Aberrant cervical thymus

Findings

Ultrasound demonstrates unique linear echogenic septa and multiple discrete echogenic foci characteristic for thymic tissue.
T1-weighted MRI demonstrates low signal intensity slightly greater than muscle.
T2-weighted MRI demonstrates high signal intensity slightly lower than that of fat but greater than muscle.
Coronal T2-weighted image identifying normal thymus in its expected location in the anterior mediastinum, as well as, the aberrant thymus in the right neck. Note that the aberrant and native thymus demonstrate the same signal characteristics on MRI.


Diagnosis: Aberrant cervical thymus

Aberrant or ectopic thymus is an uncommon entity that may be encountered in early childhood. Recognition of this entity can obviate unnecessary surgery. Aberrant and most cases of ectopic thymus do not require therapy or surgery. Ultrasound is the initial imaging modality of choice for diagnosis.
The thymus is a lymphoepithelial gland that functions in T-cell lymphopoiesis and is important for immune system development. Development usually begins in the 6th gestational week from the third and fourth pharyngeal pouches. The tissue descends along a path from the angle of the mandible to the thoracic inlet, and reaches the anterior mediastinum by the 12th week.
The thymus is typically located in the anterior mediastinum immediately posterior to the sternum and anterior to the great vessels. Each of the two lobes of the thymus is divided by primary connective tissue septa carrying blood vessels to parenchymal lobules, which are composed of a cortex and medulla, both of which are developed by the 12th week. The lobules are further divided by secondary connective tissue septa carrying blood vessels from the surface of the cortex to the corticomedullary boundary. The cortex contains only capillaries, whereas, the medulla has both small arteries and arterioles. The thymus increases in size until puberty, where it can range from 30 to 40 grams. After puberty, there is gradual fatty involution that replaces the lymphoid components.
Abnormally located thymic tissue can be categorized as being aberrant or ectopic. Patients typically present clinically with a painless, nontender mass. Aberrant thymic tissue is found along the expected normal path of descent to the anterior mediastinum from the angle of the mandible to the thoracic inlet. Aberrant thymus is commonly located in the lateral neck or in the suprasternal region, and is usually an asymptomatic, incidental finding. Ectopic thymic tissue can be found in any other location, such as the pharynx, trachea, posterior neck or mediastinum, and esophagus. Ectopic thymus can occasionally be life-threatening, such as with airway obstruction.
Ultrasound is the initial imaging modality of choice. It does not require sedation such as with MR and CT; does not involve ionizing radiation such as with CT; and does not utilize contrast media such as with MR and CT. Thymus has been found to have a characteristic and unique appearance demonstrating linear echogenic septa and multiple discrete echogenic foci throughout the gland, believed to represent connective tissue septa and their associated vasculature. The cortex of the lobules is typically hypoechoic versus the more echogenic medulla. The unique ultrasound pattern allows for easy differentiation of thymus from liver, spleen, and thyroid. In cases of thymic tissue near the mandible, one can also differentiate thymus from salivary glands (including parotid and submandibular) that are typically more homogeneous with fine internal echoes.
Occasionally MRI may be obtained for further evaluation. Normal thymic tissue in children, regardless of location, is typically homogeneously low-signal intensity on T1-weighted sequences (slightly greater than muscle) and high-signal intensity on T2-weighted sequences (slightly less or equal to fat) owing to the higher water content of the thymus. The appearance of the thymus may vary depending on age, and may also be completely or partially cystic. MRI can be useful to compare the signal characteristics of normal and abnormally positioned thymic tissue, as well as, in confirming the presence of normal thymus.
Recognition of aberrant or ectopic thymus is important because these entities usually do not require therapy or surgery, except in circumstances where there is evidence of airway obstruction. The thymus is important for immune development. Ultrasound can be used to confidently identify tissue as being of thymic origin. It is important to identify the normal thymus as well because the aberrant or ectopic thymus may be the only functioning thymic tissue within the patient. MR and/or CT may be used to supplement ultrasound findings.

