Progressive Unilateral Foot Drop in a Child due to Neuromuscular Choristoma

Abstract

An 8.5-year-old boy presented with a history of progressive unilateral foot drop. A contrast magnetic resonance imaging of the pelvic girdle and the lumbosacral plexus revealed a diffuse fusiform thickening of the right lumbosacral trunk and the sciatic nerve in the intrapelvic and gluteal regions, with findings characteristic of a choristoma. Clinical and radiologic features as well as complications of this unusual entity are described.

Introduction

Progressive unilateral foot drop in children is rare. Causes include common peroneal neuropathy, L5 radiculopathy, lumbosacral plexopathies, anterior horn cell disease, mononeuritis multiplex, holocord syrinx secondary to Chiari malformation type I, parasagittal tumors, and intracranial tuberculomas.[1] We present the case of a child with a progressive unilateral foot drop due to a lumbosacral neuromuscular choristoma (NMC).

Case Report

An 8.5-year-old boy presented to us with the history of frequent falls noticed from 4 years of age. His perinatal period had been normal. He learnt to sit at 9 months and walk at 15 months. His speech and cognitive milestones were appropriate for age. For the last 4 years, he was noticed to be dragging his right foot, and the weakness had been very slowly progressing.

On examination, he had limb length asymmetry, with the right leg being 1 cm shorter. He had wasting of right hamstrings and calf muscles with weakness of right ankle dorsiflexors. Right ankle jerk was absent, and there was mild shortening of the right Achilles tendon. There was no sensory deficit in the right leg. His higher mental functions and cranial nerve and upper limb examination were normal.

Investigations at 4 years had revealed total creatine phosphokinase (CPK) of 265 U/L (normal <195 U/L) and normal magnetic resonance imaging (MRI) of the brain and spine. Nerve conduction studies at 8 years showed normal distal latencies, but reduced compound muscle action potential (CMAP) of the right peroneal nerve recorded on the extensor digitorum brevis (698.8uV on the right vs. 13 mV on the left). CMAP was also reduced on testing the right tibial on the abductor hallucis brevis (4.3 mV on the right vs. 16.9 mV on the left) with preserved conduction velocities. The rest of the motor conduction parameters and routine sensory conduction parameters were normal.

Contrast MRI of the pelvic girdle and the lumbosacral plexus revealed a diffuse fusiform thickening of the right L4, lumbosacral trunk, and S1–S3 nerve roots, as well as the sciatic nerve in the intrapelvic, gluteal, and proximal thigh measuring 20 cm in length. The sciatic nerve was isointense to surrounding muscle tissue and intermixed with subtle areas of intermediate signal intensity on T2 and spectral adiabatic inversion recovery T2 sequences [Figure 1a and b]. There was also hypointense perineurial circumferential thickening. The intramural fascicles were unremarkable. The maximum axial diameter (2.3 × 2.3 cm) was at the level of the pyriformis. The nerve was isointense on T1-weighted images with no evidence of fat or calcification [Figure 1c]. There was minimal diffuse gadolinium enhancement of the involved right sciatic nerve [Figure 1d and e]. Denervation fatty atrophy of the ipsilateral hamstrings was seen. The pelvic bones and soft tissues were normal.

Figure 1.

(a) T2 axial image of the pelvis shows thickened right lumbosacral plexus with linear hypointense striations similar to muscle signal interspersed with intermediate signal intensity striations representing nerve fascicles. (b) SPAIR T2 axial image showing thickened fusiform right sciatic nerve with isointense signal to the surrounding muscles. (c) T1 axial image showing thickened right sciatic nerve with isointense signal appearing similar to the surrounding muscles. (d) T1 postcontrast fat-suppressed axial image showing mild diffuse striated enhancement within the thickened right sciatic nerve. (e) T1 postcontrast coronal image reveals diffusely thickened sciatic nerve with enhancement

SPAIR: Spectral Adiabatic Inversion Recovery

This imaging was characteristic of an NMC.[2] The areas of intermediate signal within the nerve were suggestive of muscle fibers within the nerve bundle. Nerve biopsy was deferred in view of the risk of precipitating aggressive desmoid-type fibromatosis (DTF) documented in literature.[3] The child has been advised close clinical and radiologic monitoring for complications including hip dysplasia and spontaneous development of DTF. He is undergoing neurorehabilitation and is using an ankle foot orthosis.

