Parents often become concerned when they observe sudden, rhythmic jerking in their newborn’s ankles, a phenomenon known as ankle clonus. This involuntary muscle contraction is a common reflex in infants and is usually a temporary sign of their developing nervous system. This article clarifies what ankle clonus is, the typical timeline for its disappearance, and when it may suggest a need for further medical evaluation.
What Exactly is Ankle Clonus?
Ankle clonus is defined as rhythmic, involuntary muscle contractions that occur when the muscle’s tendon is quickly stretched. In the ankle, this reflex is typically elicited by a rapid upward push of the foot toward the shin, which stretches the Achilles tendon. A parent might observe this as a series of rapid, repeated foot jerks or “bouncing” when the baby’s foot is held in a flexed position.
The presence of clonus in newborns is generally attributed to the temporary immaturity of the central nervous system (CNS). Specifically, the descending pathways that normally inhibit reflex activity are not fully developed and functional at birth. This lack of inhibitory control causes the stretch reflex arc to become momentarily hyperactive, resulting in the characteristic rhythmic oscillations. While clonus is most commonly tested and observed at the ankle, it is a general reflex that can potentially be found in other joints as well.
The Normal Timeline: When to Expect Clonus to Fade
In healthy newborns, ankle clonus is a very common finding, especially in the first few weeks of life. When it is observed, it is typically mild, consisting of only a few beats that quickly fade on their own. Most medical professionals consider fewer than 10 rhythmic beats of clonus to be within the range of normal for a young baby.
The majority of infants will no longer exhibit ankle clonus by the age of four to eight weeks. It is important to note that the resolution is a gradual process, and a small degree of occasional, brief clonus may persist in some healthy babies for a few months. A general expectation is that this reflex should be completely resolved by three to four months of age.
Recognizing Red Flags: When Clonus Signals a Deeper Issue
While a few beats of transient clonus in a young infant are generally benign, certain characteristics suggest the need for immediate medical evaluation. One concerning sign is the persistence of clonus beyond the typical resolution period of three to four months of age.
Another significant red flag is the presence of sustained clonus, which is defined as a rhythmic contraction of ten or more beats. Sustained clonus is a clinical indicator of an extreme hyperreflexia often associated with dysfunction in the upper motor neurons. Asymmetry is another sign that requires attention, such as clonus being present in one ankle but not the other.
Clonus becomes more concerning when it is accompanied by other signs of neurological concern. These include exaggerated deep tendon reflexes, increased muscle stiffness known as spasticity, or a failure to meet expected developmental milestones.