Motor Neuron Disease Diagnosis and Management

Clinical Features

The American Academy of Neurology suggests that UMN lesions produce spasticity, hyperreflexia, and extensor plantar responses, while LMN lesions cause flaccidity, hyporeflexia/areflexia, fasciculations, and muscle atrophy 1, 2
Fasciculations are the most characteristic sign of LMN damage, appearing as spontaneous discharges of entire motor units that originate from the motor neuron or distally along the axon 2
Muscle weakness and atrophy develop progressively due to denervation 2
Hypotonia results from interruption of normal neural input 2
Hyporeflexia or areflexia with diminished or absent deep tendon reflexes 2
Flaccid paralysis characterized by decreased muscle tone with weakness 2
Spasticity with increased muscle tone and velocity-dependent resistance 2
Hyperreflexia manifesting as brisk or exaggerated deep tendon reflexes 2
Clonus showing rhythmic muscle contractions in response to sudden, maintained stretch 2
Extensor plantar response (Babinski sign) 3

Electromyography (EMG) and nerve conduction studies are essential for confirming LMN involvement 1, 4
LMN findings on EMG: Fibrillation potentials, positive sharp waves, fasciculations, and complex repetitive discharges indicating denervation 4, 2
Nerve conduction studies in LMN disease: Normal or low compound muscle action potential (CMAP) amplitudes with relatively normal conduction velocities 4
MRI of the brain without IV contrast is the optimal initial imaging modality for suspected motor neuron disease 1
UMN findings: Abnormal T2/FLAIR signal in the corticospinal tracts, particularly in the posterior limb of the internal capsule and cerebral peduncles 1
LMN findings: Spine MRI may show abnormal T2/STIR signal in the anterior horns ("snake eyes" appearance), though this is not specific and may appear late 1, 4
Creatine phosphokinase (CK) should be measured when LMN involvement with weakness is identified, as it is significantly elevated in conditions like Duchenne muscular dystrophy 3

Multidisciplinary care including neurology, pulmonology, nutrition, physical therapy, occupational therapy, speech-language pathology, social work, and palliative care improves survival and quality of life 1
Non-invasive ventilation (NIV) for respiratory insufficiency improves median survival and quality of life 1
SLE myelopathy with UMN signs: High-dose glucocorticoids combined with intravenous cyclophosphamide, initiated within hours for optimal outcomes 5
Specific genetic forms of LMN disease: Submaximal, functional, and aerobic exercise while avoiding excessive resistive and eccentric exercise 4

Do not rely on clinical examination alone to establish anterior horn cell (LMN) involvement; electrodiagnostic studies are mandatory 4
Sensory symptoms should prompt reconsideration of pure motor neuron disease, as sensory pathways are not involved in typical UMN or LMN lesions 1
Maintain high suspicion for infections in patients with LMN disease, as they are at very high risk for pneumonia and respiratory failure 4
Perform a second lumbar puncture if initial CSF cytology is negative but clinical suspicion for leptomeningeal disease remains high 6, 7

ALS accounts for roughly 85 % of all motor neuron disease cases in the population. 8
Approximately 85–90 % of ALS cases are sporadic, with no known family history. 8
The annual incidence of ALS is estimated at 1–2 new cases per 100,000 individuals. 8

Hyperreflexia, manifested as brisk or exaggerated deep‑tendon reflexes, is a typical upper motor neuron sign in ALS. 8
The disease follows a rapid progression: most patients reach their maximum level of disability within about two weeks of symptom onset; progression that is markedly faster (within 24 h) or slower (beyond four weeks) should prompt evaluation for alternative diagnoses. 9

Median survival after symptom onset is 3–4 years; however, 5–10 % of patients survive longer than 10 years. 8

Autonomic dysfunction may occur, including instability of blood pressure or heart rate, pupillary abnormalities, and bowel or bladder disturbances. 9
Pain is frequently reported in ALS and can be muscular, radicular, or neuropathic in nature. 9

Pure motor variant – presents with limb weakness without accompanying sensory signs. 9
Pharyngeal‑cervical‑brachial variant – weakness is confined to the upper limbs. 9
Paraparetic variant – weakness is limited to the lower limbs. 9

In atypical presentations, especially the pure motor variant or when an acute motor axonal neuropathy (AMAN) pattern is seen on electrophysiology, reflexes may be normal or even exaggerated, potentially misleading the clinician. 9
Children younger than six years may present with non‑specific symptoms such as poorly localized pain, refusal to bear weight, irritability, signs of meningism, or an unsteady gait rather than classic motor weakness. 9