An Unusual Cause of Neonatal HIP Dislocation: Infantile Myofibromatosis Presenting as Developmental Dysplasia of the HIP

Introduction

Infantile myofibromatosis (IM) is a rare tumour of infancy first described by Stout et al in 1954 (1). Around 90% of cases present within the first 2 years of life, often at birth, with painless nodules most commonly arising in the head and neck (2). The disease can be solitary (70%–80% of cases) in which case single myofibromata occur in the skin, subcutaneous tissue and musculature. These lesions have an excellent prognosis, often demonstrating spontaneous regression and low rates of recurrence post-excision (2, 3). However, in around 25% of multicentric cases of myofibromatosis there is visceral involvement, which can result in significant morbidity and death, usually from mass effect or obstruction (4).

Although myofibromatosis is the most common fibrous tumour in infancy, the rarity of the disease and its varying manifestations can still be challenging to diagnose. Here we report an unusual case of infantile myofibromatosis presenting as developmental dysplasia of the hip (DDH) in an 18-day old baby.

Case discussion

The patient presented at the post-natal examination at 18 days of age with atrophy of the right leg and a dimple noted in the right gluteal region. She was born at 42 weeks by forceps delivery with no complications. Other than the right leg, she was healthy with normal growth and development and no family risk factors were present to predispose to DDH. Routine hip ultrasound was performed, which demonstrated a dysplastic right acetabulum and dislocation of the right hip with normal morphology of the left hip (Fig. 1A). Clinically, the hip was dislocated and nonreducible. There was noted to be marked atrophy of the right leg, particularly in the calf, with flaccidity of the right foot and ankle. There was, however, good movement in the right hip and knee. A Pavlik harness was applied by the paediatric orthopaedic surgeon for a putative diagnosis of developmental dysplasia of the hip, although the findings of flaccidity and atrophy of the lower limb were thought to be atypical.

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Fig. 1

Ultrasound imaging of the affected hip and buttock dimple: (A) Ultrasound image of the hip showing dysplastic acetabulum and debris posterior to the femoral head (arrowhead); (B) Targeted ultrasound images of the buttock mass show an echopoor mass with echogenic centre but no significant vascularity associated with the posterior aspect of the hip joint.

Follow-up in clinic showed nonreduction of the right hip despite the harness and treatment was discontinued to reduce the risk of osteonecrosis. Repeat ultrasound of the hip showed persistence of right hip dislocation. Targeted ultrasound of the region of the right gluteal dimple showed an echopoor mass associated with the posterior aspect of the hip joint containing internal curvilinear high reflectivity with acoustic shadowing and no significant vascularity (Fig. 1B).

Plain x-rays of the pelvis confirmed marked right acetabular dysplasia and dislocation of the hip joint. There was also heterotopic calcification noted medially and inferiorly to the hip joint which was thought to be unusual as myositis ossificans is not thought to occur in this age group (Fig. 2). MRI of the hip and thigh showed an irregular soft tissue mass in the right gluteus maximus muscle measuring 3.6 × 2.7 × 3.0 cm demonstrating low T1 signal (Fig. 3). On T2 weighted imaging, the mass showed a peripheral solid component that was hyperintense to muscle and central bright T2 signal suggesting necrosis. Flecks of low signal on both T1 and T2 imaging were seen consistent with internal calcification. There was marked surrounding desmoplastic reaction with dimpling of the overlying skin. The right sciatic nerve was inseparable from the tumour, although it appeared normal distally. MRI confirmed that the cause of the DDH was secondary to marked wasting of the posterior thigh muscles as noted clinically. Post-contrast imaging showed enhancement of the peripheral soft tissue component and also revealed other small enhancing soft tissue lesions in the lateral thigh bilaterally likely representing the same pathology.

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Fig. 2

Plain x-ray of the pelvis. There is a dysplastic, dislocated right hip. There are marked heterotopic calcifications seen medial to the lesser trochanter (arrowhead).

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Fig. 3

MRI of the pelvis with contrast. T1 and T2 weighted axial images (A, B) and T2 weighted sagittal image (C) show mass in the right gluteus maximus muscle with central high T2 signal and surrounding desmoplastic reaction causing dimpling of the skin. There are low signal foci within indicative of calcification and there is marked wasting of the posterior thigh muscles. Post contrast image (D) shows bright enhancement of the peripheral soft tissue component but no enhancement of the centre.

A radiological diagnosis of infantile multicentric myofibromatosis was suggested on the basis of the ultrasound and MRI appearances and enhancement characteristics.

