DDs of Owl's Eye _ Spinal cord T2 hyper intensities on MRI

There is not much mentioned about this sign but whatever is available in literatures and case reports where they have described this finding and in view of clinical scenario they have tried to attribute this finding to one particular condition or cause. Most of them have associated this finding with spinal cord infarction, however this finding is not specific or pathognomonic for spinal cord infarction.

I have tried to elaborate list of other remote conditions and causes which can present with such finding on MRI or follow up MRI irrespective of clinical settings. Most of them are with reference and few of them are through my personal experience.

What is Owl's Eye Sign? 
An abnormal intra medullary T2 hyper intensity in the region of anterior horn cells of spinal cord, as two white dots, one in each half of cord on axial T2w MRI images in the background of normal gray coloured spinal cord.

This is totally different from ‘Winking  Owl’  sign
Winking owl sign  is related to vertebral metastases, a reliable sign of osteolytic spinal metastases on AP radiographs corresponds to loss of the normal pedicle contour. The appearance of unilateral pedicle absence has been likened to that of a winking owl with the missing pedicle being the closed eye, the contralateral pedicle being the open eye and the spinous process being the beak.

DDs of Owl's Eye in spinal cord. 

1. Spinal cord Infarction 
The vascular supply to the spinal cord is primarily composed of one anterior and two posterior spinal arteries, which extend along the length of the spinal cord in a variable manner. The anterior and posterior spinal arteries are connected by a pial plexus that extends around the circumference of the spinal cord. At many levels, the anterior and posterior spinal arteries receive vascular contributions from the radicular arteries, which course along the nerve roots and enter the spinal canal. Thirty-one pairs of radicular arteries exist. The anterior spinal artery gives rise to central arteries at multiple levels; these arteries supply the anterior horn cells and the anterior aspect of the lateral columns on both sides of the spinal cord.
Two major forms of spinal cord infarcts are recognized. The first involves the interruption of supply by the radicular arteries or artery of Adamkiewicz and is characterized by unilateral or bilateral infarcts of the anterior or posterior spinal arteries. The second one is caused by diffuse hypoperfusion and is manifested by central or transverse infarcts.
Recognized causes include spinal and aortic surgery, hypotension, vertebral artery dissection, embolism, vasculitis, and cocaine abuse. In about half of cases, infarction occurs immediately after a movement, such as back extension, an arm movement or a Valsalva maneuver, possibly causing mechanical stress on a radicular artery.

2. Fibrocartilaginous emboli
Rapid onset of spinal cord symptoms from retrograde flow of emboli from a herniated nucleus pulposus into the anterior spinal artery or spinal veins during straining, causing an anterior spinal artery syndrome (Wilmshurst et al 1999). There is back or neck pain but often no history of trauma, followed by sudden (minutes to hours) onset of weakness and incontinence. This is more common in women than men and is associated with anterior cord lesions on MRI and anterior horn cell fallout on electrophysiologic testing. Cord swelling on MRI is associated with a collapsed disc at the level of the cord deficit, usually in the cervical region (Tosi et al 1996). The CSF is normal. There is no associated viral syndrome. Recovery is unlikely.

3. Resolved Cord Contusion
A resolved spinal cord contusion on follow up imaging can have similar appearance due to Gliosis. History of trauma particularly hyper flexion injury needs to ruled out clinically. Vertebral collapses may be an associated finding on MRI.

4. Poliomyelitis and Motor Neuron Diseases 
A disease of the lower motor neurons that affects the gray matter of the spinal cord, specifically the central horns. In a retrospective study, spinal cord segments from all patients who had had poliomyelitis showed loss or atrophy of motor neurons, severe reactive gliosis, and a perivascular and intraparenchymal inflammation even in the chronic phase, up to 20 years after infection. In acute to subacute phase (up to 8 weeks after acute illness), the ventral horn cells are characterized by a severe inflammation, neuronophagia, active gliosis, and destruction of the anterior horn cells. This correlates with the T2 signal hyperintensity in the region of the ventral horns on MRI and should be fairly specific for poliomyelitis.
(Reference : Poliomyelitis: Hyperintensity of the Anterior Horn Cells on MR Images of the Spinal Cord; Mark S. 1 Jeffrey M, Charlene A. Tate, Vladislav Zayas, and J. Donald Easton)

5. Compressive myelopathy 
A bulging disc causing mechanical compression over redicular artery leading to a chronic ischemic changes and Gliosis in the region of anterior horn cell. Disc herniation and degree of canal stenosis may be not be severe as here main culprit is radicular artery compression which is lying anterior to cord and not the direct cord compression by disc.

