Pathologic Reflexes

The pathologic reflexes are responses that are absent in the intact adult nervous system and appear only when descending control over the segmental apparatus is disturbed. Most are not new reflexes at all but primitive, developmentally early responses that the maturing corticospinal system normally holds in restraint; when that restraint is withdrawn by an upper motor neuron lesion, the suppressed response is released. Their value at the bedside is twofold. First, they are signs of corticospinal (pyramidal) tract dysfunction, and several are the most reliable indicators of an upper motor neuron lesion available to the examiner. Second, and more fundamentally, they are objective: unlike strength, which can be withheld, or sensation, which is reported subjectively, a pathologic reflex either occurs or it does not, and it does so independently of the patient's will. This article describes the principal pathologic reflexes of the limbs and face, the response each produces, and the significance of each.

The Extensor Plantar Response (Babinski Sign)

The extensor plantar response is the cardinal pathologic reflex of corticospinal disease and the single most reliable sign of an upper motor neuron lesion. It is elicited by drawing a blunt point firmly along the lateral border of the sole, from the heel toward the fifth metatarsophalangeal joint and then curving medially across the ball of the foot. In the intact adult the toes flex (the normal flexor, or downgoing, response). When the corticospinal tract is dysfunctional, the great toe instead dorsiflexes (extends), frequently with fanning (abduction) of the remaining toes. This extensor, or upgoing, response is the Babinski sign, named for Joseph Babinski, who described the toe phenomenon in 1896 specifically as an objective means of separating organic from hysterical hemiplegia.

When the plantar response is equivocal or difficult to read, the same extensor response can be provoked from other sites by a family of reinforcing maneuvers, each bearing an eponym: Chaddock (stroking around the lateral malleolus), Oppenheim (firm pressure stroked down the anterior tibia), Gordon (squeezing the calf), Schaefer (squeezing the Achilles tendon), Bing (light pinprick over the dorsum of the foot), and Gonda (forced flexion and sudden release of the fourth toe). All yield the same upgoing great toe in the presence of a corticospinal lesion, and a response obtained from more than one site strengthens the interpretation. The technique, the reinforcing maneuvers in full, and the common pitfalls, chiefly the confusion of a brisk voluntary or ticklish withdrawal with a true tonic dorsiflexion, are treated in detail on the dedicated Plantar Response and Babinski Sign page.

Upper-Limb Finger-Flexor Signs

The corticospinal signs of the upper limb are finger-flexor reflexes that, like the extensor plantar response, reflect release of a normally suppressed reflex arc. They share a common significance and a common caveat: each carries weight chiefly when asymmetric or accompanied by other corticospinal signs, since each can occur in healthy individuals with constitutionally brisk reflexes.

• Hoffmann sign. The examiner supports the patient's middle finger and flicks the distal phalanx briskly downward, allowing it to spring back. A positive response is reflex flexion and adduction of the thumb together with flexion of the index finger. The sign reflects release of the finger-flexor (C8–T1) reflex arc and suggests an upper motor neuron lesion above C5–C6. It is most meaningful when unilateral or when it accompanies hyperreflexia, spasticity, or an extensor plantar response; in isolation, and especially when symmetric, it may be a normal finding.
• Trömner sign. A variant of the same reflex, elicited by flicking the volar surface of the distal phalanx of the middle finger upward rather than flicking the nail downward. It produces the same flexion of the thumb and fingers and carries the same significance as the Hoffmann sign.
• Wartenberg sign and the finger-flexion reflex. The finger-flexion reflex is elicited by tapping the examiner's own fingers laid across the patient's slightly flexed fingertips; an exaggerated flexion response, like a brisk Hoffmann, indicates release of the finger flexors. The eponym Wartenberg sign is applied to a related sign of pyramidal dysfunction in which the thumb adducts and flexes on resisted flexion of the other fingers. (The same name is also used for an unrelated sensory sign of ulnar neuropathy, so the term should be qualified when used.)

Exaggeration of the Jaw Jerk

With the patient's mouth held slightly open and relaxed, a tap on the chin stretches the masseter and temporalis muscles and produces reflex closure of the jaw. The afferent and efferent limbs both travel in the trigeminal nerve, and the entire arc is confined to the pons. In health the jaw jerk is minimal or absent. A briskly exaggerated jaw jerk indicates a bilateral supranuclear (corticobulbar) lesion above the pons and is a characteristic feature of pseudobulbar palsy. Because the reflex arc is wholly pontine, a pathologically brisk jaw jerk carries localizing value that brisk limb reflexes do not: it places the responsible lesion above the pons. The jaw jerk is therefore particularly useful in distinguishing a high cervical myelopathy or other lesion below the pons, in which the limb reflexes are brisk but the jaw jerk is normal, from a more rostral process affecting the corticobulbar pathways bilaterally.

Sustained Clonus

Clonus is a series of rhythmic, involuntary muscular contractions provoked by a sudden, maintained stretch, elicited most reliably by briskly dorsiflexing the ankle and holding it under tension. A few unsustained beats may occur in anxious individuals with otherwise brisk reflexes and are of no consequence. Sustained clonus, in which the beating continues for as long as the stretch is maintained, is a pathologic release phenomenon. It belongs to the same disinhibited, overactive reflex apparatus that produces hyperreflexia and the extensor plantar response, and it signifies upper motor neuron hyperexcitability. Sustained clonus at the ankle, patella, or wrist is thus a member of the upper motor neuron family of signs rather than an isolated curiosity.

Why Pathologic Reflexes Matter: Objectivity

The enduring clinical value of the pathologic reflexes lies in the fact that they cannot be voluntarily produced or suppressed. A patient cannot will the great toe to dorsiflex on plantar stimulation, nor abolish a true extensor response by effort, and the same holds for the finger-flexor signs, the exaggerated jaw jerk, and sustained clonus. This is the very property that gave the Babinski sign its original purpose: a reproducible extensor response, lying outside voluntary control, offered an objective bedside means of distinguishing organic disease of the central nervous system from functional (then termed hysterical) weakness. That distinction is one neurologists still draw daily, and the pathologic reflexes remain among the most dependable tools for drawing it, because they report on the integrity of the corticospinal system without the confounding of effort, attention, or report.

Synthesis

Taken together, the pathologic reflexes form a coherent family, each the expression of a segmental reflex arc released from corticospinal restraint. An extensor plantar response, a Hoffmann or Trömner sign, an exaggerated jaw jerk, and sustained clonus are not independent findings but several views of the same underlying disturbance, and they are most informative when read alongside the other features of the upper motor neuron syndrome, hyperreflexia, spastic tone, loss of the superficial reflexes, and pyramidal-pattern weakness. Their presence establishes that the corticospinal system is dysfunctional; their pattern and their company tell the examiner where.