Benign perimesencephalic SAH

Findings

Figure 1, Figure 2, and Figure 3: Axial CT images of the brain demonstrate SAH in the premedullary, prepontine, suprasellar, and interpeduncular cisterns.
Other figures (not shown): Representative images from a 4-vessel cerebral angiogram demonstrate no evidence of aneurysm or vascular malformation.


Diagnosis: Benign perimesencephalic SAH


Trauma and aneurysm are the two most common causes of SAH. At least 80% of cases of atraumatic SAH are caused by rupture of an intracranial aneurysm. When SAH is present, many clinicians request CT or MR angiography in order to quickly and non-invasively diagnose aneurysm. If an aneurysm is not detected with one of these modalities, conventional cerebral angiography (the gold standard for exclusion of aneurysm) is necessary. If the initial angiogram is negative, a second cerebral angiogram, typically performed 1-3 weeks after the first, is mandatory. This is because occasionally an aneurysm will be missed on the initial angiogram due to spasm or partial/complete thrombosis. The diagnosis of non-aneurysmal SAH can be applied to patients who have two consecutive negative technically adequate 4-vessel cerebral angiograms. Additionally, many clinicians request MRI of the spine to exclude the possibility of spinal AVM as a source for SAH.

The classic variety of non-aneurysmal SAH is known as benign perimesencephalic SAH or pretruncal nonaneurysmal SAH. As the name implies, the hemorrhage is situated around the midbrain and anterior to the brainstem in the ambient, interpeduncular, and prepontine cisterns. The term “benign” refers to the fact that after recovery from the initial episode, there is no increased risk of repeat hemorrhage. Cerebral vasospasm is less likely in these patients, but does occur. Hydrocephalus also remains a possibility during the acute phase. Although not clearly understood, one proposed mechanism of benign perimesencephalic SAH is rupture of the venous plexus anterior to the pons (the anterior pontomesencephalic plexus). This is postulated to occur as a result of increased venous pressure from strenuous activities such as exercise. Intramural hematoma of the basilar artery and rupture of a basilar perforating artery have also been suggested as alternate hypotheses.

Although benign perimesencephalic SAH has been known as a distinct clinical entity for some time, patients may present with non-aneurysmal SAH in an atypical distribution (non-perimesencephalic). In some of these patients, the total volume of hemorrhage is increased such that blood is present throughout the basal cisterns and extends over the cerebral convexities. In other patients, the hemorrhage is confined to the convexities, quadrigeminal cistern, or other atypical locations. In today’s case, Patient #1 presented with the classic variety of benign perimesencephalic SAH. Patient #2 presented with atypical non-aneurysmal SAH. Both patients recovered, and have had no repeat episodes of hemorrhage to date.

Possible causes of SAH:
- Trauma
- Aneurysm
- AVM
- Vasculitis
- Dural AV fistula
- Extension from intraparenchymal hemorrhage
- Dural venous sinus thrombosis
- Infection
- Neoplasm
- Idiopathic

Lumbar disc extrusion with a wrapped disc

Findings

There is a left central disc extrusion at L5-S1 that causes mild to moderate left lateral recess narrowing and nerve root displacement without nerve root compression. At this level there is also contrast enhancement traversing the left laminectomy defect and encasing the disc extrusion, consistent with a wrapped disc. There is enhancement in the left lateral recess, suggesting post-operative fibrosis.

Differential diagnosis:
- Wrapped disc
- Peridural fibrosis
- Epidural abscess
- Epidural metastasis
- Nerve sheath tumor
- Disc pseudobulge
- Intervertebral disc protrusion
- Intervertebral disc extrusion
- Recurrent intervertebral disc herniation


Diagnosis: Lumbar disc extrusion with a wrapped disc


Key points: "Wrapped" disc


Disc herniation (protrusion, extrusion, or fragment) may be caused by trauma, repetitive or acute, and are a common source of pain and subsequent back surgery in the general population. In the acute phase, the herniated disc stimulates a fibrovascular response. A "wrapped disc" is the focal herniation (protrusion, extrusion, or fragment) that is encased in vascular scar tissue stimulated by this response and is evident by enhancement on contrast-enhanced T1-weighted images.