Heroin induced leukoencephalopathy

Findings

Figure 1 and Figure 2: Axial T2 weighted images demonstrate diffuse high signal throughout the supratentorial white matter involving the centrum semiovale and corona radiata. There is sparing of gray matter and subcortical U-fibers. There is focal high signal in the peripheral left frontal region which corresponded to an area of acute infarct on diffusion weighted images and ADC map.
Figure 3 and Figure 4: Axial FLAIR images demonstrate diffuse high signal in the centrum semiovale and internal capsule extending to the frontal and parietal lobes with sparing of the basal ganglia and thalami. There is focal high signal in the peripheral left frontal region which corresponded to an area of acute infarct on diffusion weighted images and ADC map.


Diagnosis: Heroin induced leukoencephalopathy ("Chasing the Dragon")


“Chasing the dragon” also known as “chinesing” or “Chinese blowing” is a practice which involves inhaling the vapor of liquefied heroin. A small amount of white powder is placed on a piece of foil which is then heated from below. A stream of vapor, which looks like a dragon’s tail, arises from the molten heroin (the “dragon”) and the drug user “chases” it with a pipe or straw. This practice has gained popularity because one forgoes the risks associated with IV drug use while still benefiting from a rapid rate of absorption and onset of effect of the drug.

Heroin induced spongiform leukoencephalopathy is a progressive disease that was first described in the Netherlands in 1982. The heroin used is often impure with many additives and there is speculation that one of these additives becomes activated when heated and is the cause of the leukoencephalopathy. Since the first reported cases, many substances in the pyrolysate have been studied as potentially leading to the leukoencephalopathy but none has been positively identified. The damage is irreversible and there is no cure for this disease. Treatment is supportive care although there is a questionable benefit to using coenzyme Q and vitamin supplements.

Heroin induced leukoencephalopathy is diagnosis often made clinically and should be suspected in patients with a history of "chasing the dragon". The natural course is variable and not well defined. There may be a latent period with a subclinical evolution of white matter degeneration. Additionally, it appears that patients with higher levels of exposures have more severe disease. Patients often present in one of three clinical stages. The first stage consists of cerebellar signs (such as ataxia), apathy and motor restlessness. The second stage is comprised of tremors or myoclonus, chorea and athetosis, and pyramidal tract signs. The third stage consists of hypotonic paresis, stretching spasms, central pyrexia, akintetic mutism, and death. Progression of the disease continues even after cessation of the toxin for up to 6 months.

On pathology, there is symmetric spongiform degeneration, specifically in the cerebral and cerebellar white matter as well as the corticospinal and solitary tracts. The MR images illustrate this distribution, showing symmetric high signal on both T2-weighted and FLAIR sequences in the white matter of the cerebellum and occipital, parietal, and temporal lobes with relative frontal sparing. Specifically, there is involvement of the cerebellum and the posterior limb of the internal capsule, with sparing of the anterior limb and subcortical white matter. FLAIR images can demonstrate regions of subtle white matter abnormality more reliably and are better at excluding gray matter involvement than the T2 weighted images. There can also be additional signal abnormality in the splenium of the corpus callosum, the corticospinal tracts, and the lemniscal pathway in the brainstem. The spinothalamic tracts are spared which help to distinguish heroin induced leukoencephalopathy from other potential causes of encephalopathy. MRS in patients with heroin leukoencephalopathy has shown abnormally elevated intracerebral lactate in the affected white matter as well as decreased levels of N-acetyl aspartate in the white matter, gray matter, and cerebellum.

Guillain-Barre syndrome (GBS)

Findings

When compared with pre-contrast T1 weighted sagittal images (Figure 3), post-gadolinium sagittal T1-weighted images demonstrate enhancement and thickening of the cauda equina (Figure 4).


