Upper vs Lower Motor Neuron Lesion Localisation

A clinical reasoning framework for neuroanatomical lesion localisation using gait analysis, postural reactions, spinal reflexes, and muscle tone — with systematic interpretation of UMN vs LMN patterns and their clinical significance.

Introduction: Why Lesion Localisation Comes First

In veterinary neurology, the single most valuable question is not 'What is the disease?' but 'WHERE is the lesion?' The answer to that question — derived entirely from the neurological examination — narrows the differential diagnosis from hundreds of possibilities to a handful. A T3–L3 myelopathy has a completely different differential list from an L4–S3 neuropathy. And the key to that localisation is understanding the difference between upper motor neuron (UMN) and lower motor neuron (LMN) signs.

This article presents a systematic, stepwise clinical reasoning framework. We begin with the anatomy of the UMN and LMN systems, then progress through gait analysis, postural reactions, spinal reflexes, and muscle tone. By the end, you will be able to examine any neurological patient and answer the four localisation questions: (1) Is it UMN or LMN — or both? (2) Which limbs are affected? (3) What spinal cord segment does that implicate? (4) What is the most likely differential?

Anatomy: UMN vs LMN — Two Neurons, One Goal

Upper Motor Neuron (UMN) System

The UMN system comprises all motor pathways that originate in the brain and descend through the spinal cord to synapse on LMNs in the ventral horn. The principal UMN tracts are: (1) the lateral corticospinal tract (voluntary skilled movement in primates; less important in quadrupeds), (2) the rubrospinal tract (facilitates flexor muscles, inhibits extensors), (3) the vestibulospinal tract (facilitates extensor muscles for antigravity support), and (4) the reticulospinal tract (regulates muscle tone and postural adjustments).

In quadrupeds, the corticospinal tract is less dominant than in primates — domestic animals rely heavily on the rubrospinal, vestibulospinal, and reticulospinal tracts for locomotion. This is why a lesion restricted to the motor cortex in a dog may produce only subtle gait deficits, whereas a comparable lesion in a human would cause dense hemiparesis.

UMNs are entirely within the central nervous system (CNS). A lesion anywhere along the UMN pathway — from the motor cortex through the spinal cord white matter — produces UMN signs CAUDAL to the lesion. The key UMN signs are: (1) exaggerated spinal reflexes (hyperreflexia), (2) increased muscle tone (spasticity), (3) disuse muscle atrophy (slow, mild), and (4) abnormal postural reactions.

Lower Motor Neuron (LMN) System

The LMN is the final common pathway — the neuron whose cell body lies in the ventral horn of the spinal cord (or a brainstem motor nucleus) and whose axon travels through the peripheral nerve to innervate skeletal muscle fibres. A lesion of the LMN anywhere along its course — cell body, ventral root, spinal nerve, peripheral nerve, or neuromuscular junction — produces LMN signs in the muscles that neuron supplies.

The key LMN signs are: (1) reduced or absent spinal reflexes (hyporeflexia/areflexia), (2) decreased muscle tone (flaccidity), (3) rapid, severe neurogenic atrophy (within 7–10 days), and (4) fibrillation potentials on electromyography (EMG) after 5–7 days.

Step 1: Gait Analysis — The First Observation

Gait analysis is performed BEFORE you touch the patient. Watch the animal walk, trot, and turn. Ask: Is there ataxia? If so, is it sensory (proprioceptive — wide-based, knuckling, scuffing), vestibular (head tilt, leaning, circling), or cerebellar (intention tremor, hypermetria, dysmetria, truncal sway)? Is there paresis (weakness)? Which limbs are affected?

Step 2: Postural Reactions — The Most Sensitive Test

Postural reactions test the entire sensorimotor pathway: sensory receptors in the paw → peripheral nerve → spinal cord → brainstem → cerebellum → motor cortex → descending UMN pathways → LMN → muscle. Because they travel the full neuraxis, postural reactions are the MOST sensitive indicator of neurological dysfunction — they deteriorate BEFORE gait abnormalities, paresis, or reflex changes.

