03 The first fork: upper or lower motor neuron
Deciding whether weakness comes from the upper or the lower motor neuron is the first and most powerful split in localization, and the examination usually makes it within minutes.
Stroke the sole of a four-month-old's foot and the big toe will often rise, which is entirely normal at that age. Do the same in a four-year-old and the identical movement tells you that something has gone wrong in the pathway coming down from the brain.
In 1896 Joseph Babinski (1857–1932) described the big toe that rises instead of curling when the sole is stroked after damage to the upper motor neuron. He also introduced the word anosognosia, for a patient with a left hemiplegia who seemed unaware of it. (DeMyer, pp. 275, 441)
By the end of this module you can
- Distinguish upper from lower motor neuron lesions by distribution, wasting, tone and reflexes.
- Tell spasticity from rigidity, and place hypotonia on the reflex arc.
- Grade reflexes, judge clonus, and recognize a true extensor plantar response at any age.
- Recognize when the usual signs mislead: neural shock, early cerebral palsy, axonal Guillain-Barré syndrome, the face.
- Decide whether a floppy infant's hypotonia is central or peripheral.
Upper and lower motor neurons
No findings yet: every level is possible.
A teaching model of classical rules (Brazis 2011; DeMyer; Peredo 2009). Each rule's exceptions are listed under its finding. Real patients can have more than one lesion.
Damage to the nerve that supplies a muscle makes its reflex fade. Damage higher up, in the pathway from the brain, makes the same reflex brisker. The reason is that every voluntary movement passes through two neurons in series. The upper motor neuron runs from the cortex down through the brainstem and spinal cord. The lower motor neuron starts in the anterior horn or a cranial nerve nucleus and runs through a root and a nerve to the muscle. The boundary is the synapse on the anterior horn cell (Pearl 2014, p. 65), and lesions on either side of it produce opposite signatures. That makes this the first and most powerful fork in localization.
Why the signs differ
The lower motor neuron is the final common path, so when it is cut the muscle loses its nerve supply. It weakens, wastes early and may twitch, and with the reflex arc broken, reflexes and tone fall. The upper motor neuron mostly holds the lower one in check. When it is cut, the surviving lower neuron is released, and reflexes and tone rise (Pearl 2014, p. 65).
The two differ under three headings (DeMyer, p. 289).
- Distribution. Upper lesions weaken movements, in a hemiplegic, quadriplegic or paraplegic pattern. Lower lesions weaken individual muscles, or the group served by a root or nerve.
- Wasting. Upper lesions cause only slight, late disuse atrophy. Lower lesions cause early, severe denervation atrophy with fasciculations.
- Reflexes and tone. Upper lesions bring brisk reflexes, clonus, clasp-knife spasticity and an extensor plantar response. Lower lesions bring reduced or absent reflexes and hypotonia.
Try the localizer above. Switch on brisk reflexes and every level of the motor unit disappears. Switch on absent reflexes and the brain and brainstem disappear, though the spinal cord stays, for a reason section 4 explains.
Two warnings for children follow in sections 4 and 5. An acute upper motor neuron lesion can look floppy, and infant hypotonia is far more often central than peripheral.
Key points
- The upper motor neuron runs from the cortex to its synapse on the anterior horn cell, and the lower motor neuron runs from the anterior horn to the muscle (Pearl 2014, p. 65).
- An upper motor neuron lesion releases the lower neuron, producing brisk reflexes, spasticity, clonus and an upgoing toe.
- A lower motor neuron lesion denervates the muscle, producing early wasting, fasciculations, reduced reflexes and hypotonia.
- Upper motor neuron lesions weaken movements in hemiplegic, paraplegic or quadriplegic patterns, while lower motor neuron lesions weaken muscles in the pattern of a root or nerve (DeMyer, p. 289).
Check your understanding
A 13-year-old has had six months of weakness in the right hand. The small hand muscles are visibly wasted and twitch with fasciculations, the right triceps and finger reflexes are absent, tone is low, and both plantar responses are flexor. Which side of the first fork does this belong to?
Select an answer to reveal the explanation
Tone: spasticity, rigidity and hypotonia
The easiest place to learn normal tone is on yourself. Rest one forearm on your thigh with the wrist dangling, relax it completely, and move the wrist with your other hand. That faint resistance is tone, the slight resistance of a relaxed joint to passive movement (DeMyer, p. 270). Abnormal tone is a departure from that feeling, and the kind of departure localizes the lesion.
Spasticity and rigidity
The speed of the movement that brings them out tells the two apart (DeMyer, p. 273).