Asymptomatic or low back pain and/or radiculopathy are most common in the lumbar spine at L4-L5 and L5-S1. A wrapped disc is a post-surgical sequela, particularly following surgery for spinal stenosis in which the surgical procedure is more extensive, involving a laminectomy and a medial facetectomy.

Best imaging modality: MR (sequences: sagittal and axial T2WI and T1WI, as well as contrast-enhanced axial and sagittal T1WI)
Other imaging modalities: CT, myelography


Imaging findings

MR: Anterior extradural mass contiguous with the disc space extending into the spinal canal
*Contrast-enhanced T1WI: Peripheral enhancement surrounding the disc herniation or fragment with/without central canal, lateral recess, or foraminal stenosis and cord or nerve root impingement. (*most helpful MR sequence)
Non-enhanced T1WI: Isointense to parent disc
T2WI: Iso- to hyper intense to parent disc
General disc hypointensity and height loss at the level of the herniation, as well as postoperative changes (laminectomy defects, etc), degenerative facet disease, and osteophytes, are common associated findings.
CT:
Non-contrast CT: An anterior extradural soft tissue mass that may displace the nerve root / indent the thecal sac
Contrast-enhanced CT: Mild peripheral enhancement of the disc herniation/fragment
Myelography: An extradural mass that indents the thecal sac and nerve root sleeves
Imaging findings of other common differential diagnoses
Peridural fibrosis: Scar within epidural space after lumbar surgery that infiltrates epidural fat, causing homogeneous enhancement that diffusely surrounds the thecal sac and nerve root; increased in T2 signal relative to adjacent disc herniation
Epidural abscess: A distinct fluid collection in the epidural space with peripheral enhancement on post-contrast images, often associated with findings of diskitis
Epidural metastasis: Elongated (cranial-caudal orientation) enhancing mass with osseous involvement and may demonstrate paravertebral extension
Nerve sheath tumor: Avid enhancement surrounding the nerve root, some of which are in a "dumbbell" shape
Disc pseudobulge: Smooth generalized extension of the disc margin without a focal defect due to "uncovering" of disc related to spondylolisthesis
Intervertebral disc protrusion: Anterior extradural mass contiguous with disc space and triangular in shape with broader base than apex; no enhancement
Intervertebral disc extrusion: Anterior extradural mass contiguous with disc space by a "neck," in which this herniated disc material then widens in the epidural space
Recurrent intervertebral disc herniation: Extradural mass contiguous with intervertebral disc margin, demonstrating enhancement peripherally but without central enhancement
Treatment
Conservative: Anti-inflammatory and pain medications, avoid trauma
Surgical: Repeat surgery to remove herniated disc (protrusion, extrusion, fragment)

Pseudotumor cerebri - Idiopathic Intracranial Hypertension (IIH)

Findings

T2W axial MRI (Figure 1) shows signs of increased ICP, but only increased fluid within the optic nerve sheaths, flattening of the posterior orbit, and a partially empty sella.
The 3D TOF MRV Towne and RPO projections (Figure 2 and Figure 3) show bilateral, right greater than left, focal transverse-sigmoid venous sinus junction narrowing’s. It is not a normal MRV given the pt’s history, with more explanation in the discussion. There is no aneurysm or collection of collateral blood vessels seen in these images.

The AP and lateral (Figure 4 and Figure 5) venous phase carotid arteriogram shows long segment stenosis at transverse-sigmoid venous sinus junction distal to the vein of Labbé. Pre procedure venography showed a venous pressure gradient across this lesion of 17 mmH2O with 37 mmH2O on transverse sinus side and 15 mmH2O on internal jugular vein side.