Ultrasound of the abdomen was undertaken to ascertain visceral involvement, and showed no other lesions. Biopsy of the lesion under general anaesthetic was performed at a tertiary paediatric centre. Haematoxylin and eosin stain showed a lesion composed of long intersecting bundles of spindle cells with bland cytoplasm and areas of collagen and smooth muscle. No necrosis or vascular invasion were identified and mitotic figures were scanty. Immunohistochemistry showed positivity for SMA and CD34 in a patchy distribution, but desmin, ALK, beta-catenin and S100 were negative, with Ki-67 highlighting 5–10% of the lesional cells. This confirmed a diagnosis of infantile myofibroma (multicentric type with no visceral involvement). FISH for ETV6 gene mutation was subsequently negative, ruling out infantile fibrosarcoma. The patient was referred to a specialist paediatric surgical centre for discussion of further treatment options.

Discussion

IM is a very rare tumour of childhood with an incidence of 1:150000 live births, but is the most common fibrous paediatric tumour (5). The solitary form shows slight male predominance and has a predilection for the head and neck (3, 4). Most lesions present as firm red/purple dermal or subdermal painless nodules, but atrophic depressed lesions such as this case, as well as pedunculated or ulcerated lesions, have also been reported. In the multicentric form, lesions can also affect the bones (6) and visceral organs, but this is extremely rare in the solitary form.

Most cases of IM are thought to be sporadic; however, congenital myofibromata and myofibromatosis have been described in various series with autosomal dominant or autosomal recessive inheritance. Genetic studies implicate several cell signalling proteins including PDGFRB (a receptor for platelet derived growth factor) and PTPRG (a tyrosine kinase signalling protein in the same pathway) (7).

Imaging characteristics previously reported by Koujok et al (8) are variable but consistent with the findings in this case. In this series, ultrasound showed an anechoic centre with septae in most patients but a hyperechoic centre as in our patient in only one case. There is invariably no central Doppler vascular flow reflecting internal fibrosis/necrosis. Although calcification was a notable observation in our case, plain film and CT have previously shown calcification in only the minority of lesions and the mass usually has similar or lower attenuation to surrounding muscle after iodinated IV contrast. MRI shows low T1 signal within the mass, but the centre of the lesion could be hyper- or hypointense on T2 weighted imaging depending on the degree of internal necrosis. The soft tissue periphery usually shows high T2 signal and enhancement with gadolinium as in our case.

Pathological appearances of infantile myofibroma typically demonstrate short fascicles and nodules of plump myofibroblasts with scant cytoplasm and hyperchromatic nuclei. Pleomorphism and mitotic activity is minimal and vascular invasion is not a feature. Calcification, hyalinisation and necrosis are commonly seen. Immunohistochemistry shows SMA and vimentin positivity and is negative for S100, cytokeratins, desmin and EMA. The lesion sampled in this case study showed typical pathological appearances for this tumour.

Lesions demonstrate a variable clinical course, with many solitary lesions demonstrating good prognosis and a significant proportion demonstrating spontaneous regression (3). The treatment of choice for solitary and non-visceral multicentric lesions tends to be surgical excision and there are variable reported recurrence rates of 7–31% after treatment. However, multicentric forms involving visceral sites have a significant mortality of 33–73% (4) and can be treated with a combination of surgery and chemotherapy such as low dose vinblastine and methotrexate.

Developmental dysplasia of the hip (DDH) describes the abnormal development of the acetabulum secondary to congenital dislocation thought to be due to a combination of genetic factors determining ligamentous laxity, foetal crowding, and breech delivery.

While DDH is a common entity in the newborn population, in this case the failure of the hip to relocate clinically or in the Pavlik harness, as well as the presence of calcification on the plain film, was suggestive of an alternative, secondary pathology. The presence of paresis and wasting of the lower limb was also not compatible with the diagnosis of typical DDH.

In conclusion, in this case study we have reported an unusual presentation of DDH. Sciatic nerve palsy causing muscular atrophy and secondary intrauterine dislocation of the hip is an extremely rare cause of DDH, and to our knowledge this is the first case in the literature where this presentation has been caused by a soft tissue tumour such as a myofibroma. Clinicians should be aware that however rare, there are other secondary causes of the clinical and imaging appearances of DDH. The presence of associated soft tissue calcifications in a neonate should be interpreted with caution, as myositis ossificans is not thought to occur in this age group and therefore their presence is nearly always pathological. Although imaging appearances on other modalities may vary, our experience together with previous case series (8) show that the signal characteristics and enhancement pattern of soft tissue myofibromata on MRI with and without gadolinium contrast appear to be the most consistent finding. When a paediatric fibrous tumour is suspected clinically or by imaging, histological confirmation is essential as it is not possible to rule out malignancy such as fibrosarcoma or leiomyosarcoma on imaging appearances alone.