- Spasticity appears when you move the limb quickly. You feel a catch and then a give, the clasp-knife phenomenon. It follows an upper motor neuron pattern, whether hemiplegic, paraplegic or quadriplegic.
- Rigidity is there even when you move the limb slowly. The resistance is even through the whole range, like bending a lead pipe, often with cogwheeling and a tremor at rest. It usually affects all four limbs and points to the basal ganglia.
So test tone twice, once slowly and once with a quick flick.
Hypotonia
Low tone tells you less on its own. Tone depends on a reflex arc from the muscle spindle through the dorsal root to the anterior horn cell and back to the muscle, and a break in any link lets it fall. The signs that come with the low tone tell the peripheral levels apart (DeMyer, p. 266).
- A dorsal root lesion lowers tone and reflexes and causes sensory loss, without weakness or wasting.
- A lower motor neuron lesion adds weakness, wasting and fasciculations, without sensory loss.
- A myopathy gives weakness, wasting and hypotonia, without fasciculations or sensory loss.
Go deeper: Dorsal rhizotomy for spasticity
The dorsal root carries the spindle's excitatory signal into the cord, so removing part of that input lowers tone. That is the logic of dorsal rhizotomy, in which selected dorsal roots are cut to reduce tone in some children with spasticity (DeMyer, p. 273).
In a child
The term newborn lies flexed, with a tight flexor tone that has eased by about three months (Utah PediNeuroLogic Exam). Hypotonia in an infant is most often central, as section 5 shows. Dystonia, a fourth kind of abnormal tone, comes from the basal ganglia and is covered in Module 8.
Key points
- Tone is the slight resistance of a relaxed joint to passive movement (DeMyer, p. 270).
- Spasticity depends on velocity, with a catch and yield on quick movement, and follows an upper motor neuron pattern (DeMyer, p. 273).
- Rigidity is present with slow movement as well, as lead-pipe or cogwheel resistance, and points to the basal ganglia (DeMyer, p. 273).
- Hypotonia can arise anywhere along the reflex arc, and companions such as sensory loss, fasciculations and wasting tell the levels apart (DeMyer, p. 266).
- In infants, flexor tone loosens over the first months, and hypotonia is usually central.
Check your understanding
In a 7-year-old, moving the elbow slowly meets almost no resistance, but a quick extension produces a sudden catch that then gives way. The same arm is weak and its reflexes are brisk. What kind of tone is this, and where does it point?
Select an answer to reveal the explanation
Reflexes and the plantar response
Briskness alone means little, because it varies and many healthy children have lively reflexes. The reflex hammer tests a circuit from the muscle spindle to the spinal cord and back, which the upper motor neuron holds in check. A brisk reflex suggests that restraint has been lost, and an absent one suggests the circuit itself is broken. The finding that carries the most weight is asymmetry that reappears on repeated testing. Grading reflexes from 0 to 4+ (DeMyer, p. 36) lets you compare findings over time and between examiners. And check for consistency, because an upper motor neuron lesion that brightens the reflexes on one side should also weaken that side (DeMyer, p. 268).
Clonus
Clonus is a rhythmic to-and-fro oscillation set off by a quick stretch, another sign of hyperactive reflexes after the upper motor neuron has been interrupted (DeMyer, p. 268). A few beats that die away, called abortive clonus, can be normal, and in mature individuals only sustained clonus counts as abnormal. Yet some normal newborns show sustained clonus, set off by movement or by gravity (DeMyer, p. 269), so the sign means different things at different ages.
The plantar response
The extensor plantar response, or Babinski sign, is the most famous sign in neurology and one of the most often misread. A true extensor toe sign has four features (DeMyer, pp. 281-282).
- The toe begins to extend only after the stroke has traveled some distance along the sole.
- It stays extended while the stroke continues.
- When the stroke stops, it floats slowly back to neutral.
- It comes with a flexion withdrawal of the leg.
There is no "negative Babinski", because the sign is either present or absent (DeMyer, p. 281). Even a true extensor response can be temporary, after a seizure or in toxic-metabolic coma (DeMyer, p. 282).
In infants
An upgoing toe is normal in the first year of life, while the corticospinal tracts are myelinating (Utah PediNeuroLogic Exam), and the infant's response varies with the stimulus (DeMyer, p. 282). At 12 months the toe may go either up or down, and what matters then is reproducible asymmetry (Utah PediNeuroLogic Exam). If the child is ticklish, stroke the outer edge of the foot instead (DeMyer, p. 616).
A very brisk ankle jerk with sustained clonus on one side, part of the upper motor neuron syndrome.
NeuroLogic Exam, Larsen & Stensaas. CC BY-NC-SA. Source · Credits
Stroking the outer sole lifts the great toe and fans the others. After the first year this points to the corticospinal tract.