Guillain-Barre syndrome (GBS)


Guillain Barre syndrome (GBS) is an acute inflammatory demyelinating polyneuropathy (AIDP). It is an autoimmune disease affecting the peripheral nervous system, usually triggered by an acute infectious process. It is frequently severe and usually presents as an ascending paralysis beginning as weakness in the legs that spreads to the upper limbs and the face along with complete loss of deep tendon reflexes. Hypo- or areflexia, autonomic dysfunction and cranial nerve involvement is commonly associated. It is generally preceded by an antecedent bacterial or viral infection. Nearly 40% of patients are seropositive for Campylobacter jejuni. Diagnosis is based on clincial signs and symptoms, lumbar puncture and electrophysiologic crtieria. CSF findings include elevated protein level without pleocytosis. Nerve conduction findings suggestive of GBS include nerve conduction slowing, conduction block, prolonged distal latency, and prolonged or absent F waves. Therapeutic options include supportive measures, plasmapharesis, and immunoglobulin.
Although ascending paralysis is the most common form of spread in GBS, other variants also exist. Miller Fisher syndrome (MFS) is a rare variant of GBS and manifests as a descending paralysis, proceeding in the reverse order of the more common form of GBS. It usually affects the ocular muscles first and presents as opthalmoplegia, ataxia and areflexia. Anti-GQ1b antibodies are found in 90% of cases. Acute motor axonal neuropathy (AMAN), also known as Chinese Paralytic Syndrome, attacks motor nodes of Ranvier and is prevalent in China and Mexico. The disease can be seasonal and recovery can be rapid. Anti-GD1a antibodies are often present. Acute motor sensory neuropathy (AMSAN) is similar to AMAN but also affects sensory nerves with severe axonal damage. Recovery can be incomplete.

Radiologic studies are typically performed in order to exclude other causes. MR of the spine is performed to define infiltrative etiologies such as tranverse myelitis and compressive causes of polyradiculopathy. In cases of equivocal CSF or nerve conduction findings or atypical clinical findings, MRI of the spine can confirm or exclude the diagnosis. Findings usually include thickening and enhancement of the nerve roots that surround the medullary cone and extend along the length of the cauda equina. In GBS, there is lymphocytic and macrophagic infiltration around endoneural vessels with associated demyelination of the affected nerves. The abnormal enhancement of the intrathecal nerve roots is suggestive of blood brain barrier breakdown. Differential diagnosis of abnormal intrathecal nerve root enhancement includes AIDS-related polyradiculopathy in patients affected by CMV, arachnoiditis, sarcoidosis, and metastatic disease, The most common site of enhancement in GBS may be anterior nerve roots rather than posterior nerve roots. MRI is a sensitive but non-specific examination. Motor weakness and sensory change are correlated with patterns of nerve root enhancement and nerve conduction findings.

Acute disseminated encephalomyelitis (ADEM)

Findings

There is patchy T2 prolongation involving the thalami, basal ganglia, and scattered areas of white matter and cortex. There was no diffusion restriction or abnormal contrast enhancement.

Differential diagnosis
- Demyelinating disease (including ADEM or multiple sclerosis)
- Progressive multifocal leukoencephalopathy
- Toxic or metabolic etiologies
- Creutzfeldt-Jakob disease
- Lymphoma


Diagnosis: Acute disseminated encephalomyelitis (ADEM)


Discussion

Acute disseminated encephalomyelitis (ADEM) is an acute, monophasic demyelinating disease. It is similar to multiple sclerosis in its clinical and pathologic features. It is most commonly seen in children, but can be seen at any age. It is likely of autoimmune etiology and typically follows vaccination or viral infection. ADEM occurs most commonly from October to March. Pathogenesis likely involves T helper cells sensitized to auto antigens, such as myelin protein. Symptoms include fever, headache, and meningeal signs. Seizures, focal neurologic deficits, stupor, and coma may develop. ADEM is often clinically distinct from MS in its association with viral exposure, presence of constitutional symptoms, presence of cortical signs, and lack of posterior column abnormalities. CSF analysis is crucial to the diagnosis.

Treatment includes steroids and full recovery is possibly if treated early. Mortality was as high as 10% to 20% although this data was obtained prior to modern ICU technology.

Acute hemorrhagic leukoencephalitis is a severe variant of ADEM that is often fatal. Pathologically, there is perivascular hemorrhagic necrosis, primarily in the centrum semiovale. The major imaging feature is a rapid progression of white matter lesions over the course of a few days.


