‘We cannot learn without pain.’– Aristotle
In cases of unexplained fractures and injuries, the differentials considered are usually non-accidental injury or osteogenesis imperfecta. This case highlights an additional possibility of congenital insensitivity to pain (CIP) being responsible for unexplained injuries.
Our patient is an intelligent and healthy 2-year-old boy with a history of lack of response to painful stimuli. As an infant, he would bite into his tongue and lips, and had even self-extracted a tooth. In 2009, he had a left proximal tibial fracture (serial X-rays, Fig. 1). This did not heal well, and he subsequently developed non-union and osteomyelitis requiring prolonged treatment with intravenous antibiotics and open debridement and washout 6 months later. At no point was he noted to be in pain. He has also fractured his right tibia/fibula three times in 2010.
Due to his recurrent hospital presentations with unwitnessed injuries, he was brought to the attention of the child protection team, and subsequent genetics and neurology involvement.
His neurological examination was normal except for a failure to withdraw to painful stimuli. Nerve conduction studies were normal.
Due to his pure sensory involvement, SCN9A-mediated CIP was considered as a diagnosis. Other pain insensitivity conditions were also considered, including the hereditary sensory and autonomic neuropathies (noted for their sensory and autonomic involvement), or a disruption to cerebral pain pathways.1 SCN9A gene testing revealed a compound heterozygous mutation.
The earliest published report of CIP was in 1932, of Edward H Gibson, making his living in the circus as ‘The human pincushion’.2 The first CIP patient with an SCN9A mutation was a 10–year-old Pakistani ‘street theatre’ performer.3 He placed knives through his arms, walked on burning coals and died after jumping off a roof. Since then, there have been 30 cases reported in the literature with recessive SCN9A CIP.4
SCN9A is a voltage-gated sodium channel on chromosome 2, expressed exclusively in the dorsal root ganglia of sensory neurons. These voltage-gated channels are responsible for transmitting peripheral sensory impulses, and loss-of-function mutations of SCN9A lead to closure of the sodium channel. Interestingly, dominant ‘gain of function’ mutations of SCN9A cause a hyper-excitable channel, leading to two autosomal-dominant extreme pain disorders: paroxysmal extreme pain disorder and primary erythermalgia. These disorders are characterised by severe paroxysmal burning pain in response to minor stimuli such as exercise, spicy foods or cold drinks.
Many people with CIP do not survive childhood due to recurrent injuries. Self-injury, burns, repeat fractures, osteomyelitis and accidental death are the main dangers of this disorder. Management consists of learning behaviours to reduce risk of injury and modifying the environment at home to minimise occult trauma. This is particularly difficult during childhood.