upper motor neuron dysfunction.

Reflex testing identifies neurological compromise between central (brain/spinal cord) and peripheral (nerve roots) systems. Key outcomes: bilateral hyperreflexia indicates upper motor neuron lesions (e.g., TBI), while unilateral hyporeflexia suggests lower motor neuron lesions (e.g., nerve root compression). Five essential tests: triceps (C7-C8), biceps (C5-C6), brachioradialis (C6-C7), Babinski, and clonus. Proper technique requires patient relaxation and comparison between sides. Babinski involves stroking from calcaneus to first digit—normal shows toe flexion, positive shows extension with fanning. Clonus applies jerking dorsiflexion; positive shows >3 rhythmic plantar flexion contractions. Mnemonic: ankle (S1-S2), knee (L3-L4), brachioradialis (C6-C7), biceps (C5-C6), triceps (C7-C8).

The Babinski sign is a critical neurological examination used to detect pyramidal tract lesions, particularly in emergency settings. To elicit it, stroke the lateral aspect of the sole from heel to toes, stopping before the first metatarsal head; in a normal response, all toes flex together, but in a pathological response (Babinski sign present), the great toe extends (dorsiflexes) while other toes extend and fan outward, accompanied by ankle, knee, and hip flexion—a universal flexion response. This sign indicates major neurological disorders like stroke but must be interpreted carefully as it can be positive in deep sleep, anesthesia, intoxication, or in young children without actual pyramidal tract damage.

The Babinski sign is a pathological reflex indicating pyramidal tract damage, characterized by hallux extension and toe fanning (or just hallux extension) when the plantar surface is stimulated, and is part of a triad with hyperreflexia profunda and spasticity; the test must be performed bilaterally to localize lesions (unilateral indicates medullary lesions, bilateral indicates encephalic lesions above the decussation), and should be followed by assessment of deep reflexes and muscle tone for comprehensive neurological evaluation.

Cerebral palsy is a non-progressive neurodevelopmental disorder caused by brain injury or malformation during early development, resulting in impaired movement and posture; it is classified into spastic (most common, involving upper motor neuron lesions with hyperreflexia, spasticity, and clonus), dyskinetic (basal ganglia injury causing involuntary movements), and ataxic (cerebellar damage causing poor coordination) types, with common causes including premature births, periventricular leukomalacia, TORCH infections, hypoxic-ischemic encephalopathy, kernicterus, and genetic factors, and management involves multidisciplinary approaches including physical therapy, occupational therapy, speech therapy, anti-spasmodic medications (baclofen, diazepam, tizanidine, botulinum toxin, dantrolene), anticholinergics for dystonia, and surgical interventions when necessary.

Spasticity is a movement disorder characterized by hyperexcitable stretch reflex, velocity-dependent muscle tone increase, and resistance to passive motion, resulting from upper motor neuron lesions. Positive features include spasticity, hypertonia, hyperreflexia, spasms, and clonus; negative features include paralysis, weakness, loss of dexterity, and muscle fatigue. Upper motor neuron pathways originate in the brain stem and cerebral cortex, regulated by glutamate (excitatory) and GABA (inhibitory); lower motor neuron pathways use acetylcholine. Spasticity progresses through paralysis (muscle fibrosis), hypertonia (increased resistance), and contractures (soft tissue tightness). Associated reactions cause involuntary movements during voluntary activation. Common causes include cerebral palsy (80-90%), spinal cord injury (80%), traumatic brain injury (40-50%), and stroke (30-40%). Pharmacological management follows three guiding questions: why treat (quality of life, hygiene, independence), when to treat (high paralysis, worsening spasticity, Modified Ashworth >2, or early in children), and how to treat (multidisciplinary approach). First-line agents include baclofen (GABA agonist, 80-150 mg/day adults, 10-20 mg/day children under 2), causing muscle relaxation with side effects including sedation, hypotonia, kidney toxicity, and potential effects on brain plasticity. Second-line agents include alpha-2 agonists like clonidine (2 mg TID, max 36 mg/day), increasing presynaptic inhibition with benefits for neuropathic pain. Third-line agents include benzodiazepines like diazepam (GABA-A receptor), improving sleep and seizure control but carrying risks of tolerance, dependence, and withdrawal. Botulinum toxin type A blocks neuromuscular transmission by inhibiting acetylcholine release, causing reversible muscle weakness with function returning to baseline in 3-5 months. Treatment goals in cerebral palsy include preserving grasp/release function, maintaining first web space for object transfer, preserving bimanual function, and enabling hand support during walking. Injection timing begins before age 2 during preschool to maintain grip function, targeting pronators, intrinsics, thumb muscles, and flexors. Dosage is 2-4 units per muscle. Injection techniques include anatomical palpation under anesthesia, EMG guidance, and ultrasonography guidance (preferred for reliability). Contraindications include hypersensitivity and active infection at puncture site. Evidence supports botulinum toxin for spasticity reduction but limited evidence for functional gains alone.

Pharmacological treatments for spasticity include oral medications (baclofen, tizanidine, gabapentin, carbamazepine, tetrabenazine), anti-epileptic medications for quick extensor spasms, and botulinum toxin injections for focal spasticity (effective for 2-6 months, requiring recurrent treatments). For severe generalized spasticity unresponsive to oral therapies, intrathecal baclofen pump delivers medication directly into the spinal fluid, allowing much smaller doses (micrograms vs. milligrams orally) and avoiding systemic side effects like sedation and bladder dysfunction. These therapies can be combined as needed.

This section provides comprehensive management of adrenomyeloneuropathy (AMN), a condition affecting men with adrenoleukodystrophy. Key topics include: (1) Multidisciplinary team approach with neurologists, endocrinologists, and genetic counselors, (2) Current treatment is supportive and symptomatic rather than disease-modifying, (3) Spasticity results from upper motor neuron lesions causing increased muscle tone, stiffness, brisk reflexes, clonus, and painful spasms, (4) Pharmacological management includes baclofen (started at low evening doses), tizanidine, and gabapentin, (5) Advanced interventions include botulinum toxin for focal muscle paralysis and intrathecal baclofen (experimental), (6) Non-pharmacological approaches include physiotherapy, splints, and functional electrical stimulation for foot drop, (7) Mobility aids are essential for maintaining independence and preventing falls, though many patients delay use due to pride.

Advanced spasticity management employs targeted pharmacological agents including baclofen (with drowsiness side effects), tizanidine (requiring blood pressure monitoring), clonazepam (lowest addiction potential), dantrium, and botulinum toxin for focal spasticity. Intrathecal baclofen pumps deliver concentrated medication directly to the spinal cord, bypassing hepatic metabolism and systemic side effects, requiring trial assessment before implantation. Cannabinoids represent emerging therapy, with cannabidiol providing non-psychoactive pain and spasticity relief supported by AAN guidelines. State laws vary significantly regarding medical marijuana access, with implementation challenges including dispensing infrastructure gaps and product quality concerns. These advanced modalities represent the therapeutic spectrum for refractory MS-related pain.