Case 4: Isolated gross motor delay with ‘sitting on air’ position in an otherwise healthy 17-month-old

A 17-month-old girl presented with an isolated delay in walking. She first sat at six months of age and was an efficient ‘bum-scooter’. She pulled to stand alongside furniture, and started cruising at 17 months of age. She did not take independent steps. The child was the product of a healthy pregnancy, with normal fetal growth and movement. She was born at term and was a well child. There was no history of developmental regression and no family history of developmental delay, or neuromuscular or metabolic disease. Her parents and older sibling were healthy. Her general physical examination was within normal limits. She had equal muscle bulk bilaterally and good range of movement in all limbs. Her reflexes were 2+ globally, with downgoing plantar responses. She pulled to stand, cruised and demonstrated controlled lowering from standing. She mobilized quickly by scooting along on her buttocks. She demonstrated hip flexion and knee extension in vertical suspension when carried around the room. She had a right hand preference but used both hands efficiently. Expressive and receptive language skills were advanced and she was a very social child. She progressed to walking independently at 19 months of age. Delayed motor development is a common referral question for general paediatricians as well as several other paediatric sub-specialists. It is often challenging to differentiate between children who fall into the realm of normal developmental variation and those who will develop significant motor disorders. Dissociation of motor maturation (DMM) is a relatively unknown and underdiagnosed clinical syndrome. Identification of DMM is reliably made when the child demonstrates the classic ‘sitting on air’ position, which is recognized by a straight back, flexion, moderate abduction and outward rotation of the hips, extension of the knees and dorsiflexion or plantar flexion of the feet when held in vertical suspension (Figure 1). The position is elicited both when the child is at ease and when the child is angry. DMM is further characterized by normal cognitive, language, social-emotional and fine motor development, with an isolated delay in gross motor skills. Children with DMM generally achieve head and arm control within the expected timeframe but are delayed in achieving gross motor milestones, including sitting, standing and/or cruising, and walking. Child demonstrating ‘sitting on air’ position. Note the flexed and abducted hips, extended knees and dorsiflexed toes when held in vertical suspension DMM was first described in 1969 and further expanded by Haidvogl (1) in 1979. The early descriptions of DMM described generalized hypotonia with preserved deep-tendon reflexes and early ambulation by means of bottom shuffling, hitching, sliding or scooting. A positive family history of similar early ambulation was observed in many of the children. Later descriptions expanded the diagnosis by describing children with “dissociated motor development simulating cerebral palsy”, which defined a condition in which the fine motor milestones appeared within the expected timeframe, while gross motor milestones were markedly delayed without obvious neurological findings. Other features that have been described with this syndrome include frequent temper tantrums and stereotyped motor movements. More recently, DiMario (2) observed a group of children referred to his paediatric neurology clinic for evaluation of developmental delay, hypotonia, weakness and to rule out cerebral palsy (CP). The children received a diagnosis of DMM if they: attained age appropriate milestones in all developmental sectors except gross motor development; had normal general and neurological physical examinations; and assumed the ‘sitting on air’ position during the consultation. Generalized hypotonia without weakness was an additional feature identified in several of these children. All children who met the inclusion criteria for DMM (n=8) ultimately had a normal motor developmental outcome and achieved independent walking by 20.1 months of age. DMM may be considered a specific outcome for children who are identified early on with congenital hypotonia with favourable outcome (formerly known as benign congenital hypotonia). It is also important to consider CP in the differential diagnosis of a child presenting with hypotonia and delayed motor development. Delayed, but ultimately normal motor outcomes, along with a normal neurological examination aside from hypotonia, will differentiate children with DMM from those who progress to receive a diagnosis of CP. Although clinical follow-up is necessary to ensure anticipated acquirement of motor skills and a normal developmental trajectory, routine laboratory testing and neuroimaging modalities are not needed and should only be obtained if there are clinical indications or physical examination findings indicating an entity other than DMM. DMM is characterized by a delay in achieving gross motor milestones, while cognitive, language, social-emotional and fine motor development are achieved within the expected timeframes. The ‘sitting on air’ position is a characteristic feature of the syndrome, and serves as a useful diagnostic sign for a normal anticipated prognosis in children presenting with delayed motor milestones. Laboratory investigations and neuroimaging modalities should be individualized, and performed only if there are associated clinical or neurological indications.

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