AP and lateral (Figure 6 and Figure 7) venous phase carotid arteriogram shows long segment stenosis at transverse-sigmoid venous sinus junction with a balloon crossing the gradient lesion.


Diagnosis: Pseudotumor cerebri - Idiopathic Intracranial Hypertension (IIH)


Pseudotumor cerebri is defined by typical clinical symptoms which occur in the setting of elevated “idiopathic” ICP and a normal composition of CSF. Classic clinical symptoms include diffuse recalcitrant headaches, vision changes (including vision loss), and hearing changes (e.g., tinnitus), and the disease is typically seen in obese women who are 20-50 years of age. Papilledema is the most common physical exam finding, but visual loss and sixth nerve palsy are also seen. Other symptoms include disabling headaches and blindness. LP opening pressure is greater than 25 cm H2O. Brain computed tomography (CT) and magnetic resonance imaging (MRI) are typically normal, however, the following suggestive non-pathognomonic findings are frequently present:

– Cerebral venous sinus stenoses
– Flattening of the bilateral posterior sclera
– Partially or fully empty sella; enlargement of the chiasmatic recess of the 3rd ventricle
– Distension of perioptic nerve subarachnoid space
– Intraocular protrusion of the optic nerve head
– Orbital optic nerve vertical tortuosity

Treatment for pseudotumor cerebri typically includes medical management with acetazolamide and pain control for headaches. Furosemide and corticosteroids have been used, as well. Surgical interventions to treat pseudotumor cerebri include lumboperitoneal shunt (LPS) and ventriculoperitoneal shunt (VPS), which often produce immediate results, however, eventual return of pseudotumor symptoms occur in approximately 50% within three years. Optic nerve sheath fenestration is also used to treat vision changes, with variable headache relief. Dominant transverse/sigmoid venous sinus angioplasty and stenting are relatively new methods for the treatment of pseudotumor cerebri for those who have significant dural sinus stenosis. Given that 80% of intracranial vascular compliance is provided from the venous vasculature, reduction of pressure in the sinuses reduces CSF pressure. Better results are achieved in patients with documented high pressure gradients, and greater efficacy is seen with regard to arrest of visual loss (>90%) than with headache relief (~50%). Long-term results are lacking. however.

In this case, cerebral angiography demonstrated bilateral high-grade transverse/sigmoid sinus stenoses distal to vein of Labbe insertions. Selective catheterization of the right transverse sinus revealed an estimated 80% narrowing to a luminal diameter of 1mm, and a pressure gradient across the stenosis of 13 mmHg (normal <5 mmHg). The contralateral sinus was smaller, but distally stenotic. A stent was placed across the right sided stenosis.
The patient was placed on antiplatelet medication to preserve stent patency immediately after the procedure. She had no headaches after the procedure and demonstrated objective visual improvement at her one- and six-week follow-up examinations.

Intracranial pseudotumor (Tolosa-Hunt syndrome)

Findings

Increased CSF signal demonstrated by T2 hyper intensity within the right optic sheath. Nodular enhancement at the superior aspect of the right optic nerve at the orbital apex, which extends with prominent nodular enhancement posteriorly along the dural reflection of the right cavernous sinus. Asymmetric dilatation of the right superior ophthalmic vein.

Differential diagnosis:
- Meningitis
- Sarcoidosis
- En plaque meningioma
- Meningeal metastases
- Meningeal Non-Hodgkin's lymphoma
- Tolosa-Hunt syndrome


Diagnosis: Intracranial pseudotumor (Tolosa-Hunt syndrome)


Discussion

Intracranial pseudotumor (Tolosa-Hunt syndrome) is a part of a spectrum of "quasineoplastic" lesions that demonstrate orbital, intracranial, or pulmonary involvement, and include such other disorders as plasma cell granuloma and hypertrophic cranial pachymeningitis. It is a chronic granulomatous disease of unknown origin, which has been hypothesized to represent a low grade fibrosarcoma of inflammatory cells versus an autoimmune phenomenon. While Tolosa-Hunt syndrome is rare, orbital pseudotumor is the third most common ophthalmic disorder, and encompasses 5-8% of all orbital masses. Histologically, the lesions of Tolosa-Hunt demonstrate mixed lymphocytic and plasma cell infiltrate, with a variable degree of fibrosis. Lesions favor the cavernous sinus and basal meninges, although falcine and tentorial lesions have been described.