Key Postural Reaction Tests

• Paw positioning (knuckling test): Turn the paw under so the animal stands on its dorsal surface. A normal animal immediately returns the paw to its normal position. Delayed correction = neurological deficit. This is the most sensitive single test.
• Hopping: Support most of the animal's weight and hop it laterally on one limb. Delayed or absent hopping = lateralised neurological deficit.
• Hemi-walking: Walk the animal with the limbs of one side lifted. Compare left vs right — asymmetry is highly significant.
• Wheelbarrowing: Support the pelvic limbs and walk forward on the thoracic limbs. Assess thoracic limb coordination. Then reverse (support thoracic limbs, walk on pelvic limbs).
• Extensor postural thrust: Lower the pelvic limbs toward the ground — the normal response is to extend the limbs backward to meet the surface. Absence indicates a pelvic limb neurological deficit.

Postural reaction deficits localise the lesion to the side (ipsilateral) and spinal cord region (cervical, thoracolumbar, or lumbosacral) of dysfunction. BUT they do not distinguish UMN from LMN disease — both will depress postural reactions. For UMN vs LMN distinction, you need spinal reflexes.

Step 3: Spinal Reflexes — The LMN Test

Spinal reflexes test the integrity of the LMN reflex arc: sensory afferent (Ia and II fibres from the muscle spindle) → dorsal root → synapse in the spinal cord grey matter → LMN → effector muscle. A spinal reflex is exaggerated (hyperreflexia) when the LMN arc is intact but UMN inhibition is lost (UMN lesion). It is reduced or absent (hyporeflexia/areflexia) when the LMN arc itself is damaged (LMN lesion).

Step 4: Muscle Tone and Atrophy — The Timeline Clue

Muscle Tone

Assess muscle tone by passively flexing and extending each limb through its normal range of motion. Increased tone (spasticity, 'clasp-knife' resistance that suddenly gives way) = UMN lesion. Decreased tone (flaccidity, limb feels heavy and limp) = LMN lesion. Normal tone with hyperreflexia still suggests UMN disease — tone alone can be misleading in acute lesions where spasticity has not yet developed.

Muscle Atrophy

The rate of atrophy is diagnostically valuable. Neurogenic atrophy (LMN lesion) begins within 7–10 days and is SEVERE — the muscle loses 50–70% of its mass within 3–4 weeks. Disuse atrophy (UMN lesion) takes weeks to months and is MILD — perhaps 20–30% mass loss. This is because the trophic influence of the nerve on the muscle is preserved in UMN disease but lost in LMN disease. Rapid, severe, unilateral muscle atrophy = LMN lesion until proven otherwise.

Putting it Together: Spinal Cord Segmental Localisation

Special Tests: Cutaneous Trunci and Deep Pain

Cutaneous Trunci (Panniculus) Reflex

Pinch the skin just lateral to the dorsal midline bilaterally, starting at the level of the ilial wings and moving cranially, segment by segment. The normal response is a bilateral contraction of the cutaneous trunci muscle, producing a skin twitch. The reflex arc: sensory fibres enter at each segment → travel cranially in the lateral funiculus → synapse in C8–T1 → motor via the lateral thoracic nerve → cutaneous trunci muscle.

A unilateral absence of the reflex at a specific level indicates a spinal cord lesion on that side, approximately 2–3 segments CRANIAL to the cut-off level. This is one of the most underutilised and powerful localising tools in the neurological examination. Document the cut-off level precisely (e.g., 'panniculus absent caudal to T12 on the right').

Deep Pain Perception

Deep pain perception (nociception) tests the spinothalamic pathway — a bilateral, multisynaptic tract in the ventrolateral spinal cord. Test by applying a haemostat to the digit (not just the skin — the periosteum) and observing a CONSCIOUS response (turning toward the stimulus, vocalisation, biting, pupil dilatation). A simple withdrawal reflex is NOT a conscious response — it is a spinal reflex and may persist even when deep pain is absent.

