Hamartomatous brain lesions (UBOs)
Unidentified bright objects (UBOs) are hamartomatous lesions in the brain tissue. They follow a characteristic age-related pattern: they appear in children under 4 years of age, actively increase in size and number between 4 and 10 years, and begin to regress in the second decade of life. They are extremely rare in adult patients.
MRI semiotics of UBOs:
- On T2-weighted images, they appear as multiple hyperintense foci.
- On T1-weighted images, the signal is usually normal. An exception is lesions of the basal ganglia (e.g., lentiform nuclei), which may be slightly hyperintense due to T1 shortening.
- They practically lack mass effect and do not enhance with contrast agents.
Typical locations include the pons, cerebellar white matter, internal capsules, basal ganglia, thalami, and hippocampi.
Differential diagnosis is performed with gliomas. Unlike tumors, UBOs do not grow after a certain age and do not enhance. However, it is important to remember that astrocytomas can develop within hamartoma zones. The appearance of contrast enhancement or increasing mass effect is a warning sign that requires follow-up MRI.
Plexiform and spinal neurofibromas
Plexiform neurofibromas are one of the major diagnostic criteria for NF-1. These are multinodular masses involving multiple trunks or bundles of a large nerve simultaneously.
Features of plexiform neurofibromas:
- Morphology: unlike regular neurofibromas, which have clear margins and cause uniform nerve enlargement, plexiform nodules grow diffusely. They can extend deep into the fascial spaces of the neck, tongue, and orbit.
- CT and MRI: on CT, the masses are hypodense and usually do not enhance with contrast. On MRI, the structure is heterogeneous—low signal on T1-weighted images and high signal on T2-weighted images. Contrast enhancement is variable, but at least part of the tumor usually enhances.
- Growth patterns: characteristic spread along neural pathways. For example, they can extend through the foramen ovale, displacing the dura mater (which appears as a black line on MRI), and grow into the pterygomaxillary fissure, orbital apex, and cavernous sinus.
Spinal neurofibromas are identified on MRI as nodules along the spinal nerves, frequently in the cauda equina region. As they grow, they pass through the intervertebral foramina, noticeably enlarging them. On T2-weighted images, they typically exhibit the target appearance (target appearance) — a central zone of hypointense signal against the background of the overall hyperintensity of the nodule.
Bone dysplasias and extraneural manifestations
Skeletal involvement in NF-1 is diverse and often detected as early as infancy.
Key bone anomalies:
- Sphenoid wing dysplasia: leads to enlargement of the middle cranial fossa and herniation of the temporal lobe into the orbital cavity. On anteroposterior radiographs, the absence of the lateral wall creates the "empty" or "bare" orbit sign ('bare orbit' sign). Clinically, this manifests as proptosis, eyeball enlargement, or pulsating exophthalmos (transmitted CSF pulsation).
- Kyphoscoliosis: characterized by a prominent sharp curve in the thoracic spine. It develops as a result of primary skeletal dysostosis, dural ectasia, scalloping of the posterior vertebral bodies (scalloping), and the formation of lateral meningoceles.
- Other changes: thinning of the cortical layer of long tubular bones and lambdoid suture dysplasia.
Additionally, patients with neurofibromatosis type 1 are predisposed to developing extraneural neoplasms. These include pheochromocytoma, carcinoid, rhabdomyosarcoma, and children also have an increased risk of developing chronic myeloid leukemia.
Briefly: answers to topical questions
What imaging features and growth patterns characterize optic nerve gliomas in neurofibromatosis type 1?
Optic nerve gliomas in neurofibromatosis type 1 are characterized by fusiform nerve thickening and variable contrast enhancement.
Growth patterns:
- Localization — primarily affects the optic nerves themselves, less frequently the chiasm.
- Clinical course (indolent) — sluggish, biological behavior ranges from static or minimal growth to rapid enlargement.
- Spread — via the expanded optic canal (canalis opticus) to the retrobulbar segment and the optic chiasm (chiasma opticum).
Imaging findings (MRI):
- Shape (fusiform) — fusiform thickening of the optic nerve.
- T1-weighted images — isointense or hypointense signal.
- T2-weighted images — hyperintense signal.
What are the clinical and diagnostic criteria for neurofibromatosis type 1 (NIH criteria)?
The diagnosis of neurofibromatosis type 1 is based on the presence of at least two major criteria.
Major Criteria:
- Café au lait spots.
- Inguinal or axillary freckling (Crowe's sign).
- One plexiform neurofibroma or two neurofibromas of any type.
- Optic pathway glioma.
- Two or more Lisch nodules on the iris.
- Specific bone lesions, such as sphenoid dysplasia or cortical thinning.
- A first-degree relative with NF-1.
Minor Criteria:
- Short stature.
- Macrocephaly.
- Scoliosis.
- Pectus excavatum.
- NF-1-specific hamartomatous lesions.
- Neuropsychological abnormalities.
What is the risk and imaging features of malignant transformation (malignancy) of a plexiform neurofibroma?
The risk of malignant transformation of a plexiform neurofibroma into a fibrosarcoma ranges from 2% to 12%. The risk of malignant transformation is higher in patients with large central lesions.
MRI signs of malignancy:
- Large, heterogeneous, lobulated mass.
- Absence of the target sign.
- Signs of hemorrhage, including a hyperintense signal on T1-weighted images.
- Contrast enhancement is more frequently peripheral than central.
Clinical signs of malignant transformation:
- Rapid enlargement of the lesion.
- Onset of new pain.
- Change in tumor consistency.
- Development of neurological deficits.
Class and exam questions
How to differentiate UBOs from gliomas on MRI?
Unlike gliomas, UBOs show minimal mass effect, do not enhance with contrast, and regress with age. However, the appearance of enhancement within an UBO indicates possible neoplastic transformation.
What is the "empty orbit" sign and why does it occur?
This is a radiological sign characterized by the absence of visualization of the lateral orbital wall on anteroposterior radiographs. It occurs as a result of sphenoid wing dysplasia.
What is the "target sign" in spinal neurofibromas?
This is a specific MRI sign on T2-weighted images, where a hypointense (low signal) zone is identified in the center of a hyperintense tumor node.
What else to study on this topic
- Diagnostic imaging of optic nerve and optic chiasm gliomas in NF-1
- Specific features of cavernous sinus involvement by plexiform neurofibromas
- Differential diagnosis of causes of kyphoscoliosis in neurofibromatosis
- Algorithm for serial MRI monitoring of brain hamartomatous lesions
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