Dandy Walker malformation

Findings

There is a large fourth ventricle which appears to be in continuation with a large posterior fossa cyst which follows CSF signal.

Differential Diagnosis:
- Arachnoid cyst
- Dandy walker variant
- Dandy walker malformation


Diagnosis: Dandy Walker malformation


Key points

Classic triad makes up the Dandy-Walker malformation
- Vermian agenesis
- Communicating with an enlarged 4th ventricle
- Posterior fossa enlargement

Dandy-Walker malformation may be isolated or can occur with other malformations of the brain.

Associated CNS abnormalities include
- Agenesis of the corpus callosum
- Lipomas
- Aqueductal stenosis
- Microcephaly
- Encephalocele

The Dandy Walker Spectrum represents a broad spectrum of cystic posterior fossa malformations.

These include (from most severe to least severe):
- 4th ventriculocele: DWM in which large posterior fossa cyst erodes occipital bone causing posterior outpouching of the cyst (encephalocele) through the occipital bone
- "Classic" Dandy Walker Malformation: Cystic dilatation of the 4th ventricle which opens dorsally to a variable sized CSF cyst which causes an enlarged posterior fossa and superiorly rotated hypoplastic vermian remnant; hydrocephalus is common
- Dandy Walker Variant: 4th ventricle opens to a smaller posterior cyst with partial rotation of the vermis, variable vermian hypoplasia; normal sized posterior fossa
- Persistent Blake Pouch Cyst: Open 4th ventricle communicates with persistent cyst that fails to regress and obliterates basal cisterns; normal vermis
- Mega Cisterna Magna: Enlarged cisterna magna, 4th ventricle is closed; normal vermis

DWM commonly presents with macrocephaly and bulging fontanel, 80% are diagnosed by 1 year of age. Early death is common.
Etiology is from hindbrain development arrest. No 4th ventricle outlet foramina are formed.
Associated with other CNS/extracranial abnormalities two-thirds of the time, such as craniofacial, cardiac and urinary tract abnormalities, polydactyly, orthopedic, and respiratory abnormalities
Treatment includes CSF diversion of hydrocephalus with VP shunt

Subacute combined degeneration

Findings

Figure 2 and Figure 4: Sagittal and axial T2-weighted images demonstrate symmetrical hyperintensity within the dorsal columns that is contiguous in length over several vertebral body lengths.
Figure 1 and Figure 3: Sagittal T1-weighted images pre and post-gadolinium demonstrate hypointense signal within the dorsal cervical spinal cord without enhancement.


Diagnosis: Subacute combined degeneration


MR findings in subacute combined degeneration include symmetrical increased signal intensity in T2-weighted sequences along the dorsal columns that is usually contiguous over multiple vertebral body lengths.
The differential diagnosis includes B12 deficiency with subacute combined degeneration, tabes dorsalis and vacuolar myelopathy in AIDS.

Vitamin B12 deficiency causes a wide range of neurological, hematological and gastrointestinal disorders. Subacute combined degeneration is one such neurological disorder that is characterized by symmetrical numbness, weakness and paresthesia. It most commonly results from pernicious anemia and is potentially reversible with B12 supplementation.

MR findings include symmetrical increased signal intensity on T2-weighted sequences along the dorsal columns that is usually contiguous over multiple vertebral body lengths. Lesions are iso or hypointense to the normal cord on precontrast T1-weighted sequences. There can be variable contrast enhancement. The differential diagnosis includes tabes dorsalis and vacuolar myelopathy in AIDS. These can be distinguished from other myelopathies on the bases of their symmetry and contiguous cord level involvement. They can be differentiated from each other on the basis of laboratory tests and clinical history.

Arachnoid cyst with acute subdural hemorrhage

Findings

Large left frontal cystic lesion with thin septations and no enhancement. Unclear whether this lesion is intra-axial or extra-axial. No diffusion restriction (image not shown), no surrounding vasogenic edema. Also present is an acute right subdural hematoma caused by the patient's fall.

Differential diagnosis for the cystic lesion:
- Arachnoid cyst
- Ependymal cyst
- Glial cyst


Diagnosis: Arachnoid cyst. Acute subdural hemorrhage.


Key points

Arachnoid cysts are CSF containing cysts which are intra-arachnoid, and do not communicate with the ventricular system.
50-60% occur within the middle cranial fossa.
May arise as a developmental anomaly, or may be acquired as a complication of adhesions.
Small number are associated with neoplasms.
Walls of cyst are formed by splitting of the arachnoid membrane.
Usually asymptomatic. Can have headache or seizures.
The most effective surgical treatment appears to be excision of the outer cyst membrane and cysto peritoneal shunting.

Capillary telangiectasia of the pons

Findings

There is an irregular FLAIR hyperintense focus located along the left anterior aspect of the pons, demonstrating mild lacy contrast enhancement. There is no associated mass effect or restricted diffusion.