Radiologic overview of the diagnosis

CT scan is relatively insensitive, but may show scattered low density areas. T2WI and FLAIR usually show multiple regions of hyper intensity at the gray-white junction, in the brainstem, cerebellum, and basal ganglia. Optic neuritis is also common. Solid or ring enhancement can be seen. There can be variable diffusion restriction. There can be high signal on ADC maps in regions of demyelination. Spectroscopy can show low NAA. Imaging findings can lag behind clinical condition during both onset and resolution. The lesions regress with successful treatment, correlating with clinical improvement. Imaging findings are rarely pathognomonic.


Key points

ADEM is a monophasic demyelinating disease that has similarities to MS.
Imaging findings are rarely pathognomonic. CSF analysis is key to diagnosis.
MRI shows scattered areas of T2 prolongation predominantly in white matter, but may also affect gray matter.

Endolymphatic Sac Tumor

Findings

CT images showing an erosive lesion of the right petrous bone, centered at the right vestibular aqueduct opening (endolymphatic sac). The lesion is eroding the posteriolateral wall of the jugular fossa. There is also erosion of the posterior wall of the right IAC, however the right IAC itself is still intact. There are scattered bony spicules seen within the lesion.
Axial T1 and axial T2 precontrast images, respectively, at the level of cerebellopontine angle. There is a hyperintense multilobulated lesion occupying the right endolymphatic sac space. There is also a central area of low signal intensity which enhances post-gadolinium.
T1-weighted post-gadolinium axial and coronal images, respectively. There is a central area of enhancement indicating hypervascularity. Low-signal intensities within the lesion represent intratumoral bony spicules.

Differential Diagnosis:
- Paraganglioma (glomus jugulare)
- Cystic and papillary adenocarcinoma
- Chondroid lesions
- Cholesterol granuloma
- Metastasis
- Cystic schwannoma


Diagnosis: Endolymphatic Sac Tumor (Papillary Adenomatous Tumor of the Temporal Bone)


The endolymphatic sac is part of the membranous labyrinth derived from the neuroectoderm and located within the petrous portion of the temporal bone where it also contacts the dura. Endolymphatic sac tumors (ELST) were described in 1989 by Heffner as a unique pathologic entity originating from the epithelium of the endolymphatic sac, specifically the rugose or middle portion of the sac. ELST histology is benign and exhibits two patterns; mixed type and papillary adenomatous type. The former is generally confined while the latter demonstrates a more aggressive nature by locally invading the temporal bone. Patients most often present with sensorineural hearing loss and may also present with tinnitus, facial nerve palsy or vestibular dysfunction. ELST is a rare skull base lesion and most lesions are sporadic. However, there is an increased incidence seen in patients with von Hippel-Lindau disease (vHL), 7% of whom are diagnosed with this lesion. If ELST is seen bilaterally, vHL becomes a likely diagnosis.

ELST is a slow growing lesion which erodes the temporal bone and often presents with intratumoral bony spicules. In addition, due to the tumor’s slow growth, the petrous bone cortex expands and surrounds the lesion with a thin shell of reactive bone. The lesion may also exhibit hypervascularization, usually from a branch of the external carotid artery.

CT may show a soft tissue mass in the endolymphatic sac between the sigmoid sinus and internal auditory meatus. On MR imaging, this lesion exhibits a hyperintense focus on T1WI with an inhomogeneous signal on T2WI. Focal low signal intensities within the lesion represent bony fragments. Tumor foci may enhance on T1WI with gadolinium due to hypervascularity. Advanced lesions may spread far, making it difficult to identify the origin of the tumor and expanding the differential. Treatment is surgical resection.

Posterior reversible encephalopathy syndrome (PRES)

Findings

The head CT demonstrates multiple, bilateral, patchy foci of hypo attenuation within the subcortical white matter with some minimal cortical involvement of the occipital, anterior parietal, and posterior frontal lobes. Axial T2 and FLAIR sequences demonstrate multiple symmetrical, patchy foci of increased signal within the subcortical white matter of the bilateral occipital, anterior parietal, and posterior frontal lobes. No corresponding areas of restricted diffusion were seen on DWI or ADC sequences.