Patients with intracranial lesions are more frequently young adults, who present initially with chronic headaches or cranial nerve palsies. Patients with orbital involvement are more frequently middle-aged, and may present with painful proptosis and vision loss . Symptoms may be intractable, leading to severe disability. Untreated or unresponsive disease may progress to death. First-line treatment is invariably high-dose steroids, with radiotherapy or surgical resection reserved for patients with incomplete response to steroids .


Radiologic Overview of the diagnosis

The imaging hallmarks of Tolosa-Hunt syndrome are characterized by an enhancing, infiltrating meningeal mass, which favors the cavernous sinus or basal meninges, although falcine and tentorial involvement has been described . Focal meningeal thickening may range from just a few millimeters to a greater than 2 cm rind. With intracranial pseudotumor, orbital involvement is spared more than 90% of the time. Tolosa-Hunt remains a diagnosis of exclusion, once meningitis, en plaque meningioma, and meningeal metastases are ruled out.

The imaging modality of choice for imaging patients with Tolosa-Hunt remains MRI, although useful information may be gleaned from other modalities. On non-contrast enhanced CT (NECT), there are no specific findings to suggest the diagnosis; however, this modality may be of some value in differentiating the lesion from en plaque meningioma. On contrast enhanced CT (CECT), salient imaging findings include enhancing, thickened meninges or a curvilinear appearance of a single meningeal region. As aforementioned, MRI remains the primary modality for diagnosis of Tolosa-Hunt syndrome, and each sequence may provide critical information required to make the diagnosis. On T1WI, one may find focal thickening of the meninges that is isointense to gray matter. On T2WI, lesions are characterized as iso- to hypo intense regions of focal meningeal thickening, which may be more hypo intense as they become more fibrotic. FLAIR is of little help in making the diagnosis, but it is unlikely to demonstrate focal brain edema underlying the lesion . Contrast enhanced T1WI is the single most valuable sequence for evaluation of Tolosa-Hunt syndrome, and is characterized by diffusely enhancing region of meningeal thickening, which may range from a few millimeters to greater than 2 cm in some cases. Diffuse boney infiltrates may be appreciated on fat saturated contrast enhanced T1 sequence. While angiography is not considered a primary modality, severe disease may result in carotid artery narrowing, thus MRA may be a useful adjunct in the appropriate clinical setting.

The appropriate differential diagnosis of Tolosa-Hunt syndrome includes meningitis, sarcoidosis, en plaque meningioma, meningeal metastases, and meningeal Non-Hodgkin's lymphoma.

Hypertensive intracranial hemorrhage

Findings

Axial CT of the head shows a large hyper dense focus with peripheral hypo density in the left frontal lobe, causing sulcal effacement at the frontal cortex but no significant midline shift. Surrounding rim of low density represents edema (image 1). Unenhanced MRI of the brain shows an the same mass-like focus in the left frontal lobe, which has an isointense center with a hyper intense rim (image 2). Post gadolinium-enhanced T1 image of the brain shows no internal enhancement of this lesion (image 3).


Differential diagnosis:
- Hypertensive intracranial hemorrhage
- Ruptured arteriovenous malformation (AVM)
- Hemorrhagic intracranial mass
- Posttraumatic cerebral contusion


Diagnosis: Hypertensive intracranial hemorrhage


Acute blood appears hyper dense of unenhanced head CT.
Without a history of trauma, intraparenchymal brain hemorrhage on head CT could represent a hemorrhagic mass, a ruptured AVM, or a hemorrhagic brain tumor (primary or metastatic).
MRI with and without contrast is the best diagnostic tool for determining if a mass lesion is present, and for evaluating the age of the intracranial hemorrhage.
Acute blood products on T1 appear hypo intense to isointense (image 2), whereas subacute and chronic hematoma are hyper intense.
Neoplasms should enhance on post contrast T1. The lack of enhancement in this patient on post contrast T1 excludes neoplasm (image 3).