Differential diagnosis:
- Developmental venous anomaly
- Cavernous malformation
- Neoplasm
- Inflammation
- Demyelination
- Capillary telangiectasia


Diagnosis: Capillary telangiectasia of the pons


Key points

Capillary telangiectasia
- Represents clusters of dilated capillaries within normal brain
- Usually an incidental finding at imaging
- Represents 15 to 20 percent of brain vascular malformations
- Increased risk for development after brain radiation
- Associated with other vascular malformation, such as cavernous malformation or venous angioma
- Associated with hereditary hemorrhagic telangiectasia

Clinical:
- Usually an incidental finding
- Rarely associated with headache or vertigo
- Found at any age
- No treatment necessary


Imaging

Characteristically found in the pons, but may also be seen in the midbrain, medulla, and elsewhere
Usually small, average 3 cam in diameter
Poorly defined, without mass effect, edema, or restricted diffusion

CT: Usually normal

MRI:
- T1 weighted sequence = usually normal
- T2 weighted sequence = half are normal, half show fine hyperintensity
- Gradient echo = hypointense
- FLAIR = usually normal, but may show foci of hyperintensity
- Diffusion weighted image = usually normal
- Post contrast images = faint lacy or speckled enhancement
- May have associated linear vessels or draining veins if associated with venous angioma

Angiography: Usually occult

Failed back surgery syndrome

Findings

Figure 1: Axial T1 post-contrast weighted image demonstrates a low signal rim-enhancing collection with posterior compression of the thecal sac.
Figure 2: An axial T2 weighted image shows a high-signal epidural collection with posterior compression of the thecal sac.
Figure 3: Sagittal T1 weighted post contrast imaging demonstrates a prior laminectomy with an elongated, peripherally enhancing epidural fluid collection with displacement of the thecal sac anteriorly.
Figure 4: Sagittal T2 weighted image reveals a septated fluid collection with posterior compression of the thecal sac consistent with an epidural abscess.

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Figure 1: Axial T1 weighted image demonstrates a low signal epidural mass in the right neuroforamen with some mass effect on the thecal sac.
Figure 2: Axial T1 post contrast fat suppressed image demonstrates high signal scar or granulation tissue surrounding a low signal right neuroforaminal mass with mass effect on the thecal sac.
Figure 3: One slice below, an axial T1 post contrast fat suppressed image displays the right nerve root surrounded by high signal scar and/or granulation tissue.
Figure 5 and Figure 7: Pre- and post-contrast sagittal T1 image reveals the low signal right neuroforaminal mass to be continuous with the L5-S1 disk with rim enhancement and compression of the thecal sac consistent with a recurrent disk herniation.
Figure 4 and Figure 6: Axial and sagittal T2 weighted images demonstrate a low-signal herniated disk in the right neuroforamen with compression of the thecal sac.


Diagnosis: Failed back surgery syndrome


Failed back surgery syndrome (FBSS) presents with recurrent or persistent low back pain after lumbar spine surgery. Patients may present with weakness and numbness related to lumbar radiculopathy or generalized low back pain. The etiologies of FBSS are vast and include early or late phenomenon. “Early” phenomena include post-operative hemorrhage; infection leading to diskitis, osteomyelitis, meningitis, or epidural abscess; dural tear with pseudomeningocele; sequestered disk fragment; hardware malposition and surgery at the wrong level. These entities demonstrate typical imaging findings depending upon the cause. “Late” etiologies of FBSS include: foraminal or central stenosis (20-60%), instability (14%), recurrent disk herniation (7-12%), epidural fibrosis (5-25%) and arachnoiditis.

Causes of Failed Back Surgery Syndrome:
- Arachnoiditis
- Central or foraminal stenosis
- Epidural Fibrosis
- Infection
- Hematoma
- Surgical trauma to roots
- Insufficient decompression
- Mechanical instability
- Pseudoarthrosis
- Residual or recurrent disk
- Spondylolisthesis
- Surgery at wrong level


Diagnosis of FBSS is made with contrast enhanced MR imaging of the lumbar spine. Canal stenosis is manifest on MR imaging by a “trefoil” appearance of the spinal canal with a thickened ligamentum flavum. Enhancing nerve roots may be present on contrast enhanced imaging. Vertebral body instability can be diagnosed on flexion and extension views on plain radiographs by demonstrating a “dynamic slip” of greater than 3 millimeters. MR findings of instability include antero- or retrolisthesis or loss of disc height with loss of disc signal on T2 weighted images. Epidural fibrosis appears low in signal on T1 weighted images with slightly higher signal than disc on T2 images with diffuse contrast enhancement. This is in contrast to herniated disc material which demonstrates little or thin peripheral enhancement after contrast administration. Post-operative arachnoiditis appears on MR imaging as peripheral or central clumping of nerve roots with variable enhancement.

Because post-operative scarring does not warrant additional surgery, its differentiation from herniated disk is crucial. This is best demonstrated on contrast-enhanced T1 fat saturated weighted images as disc material will demonstrate no or peripheral enhancement while scar tissue will avidly enhance. Additional differentiating features include traction of the dural tube secondary to fibrosis and compression of nerve roots from disk material. MR is 96-100% accurate in detecting epidural fibrosis versus disc material.