NeuroLogic Exam, Larsen & Stensaas. CC BY-NC-SA. Source · Credits
Key points
- Grade reflexes from 0 to 4+ (DeMyer, p. 36), and give reproducible asymmetry more weight than absolute briskness.
- Brisk reflexes on one side from an upper motor neuron lesion should come with weakness on that side (DeMyer, p. 268).
- Only sustained clonus is abnormal in mature individuals, and normal neonates may have sustained clonus (DeMyer, p. 269).
- A true extensor toe starts after the stroke has moved, stays up, floats back and comes with flexion of the leg, and there is no such thing as a negative Babinski (DeMyer, pp. 281-282).
- An upgoing toe is normal in the first year, and at 12 months it is asymmetry that matters (Utah PediNeuroLogic Exam).
Check your understanding
An anxious 9-year-old's big toe jerks up the instant your key touches the sole, and he pulls his whole foot away, laughing. When you re-test with a slow stroke along the outer edge of the foot, both toes go down. How should you record the plantar responses?
Select an answer to reveal the explanation
Exceptions to the rules
The upper and lower motor neuron signatures have four exceptions worth knowing, and three of them matter especially in children.
1. Shock, an upper lesion that looks lower
Right after an acute, severe upper motor neuron lesion, the patient may be flaccid and areflexic. This stage is called neural shock, spinal or cerebral depending on the site, and brisk reflexes, spasticity and an upgoing toe emerge only as recovery begins (DeMyer, pp. 273-274). In the first days, a child with transverse myelitis or a spinal cord injury can look like a child with a neuropathy. What keeps the cord on your list is the sensory level and the early loss of bladder control (DeMyer, p. 290). Upper motor neuron lesions that evolve slowly skip this floppy phase and are spastic from the start (DeMyer, p. 274).
2. The floppy infant who will become stiff
Infants with cerebral palsy may be hypotonic when they first present, before spasticity evolves, and some stay hypotonic (DeMyer, p. 273). Low tone with normal or brisk reflexes, depressed alertness and fairly good limb strength should turn your thinking toward a central cause, as section 5 shows.
3. Guillain-Barré syndrome with preserved reflexes
Guillain-Barré syndrome typically brings ascending flaccid weakness with areflexia. In some patients with the axonal form, however, the reflexes are preserved or even brisk. Telling a neuropathy from a myelopathy at the bedside then becomes difficult, and electrodiagnostic studies help to settle it (DeMyer, p. 290).
4. Forehead sparing in facial palsy
Students are taught that a central facial palsy spares the forehead and a peripheral one weakens the whole face. The distinction is useful, but forehead sparing is best treated as a strong hint that falls short of proof.
Go deeper: The anatomy behind forehead sparing
The classic explanation is that the forehead receives input from both hemispheres, so one intact hemisphere is enough to keep it moving. Anatomical studies suggest a gradient instead, in which the forehead receives less cortical input rather than more (Pearl 2014, p. 49). The strict version of the teaching therefore overstates the case.
When a single sign contradicts the rest of the picture, ask what could make that one sign misleading before you discard the whole pattern. The usual answers are the time since onset, the age of the nervous system and the kind of axonal injury.
Key points
- An acute, severe upper motor neuron lesion can be flaccid and areflexic at first, the stage of neural shock (DeMyer, pp. 273-274).
- Slowly evolving upper motor neuron lesions show spasticity and brisk reflexes from the start (DeMyer, p. 274).
- Infants with cerebral palsy may be hypotonic before spasticity appears (DeMyer, p. 273).
- The axonal form of Guillain-Barré syndrome can preserve reflexes, and electrodiagnostic studies help to decide (DeMyer, p. 290).
- Forehead sparing in a central facial palsy is a strong hint rather than a law (Pearl 2014, p. 49).
Check your understanding
A 12-year-old woke with weak, floppy legs that worsened over a day. Knee and ankle reflexes are absent, neither big toe moves when the soles are stroked, he cannot feel a pin below the umbilicus, and he has not passed urine since the morning. What is the most likely level?
Select an answer to reveal the explanation
The floppy infant: central or peripheral?
No findings yet: every level is possible.
A teaching model of classical rules (Brazis 2011; DeMyer; Peredo 2009). Each rule's exceptions are listed under its finding. Real patients can have more than one lesion.
No problem in child neurology tests the first fork more often than the floppy infant. The evaluation begins with two words that are easy to blur. Hypotonia is reduced resistance to passive movement, and weakness is reduced maximum power. The two can occur together or apart, and telling them apart is the key to localization (Peredo 2009).