Sequestered disk

Findings

There is an non- enhancing ovoid mass slightly hyper intense to muscle on both T1 and T2 sequences, in the anterior epidural space at the L3 level, measuring approximatelyl 12 x 8 x 12 mm. This is not contiguous with any adjacent disks. No signal dropout on fat-saturated sequences. The mass causes severe stenosis of the left half of the spinal canal at the L3 level, compressing the left descending nerve roots. T1 and T2 hyper intensity at the endplates abutting L2-L3 disc space representing Modic Type II changes. There is intervertebral disk space height loss at L2-L3 with severe disk desiccation changes.


Differential diagnosis:
- Sequestered disk
- Extruded disk
- Failed back surgery
- Epidermoid
- Epidural abscess
- Epidural hematoma
- Lipoma


Diagnosis: Sequestered disk


A focal disk protrusion is an extension of intervertebral disc material (nucleus pulposus) beyond the vertebral margin (AP diameter < mediolateral diameter). An extruded disk is one in which the nucleus pulposus has herniated through a rent in the annulus fibrosis. The AP diameter > ML diameter, and the disk may migrate craniocaudally, but maintains attachment to the parent disk (frequently symptomatic).
When extruded disk material loses its attachment to the parent disk, it is referred to as a sequestered disk. Sequestered discs usually lodge in the anterior epidural space (AES), just anterior to the posterior longitudinal ligament, and migrate either cephalad or caudad (with equal frequency). Because there is a midline septum associated with the PLL in the AES, the fragment is usually just off midline (to the right or left). Rarely, the sequestered fragment may migrate beyond the PLL into the posterior epidural space, through the dural ( intrathecal location), or into the paraspinal muscles.
They usually resemble the parent disk on MR, with T1 hypo intense and T2 iso- / hypo intense. There may be surrounding T2 hyper intensity and a rim of enhancement from inflammatory changes.
This is a crucial diagnosis to make, as a sequestered disk is a contraindication to limited disk procedures (e.g. Percutaneous discectomy) and may result in failed back surgery.

Myxopapillary ependymoma

Findings

Figure 1: Sagittal T1-weighted images reveals an isointense lobulated intradural mass at the level of the conus medullaris.
Figure 2: Sagittal T2-weighted images shows a hyperintense lobulated intradural mass extending from T11 through L2 with numerous small flow voids.
Figure 3: Sagittal T1 post-contrast images demonstrates intense enhancement of the intradural mass centered around the conus.


Diagnosis: Myxopapillary ependymoma


Myxopapillary ependymoma is a slow-growing tumor arising from the ependymal cells of the filum terminale. These tumors compromise 13% of all spinal ependymomas, and they occur almost exclusively in the conus, filum terminale, and cauda equina although extradural occurence in the sacrum and presacral region has also been reported.

The lesions tend to span two to four vertebral segments, and appear as a well-circumscribed intradural masses. In most cases the tumor is intrinsic to the conus medullaris but this is often difficult to recognize on imaging as the bulk of the mass is extramedullary. Typical MR characteristics include T1 isointensity, T2 hyperintensity, and avid enhancement on post-contrast images. As these tumors are prone to hemorrhage, a hypointensity at the tumor margin is often seen indicative of hemosiderin. Calcification and cyst formation within the mass are not infrequent.

On radiography and CT, vertebral changes can be seen which include widened interpediculate distance, thinned pedicles, posterior vertebral scalloping, and intervertebral foraminal widening due to tumor extension.