Discussion

NMCs are developmental tumors containing normal muscle tissue within a peripheral nerve. There have been two case series of seven patients each, one from Mayo Clinic, Rochester and the other from Peking University, China. NMCs typically involve large peripheral nerves such as the sciatic nerve, brachial plexus, median nerve, etc. Symptoms are related to either neuropathy or stunted growth of tissues in the nerve territory.[4] Sciatic NMCs typically present in the first decade with an asymptomatic hard lump, asymmetric leg weakness, muscle atrophy, or foot drop. There appears to be a notable role of peripheral nerves in bone mechanosensing and adaptation to mechanical stimuli.[5] Hence, NMCs may be associated with pes cavus, limb shortening, or hip dysplasia. Children presenting below 4 years often have an initial normal gait with secondary development of a limp. Those presenting above 4 years manifest with a pes cavus. Pain has been documented in children above 11 years of age. Patients diagnosed in adulthood have been diagnosed to have a congenital hip dysplasia, which is a long-term manifestation of the disease. Kumar et al.[6] noted that when the proximal lumbosacral plexus was involved in sciatic NMC, tissues supplied by the nerve manifested as ipsilateral reduced growth of the pelvis and hip dysplasia in 4 of 7 patients.

A major complication of NMC is DTF associated with the NMC, which appears to be precipitated by surgical intervention. In a case series from China, all five of seven patients with NMCs who underwent surgical intervention developed DTF.[7] These locally aggressive and infiltrative tumors may need repeated surgeries, radiation, chemotherapy, and even amputation.[7] Somatic mutations of CTNNB1 gene (exon 3) have been documented in 6 / 7 patients with DTF associated with NMC, as well as in sporadic DTF.[3] Elevation of CPK has not been previously reported in NMC. The mechanism may be similar to that seen in amyotrophic lateral sclerosis, where muscle atrophy secondary to neuronal degeneration and upregulation of muscle metabolism to provide energy have been proposed to explain elevations of CPK.[8]

Differential diagnosis of NMC includes benign conditions like fibrolipomatous hamartomas and perineuriomas. On MRI, lipomatous hamartomas have more intralesional fat (more than 50%) and are associated with nerve territory overgrowth, unlike undergrowth associated with NMC, and perineuriomas have consistent contrast enhancement.[9]

Since resection of the lesion would necessitate nerve resection and may result in significant deficits, surgery is fraught with problems and often necessitates nerve grafting. Theories underlying the pathogenesis of NMC include trapped muscle fibers originating from the limb mesenchyme within developing nerves, mesenchymal tissue developing from neuroectoderm, and induction of surrounding mesenchyme into muscle tissue by nerves.[9]

Long-term prognosis has been variable. Most are slow growing with mild symptoms. Complete regression of the lesion with no deficit has been reported by Louhimo et al. from Finland in a newborn child with buttock mass and in a brachial plexus NMC in a boy aged 1 year 9 months.[10] A 17-year follow-up of a neonate with NMC did not show any major progression and only mild weakness of one leg.[11]

In summary, we present a case of a child with progressive unilateral foot drop with secondary limb length asymmetry, whose diagnosis was clinched due to characteristic magnetic resonance findings of an NMC. Avoiding biopsy in these rare tumors will prevent development of serious complications such as DTF.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient’s guardian has given consent for the child’s images and other clinical information to be reported in the journal. The patient’s guardian understands that the child’s name and initial will not be published, and due efforts will be made to conceal the identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.