Central causes are commoner
Central causes account for about 60 to 80% of infant hypotonia, and peripheral causes for about 15 to 30% (Peredo 2009). The floppy infant is therefore more often a brain problem than a muscle problem. That seems backwards at first, until you remember that the motor unit is only one of the places where tone can fail.
Signs of a central cause
The features that favor a central cause are a depressed level of consciousness, mainly axial weakness, relatively normal limb strength despite the hypotonia, and normal or brisk reflexes (Peredo 2009). Dysmorphic features, seizures, or poor feeding in a sleepy, disengaged baby add weight. The peripheral pattern is close to the mirror image. The infant is alert and bright-eyed but cannot lift the limbs against gravity, the reflexes are absent, and the tongue sometimes fasciculates.
History and examination
About half of infants with hypotonia are diagnosed from the history and examination alone (Peredo 2009), and the localization chooses the first tests. A central picture calls for brain imaging and genetic testing, and a peripheral one calls for studies of the motor unit.
Infant botulism
Infant botulism localizes to the neuromuscular junction, and it is treatable. Suspect it in an infant under 6 months who has an acute or subacute floppy illness with constipation, listlessness, poor feeding, a weak cry and a reduced gag (Peredo 2009).
Use the presets in the figure above. The sleepy infant with brisk reflexes leaves the brain and brainstem standing. The alert infant with tongue fasciculations and no reflexes leaves the anterior horn cell on top, which is the picture of spinal muscular atrophy. For the constipated 3-month-old with ptosis and a weak suck, the junction is the only level that fits everything. From there you can follow the trail into the motor unit in Module 4, or into the neuromuscular disease index.
A "flat on the mat" posture with few movements against gravity, the first clue to low tone.
Pediatric NeuroLogic Exam, Larsen & Stensaas. CC BY-NC-SA. Source · Credits
In vertical suspension the same infant slides through the examiner's grip because of low shoulder-girdle tone. Utah's full examination of this baby points to a central cause.
Pediatric NeuroLogic Exam, Larsen & Stensaas. CC BY-NC-SA. Source · Credits
Key points
- Hypotonia is reduced resistance to passive movement and weakness is reduced power, and the two should be assessed separately (Peredo 2009).
- Central causes account for about 60 to 80% of infant hypotonia, and peripheral causes for about 15 to 30% (Peredo 2009).
- Depressed consciousness, axial weakness, relatively normal limb strength and normal or brisk reflexes point to a central cause (Peredo 2009).
- About half of hypotonic infants are diagnosed from the history and examination alone (Peredo 2009).
- Infant botulism should be suspected under 6 months when floppiness comes with constipation, a weak cry, poor feeding and a reduced gag (Peredo 2009).
Check your understanding
A 4-month-old is bright, smiles and follows faces, but cannot lift her arms or legs against gravity. Her reflexes are absent and her tongue quivers at rest. What is the most likely level, and what should happen next?
Select an answer to reveal the explanation
Module quiz
5 vignettesRead every section first. The quiz unlocks when the last section has been on screen.
Sign-off · with a faculty reader
On five patients, classifies the weakness as upper or lower motor neuron from the examination alone, agreeing with faculty.
Read and watch
- PediNeuroLogic Exam: the newborn ↗Tone, reflexes and the plantar response in a normal newborn.
- PediNeuroLogic Exam: abnormal newborn ↗Examples of abnormal tone and reflexes in the newborn.
- PediNeuroLogic Exam: 12 months ↗Reflexes on the parent's lap; the plantar response at 12 months.
- NeuroLogic Exam (University of Utah, adult) ↗Motor examination, tone and reflexes in adults, with abnormal examples.
These sites are free but grant no reuse licence, so this module links to their pages rather than copying their figures.
Sources
- Pearl PL, Emsellem HA. Neuro-Logic: A Primer on Localization. New York: Demos Medical; 2014.
- Biller J, Gruener G, Brazis PW. DeMyer's The Neurologic Examination: A Programmed Text. 6th ed. New York: McGraw-Hill.
- Peredo DE, Hannibal MC. The floppy infant: evaluation of hypotonia. Pediatr Rev. 2009;30(9):e66-e76. PMID 19726697 ↗
- Larsen PD, Stensaas SS. PediNeuroLogic Exam: A Neurodevelopmental Approach. University of Utah; updated 2020. Videos CC BY-NC-SA. link ↗
The interactive figures are calculated from our own models of the anatomy, and exam videos are from the University of Utah NeuroLogic Exam (CC BY-NC-SA); see credits. For education. Not for clinical decision-making; consult primary sources and your attending.