Differential diagnosis
- Posterior reversible encephalopathy syndrome (PRES)
- Acute cerebral ischemia
- Atypical viral encephalitis
- Acute demyelinating disease


Diagnosis: Posterior reversible encephalopathy syndrome (PRES). Also known as reversible posterior leukoencephalophy syndrome (RPLS)


Key points

Consider the diagnosis when you see patchy cortical/subcortical posterior territory lesions in a patient with acute severely elevated BP
95% will have parietal occipital subcortical/cortical lesions that are hyper intense on T2/FLAIR sequences
Additional common locations include the cerebellum, junctions of vascular watershed zones. Can less commonly be seen in the basal ganglia; rarely in the brainstem or deep white matter
Usually bilateral
Extent of findings is highly variable
Usually has a patchy morphology as opposed to a confluent one
Patchy, bilateral hypo dense foci with a predilection for the posterior circulation on CT
The most important differential diagnosis is cerebral ischemia. Acutely, diffusion will be decreased in cerebral ischemia, but increased in PRES. Diffusion, can, however, be increased if PRES is untreated and results in irreversible ischemia.
Usually reverses with control of BP but can occasionally result in death or permanent neurologic disability

Antrochoanal polyp

Findings

Noncontrast axial CT image in bone window demonstrating a low density mass opacifying the left maxillary sinus extending to the posterior choana. No bony destruction is present.
Noncontrast coronal CT image in soft tissue window better demonstrating the lesion extending through the middle meatus. Contralateral maxillary antral mucosal thickening is present.


Diagnosis: Antrochoanal polyp


Antralchoanal polyps present as mucoid masses originating at the maxillary antrum extending through a sinus ostium to the choana.
Antralchoanal polyps demonstrate non aggressive features with only peripheral enhancement.
Treatment is complete surgical resection.
Originally described by Killian in 1906, antrochoanal polyps result from edematous hyperplasia of respiratory epithelium that herniates through the maxillary infundibulum or the ostium of the maxillary sinus to the level of the posterior choanal (boundary of the nasal cavity and nasopharynx). They commonly arise from the maxillary antrum, hence antrochoanal; however nasochoanal, sphenochoanal, and ethmochoanal polyps have been reported. They comprise 4-6% of all sinonasal polyps and are frequently associated with bilateral maxillary sinusitis. Younger males with a mean age approximately 10 years, presenting with unilateral nasal obstruction is the typical clinical scenario.

Non-contrast CT imaging will usually reveal a large dumbbell shaped, up to 5-6 cm, mucoid density mass occupying the maxillary sinus, exiting via a widened ostium, and into the nasopharyngeal airway. Rarely will antrochoanal polyps extend deep into the nasopharynx or protrude through a nostril. Peripheral enhancement is frequently seen without central enhancement. MR imaging reveals similar findings with hypointense to variable T1 signal. The lesion is hyperintense on T2WI with only peripheral enhancement.

Differential diagnosis includes intranasal glioma, nasoethmoidal encephalocele, juvenile angiofibroma, inverted papilloma, and esthesioneuroblastoma. Intranasal gliomas usually present at birth or the very young as a soft tissue mass centered at the nasal dorsum. Nasoethmoidal encephaloceles present at birth with a frontal, nasal or medial orbital soft tissue mass contiguous with intracranial brain parenchyma extending through a bony defect. Juvenile angiofibromas present in adolescent males as an intensely enhancing mass originating at the sphenopalatine foramen extending into the posterior nasopharynx. Inverted papillomas are usually found in older males as a locally aggressive mass centered at the middle meatus extending into the maxillary sinus and nasal cavity. Esthesioneuroblastomas present in the second and sixth decades as a solid enhancing mass centered within the superior nasal cavity extending into the cribriform plate. Complete surgical resection is the treatment of choice with mean time to recurrence of 45 months.