Treatment of FBSS is dependent on the etiology. Therapeutic modalities include physiotherapy, epidural steroids, spinal cord stimulators and when warranted, repeat surgery for treatment of recurrent herniation, foraminal stenosis and instability.

Pilocytic astrocytoma

Findings

Large posterior fossa mass with central cystic portion and peripheral thick rind with enhancement and effacement of the 4th ventricle and cerebellar tonsillar herniation. There is secondary mild ventricular enlargement with transependymal edema.

Differential diagnosis:
- Medulloblastoma
- Pilocytic astrocytoma
- Ependymoma


Diagnosis: Pilocytic astrocytoma, atypical


Key points

Juvenile pilocytic astrocytomas are typically benign tumors that arise in the cerebellum, brainstem, hypothalamic region, or optic tracts. The tumor is classified as a WHO grade 1 benign lesion. Their most common site of origin is the cerebellum. It is the most common tumor to occur in the cerebellum. The medulobllastoma is more common in the posterior fossa but it has 4th ventricle origins rather than the cerebellum itself. The tumor generally is found in patients aged 5-14, but can be seen at any age more rarely. There is no sex predilection. The tumors are associated with neurofibromatosis type 1. Common presentations would include ataxia like in this patient, headache, nausea, vomiting or visual disturbances depending on where it is located. Hydrocephalus secondary to mass effect on the fourth ventricle is responsible for the majority of the presentations.

On imaging the tumors are typically well circumscribed with a large cystic component and a mural nodule. The nodule typically enhances. There can be calcification present but only up to 25% of the time. The tumors have a lot of variability in their appearance; they can be entirely solid with or without cystic degeneration. Vasogenic edema is a very uncommon finding and can be used to differentiate this tumor from others in the differential. They are typically hypo attenuating on CT which is helpful in differentiating from a medulloblastoma which is usually hyper attenuating if not at least iso- to the brain secondary to its closely packed cells.

Prognosis for the lesions is quite good. Surgical resection is the first line therapy if the lesion is accessible. Total resection is often possible in the posterior fossa lesions which can yield a 10-year survival rate is as high as 90%. No adjuvant therapy is utilized in that scenario. Even without total resection the 10-year survival rate is still as high as 45%. Radiation therapy had been employed for incomplete resections, but it remains controversial due to risk of brain radiation. Stereotactic radiation is now more commonly used and has shown some promising results. Chemotherapy is reserved for the rare pilocytic astrocytoma that continues to grow post radiation therapy.

Frontoethmoid encephalocele

Findings

Figure 1: Sagittal MR image demonstrates frontal lobe tissue protruding through a defect in the anterior calvarium. A portion of the umbilical cord is seen wrapping around the fetus’s neck.
Figure 2: Axial MR image demonstrates a soft tissue mass protruding between the orbits, with hypertelorism and bilateral hydrocephalus.


Diagnosis: Frontoethmoid encephalocele


A frontoethmoid encephalocele is herniation of intracranial contents through a defect of the skull in the nasofrontal region (1). These are usually detected first on prenatal screening or diagnostic US performed for further evaluation of prenatal screening lab abnormalities, usually elevated maternal serum alpha-fetoprotein levels (2). Sonographic clues include hypertelorism and a facial mass. The best diagnostic clue is visualizing brain parenchyma herniating through an anterior skull defect. Prenatal MR can be used to better delineate the anatomy in a suspected encephalocele, and can better determine size, location, and communication with intracranial structures (1).

Small encephaloceles can be missed on prenatal ultrasound. Other imaging clues include hypertelorism and other associated anomalies which include dysgenesis or absence of the corpus callosum, heterotopia, microcephaly, hydrocephalus, microophthalmos, beaked tectal plate, flattened basiocciput and an interhemispheric lipoma (2). In the postnatal period, they can present as a skin-covered facial or nasal mass (3).

The pathogenesis of an encephalocele is a late neurulation defect during the fourth gestational week. There is a disturbance of separation of the surface ectoderm and neuroectoderm in the midline just following neural fold closure (3). As a result there is failure of the fonticulus frontalis to close with herniation of intracranial parenchyma through persistent embryologic relationships. For unclear reasons, occipital lesions are most common (80%) in North America and Europe (1:35,000 live births) and frontoethmoidal lesions are more common in Southeast Asia (1:5000 lives births) (3).

Prognosis depends on the presence of other associated congenital or developmental brain anomalies. Prognosis is often better with frontoethmoidal lesions than occipital or parietal lesions. An overall mortality of 7-20% has been seen, with a favorable developmental outcome (3). Treatment to prevent infection and further herniation of intracranial contents involves excision of the dysplastic brain tissue with watertight closure of the dura and reconstruction of the skull defect (3). Most of the time, the glial tissue can be resected without causing additional neurological deficits. The exception is in the case of transphenoidal encephaloceles which can contain pituitary gland, hypothalamus or part of the Circle of Willis (3). Hydrocephalus should be treated first (3).