They are more common in males (M:F=2:1) with a mean age of 35 at diagnosis. Clinically, they present with back pain, paraparesis, radiculopathy, and occasionally bowel and bladder dysfunction. Because these symptoms can mimic those of disc herniation, there is often a delay in diagnosis. Treatment consists of surgical resection, and the prognosis is excellent with complete resection. Leptomeningeal seeding metastasis in myxopapillary variety is not as frequent as it is in classic spinal cord ependymomas and associated with poorer prognosis when present. Radiotheraphy after surgery improves outcome.

Wernicke’s Encephalopathy

Findings

On axial images, abnormal FLAIR signal is demonstrated at the pontomedullary junction adjoining the fourth ventricle, periaqueductal gray matter in the pons and midbrain (Figure 1), the superior aspect of the mamillary bodies (Figure 2), the tissue surrounding the third ventricle and the medial thalami (Figure 3).
On coronal slices, abnormal FLAIR signal again appears in the mamillary bodies (Figure 8), in the tissue surrounding the third ventricle (Figure 8 and Figure 10), medial thalami (Figure 10), and periaqueductal gray matter (Figure 11).


Diagnosis: Wernicke’s Encephalopathy


Wernicke’s encephalopathy is caused by thiamine deficiency, most often seen in chronic alcohol abuse. It has also been described in anorexia nervosa, prolonged starvation, hyperemesis gravidarum, patients on long-term hemodialysis, and patients with AIDS. Patients with this condition classically present with the triad of ataxia, acute mental confusion, and oculomotor dysfunction, although a minority (16-38%) of patients with the condition present with all three elements. If the symptoms also include amnesia and confabulation, then these manifestations are called Korsakoff syndrome. Wernicke’s encephalopathy is a significantly disabling and potentially lethal condition that can be prevented and reversed if treated early with thiamine supplementation.

On CT and MR imaging the brain demonstrates diffuse cerebral and cerebellar atrophy. Mamillary body enhancement or abnormal T2 signal may be the sole manifestation of Wernicke’s encephalopathy. Other typical MR findings include symmetric high T2 signal and variable enhancement within the periaqueductal gray matter of the midbrain, the tectal plate, the mamillothalamic tract, the thalami, and the tissue surrounding the third ventricle. The mamillary bodies may also show atrophy in patients with chronic Wernicke’s encephalopathy, though this finding can also be present in chronic alcoholic patients without Wernicke’s syndrome. Atypical changes may also be seen, almost always in non-alcoholic patients, and may include signal changes in cranial nerve nuclei, basal ganglia, cerebellum and dentate nuclei, the splenium, and frontal and parietal cortex. These atypical findings are very similar to the pattern seen in metronidazole-induced encephalopathy, and it is has been hypothesized that the two syndromes share a common metabolic pathway. The reason why these brain regions are more affected by thiamine deficiency is poorly understood, but it is speculated that they may be characterized by more intense thiamine metabolism.

Capillary Telangiectasia

Findings

There is an ill-defined enhancing focus in the medial right temporal lobe on post gadolinium contrast T1-weighted imaging (Figure 4). There is no corresponding signal abnormality or mass on the precontrast T1-weighted, T2-weighted, or FLAIR images (Figure 1, Figure 2, and Figure 3, respectively). There is no mass effect. On susceptibility-weighted imaging (SWI) the lesion shows hypointensity (Figure 5).


Diagnosis: Capillary Telangiectasia


Brain capillary telangiectasias are benign vascular malformations which are often found incidentally.
They can be visualized by gadolinium contrast and gradient-echo susceptibility or susceptibility weighted imaging, but not through catheter angiography, and may often not be visible on conventional T1/T2, FLAIR, or diffusion-weighted imaging.
Often asymptomatic and usually no treatment is required.

