In developmental medicine and child neurology, the approach to the ‘floppy infant’ classically focuses on determining whether the origin is in the muscles, peripheral or central nervous system (CNS), or a combination thereof, and whether it signals a genetic disorder. A few conditions respond to early targeted treatment (e.g. Pompe disease, spinal muscular atrophy).1 Origins in the muscle or lower motor neuron have been rightly emphasized, and the mechanisms for peripheral hypotonia as well as indication for therapy and secondary prevention have been well established in those contexts. However, the vast majority of cases of developmental hypotonia appear to originate in the CNS (i.e. developmental central hypotonia), but the pathophysiology and specific implications for management have been surprisingly little investigated.

Central hypotonia is often an early, non-specific feature of neurodevelopmental disorders. Examples include congenital blindness, autism spectrum disorder, intellectual disability, Down syndrome, fragile X syndrome, Prader–Willi syndrome, and even categories of cerebral palsy typified by hypertonia, not forgetting cases deemed ‘isolated’ developmental hypotonia, including so-called benign congenital hypotonia with a remitting course, but not connective tissue disorders with no neurological component.

It is likely that both developmental features of neural control and musculoskeletal properties contribute to perceived hypotonia. These factors may cause or contribute to delayed and impaired development of posture and movement, interfere with the ability to activate muscles, limit force transduction, perturb movement accuracy and balance, and lead to the emergence and reinforcement of atypical strategies. There may also be oromotor impairments leading to drooling, swallowing and feeding problems, or speech difficulty. All these may result in activity limitation and restricted participation.

As for underlying mechanisms, it remains insufficiently clear how networks within and between the cortex, basal ganglia, thalamus, hypothalamic nuclei, brainstem, cerebellum, and spinal cord interact during development to process afferent input from stretch receptors and facilitate descending influences controlling muscle spindle sensitivity and other peripheral factors that regulate muscle tone. For a start, the notion of muscle tone is rarely defined in spite of being systematically invoked in clinical practice and studies of motor disorders. Many people using that term (muscle tone) imply that it refers to the state of muscle contraction associated with ‘spontaneous’ CNS resting activity. Obviously, the state of muscle contraction also results from modulation of the excitability of neuromotor system components during active control of movement and posture, and clinicians tend to call this muscle tone too. Mechanisms of this modulation both at rest and during activity include CNS setting of the tonic stretch reflex threshold.2 In addition, tissues offer diverse resistance to stretch, independent of neural signals. While muscle and tendon form a functional unit,3 they show different viscoelastic properties, so that the muscle will be deformed mainly in a relaxed state and the tendon in an activated state.2 We typically assess muscle tone by manually gauging resistance to passive movement at rest or by guessing from observing posture.4 Most objective devices (which are rarely used) measure overall stiffness, which also results from factors other than muscle and neutral control. A common caveat is that measures of low resistance to externally applied movements can reflect ligament laxity, which is a separate construct. Like other features associated with hypotonia, it can be hypothesized to result (in part) from biomechanical and sensorimotor-driven changes in the musculoskeletal and nervous systems relating to life lived with hypotonia.

At this stage, it is important to clarify the notion and relevance of developmental central hypotonia in order to understand the motor phenotype of children with neurodevelopmental disorders more fully, to help them optimize both primary and non-primary motor aspects of their functioning and ongoing development.