Painful Leg in a Toddler: Consider Idiopathic Neuralgic Amyotrophy

    

    

    Figure 1:  Plain X-ray of the pelvic area. The vastusexternus of the quadriceps
femoris muscle on the right side measures 1,58 cm compared to 2,00 cm on
the left side.
    
    

At follow-up visit, 10 weeks after onset of the complaints, there was clear improvement. Proximal atrophy of the right leg was still visible but ambulation was normal. Tendon reflexes were restored to a normal symmetrical patron. The diagnosis of idiopathic neuralgic amyotrophy (INA) was considered knowing that this disease does not exclusively present with a neuropathy of brachial plexus. The natural course is consistent with the diagnosis.

Discussion

INA is a plexopathy with acute onset of severe pain followed by loss of muscle strength and atrophy of the muscles innervated by the involved part of the plexus. The exact incidence of INA is not known. INA can present at any age, ranging from the neonatal period till geriatric age [1]. Pain is less often a presenting symptom of INA in children, making it more difficult to recognize [1]. Our patient did present with pain but the pain was solely located in the lower limb. It is less known that INA can also involve the lumbosacral plexus, in which case the disorder is called ‘idiopathic lumbosacral plexus neuropathy’ [2]. Involvement of the lumbosacral plexus in INA patients is less prevalent. It is estimated that less than ten percent of INA patients have involvement of the lower limb [3]. However this is more often than involvement of other peripheral nerves like the phrenic nerve, recurrent laryngeal nerve or peripheral autonomic nerves [3].

Even as a recognizable clinical picture, INA remains a diagnosis by exclusion. Lumbosacral plexopathies have a broad range of causes. The most important causes are vascular (proximal diabetic neuropathy), infectious (neuroborreliosis or HIV infection), spaceoccupying lesion (for example neoplastic invasion or retroperitoneal hematoma) and trauma. The diagnostic evaluation includes laboratory investigations, electrophysiological studies (nerve conduction and myography) and neuroimaging (MRI). Further, CSF examination can be necessary to exclude an occult neuroinfection or neoplasia if suspected. In approximately ten percent of INA patients CSF analysis shows an elevated protein level which is a nonspecific fact [3]. Nerve conduction studies can confirm an axonal plexopathy, and exclude a monoradiculopathy, pressure neuropathy or demyelinisation. An important pitfall with myography is that it can be normal if the affected muscles are not tested. On top of this electrophysiological studies can be a challenge in young children [4]. In a minority of patients MRI scan of the plexus shows focal T2 hyperintensity or focal thickening of the plexus [3].

INA is considered an auto-immune disease. In support of this hypothesis, attacks are often preceded by activation of the immune system, for instance infection, operation or vaccination. Physical stress is identified as a risk factor [3]. The conception is that mechanical disturbance leads to disruption of the blood nerve barrier which gives opportunity for attack to the peripheral nerves [3].

Besides idiopathic neuralgic amyotrophy, a hereditary form exist which is less common then INA. In hereditary neuralgic amyotrophy (HNA) neuralgia attacks will recur more often. Thus family history is a relevant part of history taking and discussing the prognosis in patients with INA. HNA has an autosomal dominant inheritance pattern. In some affected families the associated point mutation or duplication in the SEPT9 gene on chromosome 17 can be found [5]. Typical dysmorphic features are seen often in these families like small, round faces, hypotelorism and short stature [6]. More than half of patients with HNA have involvement of nerves besides the brachial plexus [3]. On average the first appearance of HNA is earlier in life compared to INA [3].

Given the hypothesis that INA is an immunologically mediated syndrome, treatment with immunosuppressive medication would be rational. Indeed oral prednisolone seems effective for pain relief and strength recovery in observational studies in the acute phase of INA [7,8]. This effect needs to be verified in a prospective, randomized trial. Symptomatic treatment needs to be adapted to the stage and type of pain. Usual care comprises treatment of the acute and often severe (neuropathic) pain and guidance for supportive care. Prevention of contractures is of uttermost importance. In later stages residual neuropathic and mechanical hypersensitivity pain can remain. Paretic muscles and compensatory mechanisms can lead to musculoskeletal pain and overburden is a hazard. Unfortunately recovery for children is less favorable, less than half of patients gain full recovery [1]. However the average follow-up time in this evaluation was relatively short (10 months). In adults it is known that recovery time can extend for over more than two years.

References

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