Brain capillary telangiectasias (BCTs) are one of four major types of vascular malformations which occur in the brain (the other three are arteriovenous malformations, cavernous malformations (cavernous angiomas), and developmental venous anomalies (venous angiomas), and represent up to 20% of all intracranial vascular lesions. BCTs consist of multiple ectatic capillaries surrounded by normal brain parenchyma and are usually devoid of calcification, gliosis, extraluminal hemorrhage, and hemosiderin-laden macrophages. BCTs are most common in the midbrain, pons, medulla, and spinal cord, but they are found throughout the central nervous system. Multiple BCTs are possible, especially in certain syndromes (e.g.; ataxia telangiectasia, Osler-Weber-Rendu, or Sturge-Weber syndrome).

Often found incidentally, BCTs are usually benign, small in size, and rarely grow over time. They are rarely symptomatic and are not associated with any particular clinical feature but have been reported to be associated with headache, vertigo, and tinnitus.

BCTs are relatively well visualized through susceptibility weighted imaging where they demonstrate marked signal intensity loss due to deoxyhemoglobin present in slow flowing blood. They are also well visualized through gadolinium-enhanced T1-weighted imaging sequences where they are seen as small faint lesions. BCTs are difficult to visualize through conventional T1/T2, FLAIR, or diffusion-weighted imaging and are considered to be one of the “angiographically occult vascular malformations” due to their small size, tendency to occlude, and sluggish flow.

Methotrexate neurotoxicity

Additional clinical history: Patient was diagnosed with acute lymphocytic leukemia 2 months previously. He is status post induction therapy with a negative bone marrow biopsy, and is currently receiving consolidation chemotherapy with methotrexate, and presents with right upper extremity weakness.


Findings

MR images of the brain demonstrate a focal area of diffusion restriction involving the left frontoparietal white matter. There is minimal associated T2/FLAIR hyperintensity. No associated enhancement. Remainder of the brain was within normal limits.
Imaging done four months later shows lesion has nearly resolved.


Diagnosis: Methotrexate neurotoxicity


Discussion

Methotrexate is a folic acid analogue. Its cytotoxic effects are carried out through inhibition of the enzyme dihydrofolate reductase, which reduces tetrahydrofolic acid levels, ultimately inhibiting cell division.

From bone marrow cell precursors to the quickly dividing cells of the intestinal tract, methotrexate exerts its effects on all dividing cells in the body. One of its rare side effects is CNS toxicity. The decreased folate levels achieved with methotrexate have implications on metabolism of adenosine, homocysteine, and biopterin. Low folate levels lead to a subsequent decrease in S-adenosyl-methionine(SAM) concentrations. This eventually leads to chronic demyelination and neurologic symptoms.

An additional side effect of MTX is the elevated levels of adenosine in the CSF. Adenosine is a vasodilator, which causes dilatation of cerebral vasculature resulting in neurotoxicity. The increased homocysteine levels caused by MTX have been shown to damage vascular endothelium and lead to subsequent strokes and thromboemboli. Methotrexate has also been found to cause cytotoxic edema, which is the most common cause of lesions that enhance on MRI DWI.

The neurotoxicity caused by MTX can be immediate, acute to subacute, or delayed. Symptoms of the disease can range from headache, nausea, vomiting, and fever, to transient or permanent focal neurologic symptoms. The immediate form occurs within a day of MTX administration and presents as a chemical meningitis. The acute to subacute form presents from days to weeks after administration of MTX, and presents with seizures or focal neurologic symptoms. The delayed form presents as leukoencephalopathy and a generalized decrease in higher cognitive function.


Radiological findings

A case series containing nine cases of MTX neurotoxicity revealed that lesions found in this disease tend to be focal and show up on DWI as well as T2 and FLAIR imaging. These abnormalities can continue to persist on imaging long after the symptoms have resolved. The DWI shows diffusion restriction with T2/FLAIR hyperintensity being less conspicuous.

In another independent case study on MTX neurotoxicity, MRI demonstrated restriction diffusion with no significant T2 or FLAIR signal abnormality. Based on a combination of these imaging findings, it was determined that cytotoxic edema was likely the cause of focal neurologic symptoms on the patient, and demyelination was a less likely cause based on the MRI findings.

