Presacral plexiform neurofibroma with urinary retention

Clinical History

A 7-year-old boy with neurofibromatosis type I presented with one day of painful urinary retention requiring in & out catheterization. He has had 3 prior episodes over the past 2 years, typically resolving after 1-2 days. Urinalysis was normal.

Imaging Findings

The abdominal ultrasound showed bilateral grade 2 hydronephrosis. Within the pelvis were multiple hypoechoic perirectal and pelvic masses (Fig. 1). These were initially thought to represent a conglomeration of enlarged lymph nodes. The clinical history of neurofibromatosis brought up the possibility of plexiform neurofibroma and prompted further imaging.
Subsequent abdominopelvic MRI revealed a large (9.4 x 7.1 x 10.0 cm) pre-sacral/perirectal mass which was T2 hyperintense with central areas of T2 hypointensity (Fig. 2). The bladder was compressed and displaced anteriorly while the rectum and sigmoid colon were surrounded and partially compressed by the mass (Fig. 3).The mass was heterogeneously enhancing and involved multiple sacral nerve roots with associated neuroforaminal widening (Fig. 4).

Discussion

Neurofibromatosis type 1 is an autosomal dominant disorder in which the NF1 gene (a tumour suppressor) is mutated. This disorder is characterized by full penetrance and variable expression leading to a wide variety in presentation. [3, 4] In this patient, neurofibromatosis was diagnosed first clinically; he has more than 6 café-au-lait spots, bilateral inguinal and axillary freckling, delayed development and learning disability, and one small cutaneous neurofibroma. His diagnosis of NF-1 was confirmed at 5 years of age with genetic testing. Other clinical manifestations of this disease include sphenoid wing dysplasia or cortex thinning of long bones and Lisch nodules, hamartomas arising from the surface of the iris. [2, 3]
The pelvic MRI confirmed the suspicion of a neurofibroma causing urinary obstruction. Classic MRI appearance of a neurofibroma is a T2 hyperintense mass with central areas of T2 hypointensity (target sign) corresponding to a denser collagen stroma. Diffuse nerve involvement with soft tissue infiltration indicates a plexiform neurofibroma, which is pathognomonic of NF1. Although these fibrovascular tumours are benign, malignant transformation can occur in a small percentage (2-5%) with a significantly higher risk for pelvic plexiform neurofibromas (12-29%) [1, 3, 4]. Plexiform neurofibromas are more common in children and predominantly arise from the head and neck as well as pelvic and sacral regions [2, 4]. They cause morbidity due to mass effect and local invasion. Malignant progression typically manifests as malignant peripheral nerve sheath tumour (MPNST), a leading cause of premature death in those with neurofibromatosis, thus plexiform neurofibromas require continued observation. [5]

In summary, large heterogenous masses that follow the course of central and peripheral nerves bring up the possiblility of plexiform neurofibromas.

Differential Diagnosis List

Final Diagnosis

Presacral plexiform neurofibroma

URL:	https://www.eurorad.org/case/13302
DOI:	10.1594/EURORAD/CASE.13302
ISSN:	1563-4086