Clinical Reasoning: A Case-Based Flowchart

Here is a clinical reasoning sequence for the most common presentations:

1. Pelvic limb ataxia + paresis → Check patellar reflex → NORMAL/HYPER = UMN (T3–L3); REDUCED/ABSENT = LMN (L4–S3).
2. All four limbs ataxic + paretic → Check thoracic limb reflexes → NORMAL/HYPER = C1–C5; REDUCED = C6–T2 (LMN to thoracic limbs, UMN to pelvic limbs).
3. Monoparesis (one limb) → Check spinal reflexes in that limb → REDUCED = LMN (peripheral nerve, nerve root, or ventral horn); NORMAL = UMN (brain or cervical lesion — but unilateral spinal cord lesions usually cause ipsilateral deficits).
4. Non-ambulatory paraplegia → The single most important question: Is deep pain perception PRESENT? → YES = good prognosis (80–95% recovery with surgery); NO = guarded to poor (0–50% recovery).
5. Ataxia without paresis → Sensory (proprioceptive) ataxia vs cerebellar ataxia vs vestibular ataxia — each has a unique examination profile. Use postural reactions and cranial nerve exam to differentiate.

Neuroanatomical Lesion Localisation: Beyond UMN and LMN

Vestibular vs Cerebellar Ataxia — Two Types of Incoordination

Vestibular ataxia is characterised by head tilt, leaning/falling to one side, circling, and positional nystagmus. The lesion localises to the vestibular apparatus (peripheral — inner ear, CN VIII) or brainstem vestibular nuclei (central). Key differentiating features: (1) Central vestibular disease causes VERTICAL or direction-changing nystagmus, while peripheral disease causes horizontal or rotatory nystagmus that does NOT change direction. (2) Central disease often has accompanying postural reaction deficits and cranial nerve deficits (CN V, VI, VII). (3) Horner's syndrome (miosis, ptosis, enophthalmos, third eyelid protrusion) can accompany peripheral vestibular disease (otitis media/interna) due to sympathetic fibres travelling through the middle ear. Cerebellar ataxia produces: intention tremor (worsens as the target is approached, absent at rest), hypermetria (over-reaching — high-stepping gait), dysmetria, truncal sway (broad-based stance), and a lack of the normal physiological nystagmus (menace response deficit). These two syndromes are clinically distinct from the proprioceptive ataxia of spinal cord disease — and their lesion localisations are completely different.

Neuropathic Pain — When the LMN Itself Signals Damage

LMN disease can produce neuropathic pain — a burning, electric-shock quality that is disproportionate to the apparent injury. Mechanisms: damaged nerves express abnormal sodium channels (Na_v1.3, Na_v1.7) → spontaneous firing → ectopic impulse generation in the dorsal root ganglion and along the injured axon. This produces spontaneous pain and allodynia (non-painful stimuli perceived as painful). Clinical examples: (1) Cervical syringomyelia in Cavalier King Charles Spaniels — the syrinx damages dorsal horn neurons and spinothalamic tract fibres → phantom scratching ('air guitar') and apparent neck pain. (2) Brachial plexus avulsion — deafferentation pain → self-mutilation of the affected limb. (3) Feline orofacial pain syndrome — likely trigeminal neuropathy. Management: gabapentin/pregabalin (binds α2δ subunit of voltage-gated Ca²⁺ channels → reduces neurotransmitter release), amantadine (NMDA receptor antagonist — reduces central sensitisation), and tricyclic antidepressants (amitriptyline — inhibits noradrenaline and serotonin reuptake → enhances descending inhibitory pathways).

Intervertebral Disc Disease: A Lesion-Localisation Masterclass

Intervertebral disc disease (IVDD) provides the perfect platform for practising lesion localisation. Consider a Dachshund presenting with acute pelvic limb paresis. If the lesion is T3–L3 (80% of thoracolumbar disc extrusions): UMN signs in pelvic limbs (normal to exaggerated patellar reflex, increased tone, proprioceptive ataxia), NORMAL thoracic limbs, and a cutaneous trunci cut-off (localising to within 2–3 segments). If the lesion is L4–S3 (lumbosacral): LMN signs in pelvic limbs (reduced patellar reflex, reduced withdrawal, flaccid tail, ± urinary incontinence from pudendal nerve involvement). The difference determines the surgical approach (hemilaminectomy for T3–L3; dorsal laminectomy for L4–S3) and the prognosis.

Cranial Nerve Examination: Lesion Localisation Above the Foramen Magnum

The cranial nerves provide a window into the brainstem — each nerve or nucleus localises to a specific region, and their systematic examination is as powerful for intracranial lesion localisation as spinal reflexes are for spinal cord localisation.

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