A different case study had MRI findings showing subtle signal changes in the left centrum semiovale, with an obviously abnormal area of restricted diffusion, indicating the presence of increased fluid. The authors of this case also mentioned a relation between elevated choline levels in lesion areas with myelin breakdown.

The lesion in this disease is similar in appearance to ischemic stroke, but differs in distribution. The lesions in MTX neurotoxicity can show up in many different patterns, whereas ischemic strokes often follow a vascular distribution, helping differentiate the two.


Radiology

MRI:
MRI with DWI is the gold standard for diagnosis
Will show focal areas of demyelination and/or edema throughout the brain
Can be normal, even in the presence of symptoms
Must perform early to avoid unnecessary workup


CT:
Can be used to rule out other etiologies that may cause focal symptoms, but is not a sensitive test for demyelination and edema found with MTX neurotoxicity
Ultimately need MRI to make diagnosis as CT is often negative
Angiography
Not very useful as it is usually normal

Basilar invagination secondary to rheumatoid arthritis

Findings

Axial and sagittal CT images demonstrate severe basilar invagination (Figure 1). The tip of the odontoid process measures 2.3 cm above Chamberlain’s line (yellow line in Figure 2). McGregor's line (red line in Figure 2) is also shown. Incidentally noted are right-sided opacified mastoid air cells (Figure 1).
Once again, severe basilar invagination is evident. On the sagittal T2 image the foramen magnum is narrowed and obliteration of the CSF space is noted at the C2-C3 level (Figure 3). On the axial T2 weighted image increased T2 signal (Figure 4) is seen within the cord at the C2-C3 level indicating edema versus myelomalacia.



Diagnosis: Basilar invagination (impression) secondary to rheumatoid arthritis.


Basilar invagination refers to a condition in which the odontoid process protrudes upward into the intracranial space. Basilar invagination may be classified as primary (congenital) or secondary (acquired). Down syndrome, Klippel-Feil syndrome and Chiari malformations are congenital causes of basilar invagination. Acquired basilar invagination, also known as basilar impression, is associated with softening of the skull base and is often due to rheumatoid arthritis, Paget disease, osteomalacia, hyperparathyroidism and osteogenesis imperfecta. Basilar invagination is probably better described as a radiologic finding rather than a diagnosis. Once the finding is identified, a cause of basilar invagination should be diligently pursued.

Plain lateral radiographs with odontoid views, although not 100% sensitive, are often the initial study used to diagnose basilar invagination. MRI is the optimal study, which also assesses the cervicomedullary junction and cervical cord. Two craniovertebral junction lines are particularly useful in defining basilar invagination. Chamberlain’s line extends between the posterior pole of the hard palate and the posterior edge of the foramen magnum (opisthion). If the dens is >3.0 mm above this line basilar invagination is present. McGregor’s line, a modification of Chamberlain’s line was developed because the opisthion could not always be seen on plain radiographs. This line extends from the posterior pole of the hard palate to the undersurface of the occiput. If the dens extends >4.5 mm above this line basilar invagination is present.

Clinical manifestations of basilar invagination include posterior skull pain, headache, signs and symptoms of brainstem and upper cervical cord compression or disturbances of CSF circulation causing obstructive hydrocephalus. The brainstem may be compressed at the level of the foramen magnum possibly resulting in compromise of the autonomic centers resulting in labile blood pressures, arrhythmias, or sudden death. Neurosurgery is recommended in patients that are symptomatic with concomitant MRI findings indicating compression. Although asymptomatic patients are often followed conservatively, many authors favor surgery even if no symptoms of cord compression are evident in rheumatoid patients.

Although often appearing together, basilar invagination or impression should not be confused with platybasia; which literally means “flattening of the base of the skull”. Platybasia, which can be seen in Klippel-Feil anomalies, cleidocranial dysplasia and achondroplasia, is present when the basal angle formed by intersecting lines from the nasion to the tuberculum sellae and from the tuberculum along the clivus to the anterior aspect of the foramen magnum (basion) is greater than 143 degrees.