6. Hopkins syndrome
Flaccid paralysis of one or more limbs, 4 to 7 days after an asthma attack. Anterior cord lesions in 2 to 12 year-old children with onset over 1 to 2 days are followed by permanent paralysis. CSF typically contains 20 lymphocytes and 20 polymorphonuclear neutrophils (Hopkins 1974).

7. Radiation myelopathy
Possible with an exposure over 50 Gy. Damage is delayed up to 15 years after exposure but is typically 10 to 16 weeks later (Yamada et al 1987). Radiation myelopathy causes vasculopathic and sometimes anterior horn cell changes with high MRI T2 signal owing to Gliosis in corresponding region on follow up studies. 

Purely Intracanalicular Acoustic Schwannoma MRI

MRI Brain FIESTA (3D CISS) sequence shows an intra canalicular nodular low signal intensity of a 8th CN Schwannoma confined to Internal Auditory Canal on left side.

Comparison of FIESTA and Contrast enhanced study in MRI screening for Acoustic Schwannoma

Acoustic schwannomas are a treatable cause of sensorineural hearing loss.
Currently, MRI is the gold standard examination and screening test for exclusion of acoustic schwannoma particularly those confined to internal auditory canal.

In MRI we have two main options  in addition to the routine MRI sequences, one is Contrast Enhanced study and second is FIESTA (fast imaging employing steady-state acquisition) sequence.
Contrast Enhanced study consist of plain T1 and Gadolinium-enhanced T1-weighted images, both in multiple planes, adds extra time and cost to the examination, and possibility of contrast reaction exists.
Whereas FIESTA is a single sequence, a true-FISP (free induction steady-state precession) sequence that provides high-resolution fluid-bright images of the CPAs and basal cisterns.

In a study of 50 patients, results of contrast enhanced study and FIESTA images were compared. The hypothesis was that the FIESTA sequence can replace contrast enhanced study for screening and diagnosis of AS confined to internal auditory canal. The results showed that in 98% of cases, this was possible. So the FIESTA sequence can be employed in isolation for screening of AS. Same can be equally beneficial in cases where gadolinium is contraindicated such as pregnancy. However, Contrast enhanced study should be employed when pathology is seen and to follow-up post-surgical patients.

Conclusion: Use of the FIESTA (CISS) is sufficient to exclude AS confined to internal auditory canal without the need for gadolinium-enhanced sequences. 

Reference : Comparison of FIESTA and gadolinium-enhanced T1-weighted sequences in magnetic resonance of acoustic schwannoma; Paul J. Rigby

Measles Encephalitis MRI

 A 12 years old patient with characteristic morbiliform rash.

Admitted in our neuro institute with symptoms of encephalitis occurred 10 days after appearance of the rash, altered sensorial with lower limb flaccid paralysis now.

Csf report positive for specific IgM and IgG antibodies.  

Clinical diagnosis was measles encephalitis.

Here is her on admission MRI Brain Axial FLAIR, T2w and Diffusion. 

MRI Brain Axial FLAIR shows confluent bilateral cerebral white matter hyper intensity extending along external capsules with faint restricted diffusion. T2 w images show bilateral basal ganglionic  symmetric T2 hyper intensity with focal parenchymal swelling consistent with clinical diagnosis of measles encephalitis.

Imaging findings of Measles Encephalitis mentioned in literatures are 1. Multifocal high signal in bilateral cerebral hemispheres, swelling of the cortex, 2.  Bilateral, symmetrical involvement of the putamen and caudate nucleus, lesions showing low apparent diffusion coefficients. 3. Sub acute gyriform hemorrhage, asymmetrical gyriform contrast enhancement mentioned in severe cases. 4 Diffuse cerebral cortical atrophy, gliosis and encephalomalacic changes on follow up MRI studies. 

Histopathological findings of Measles Encephalitis mentioned in literatures are perivascular mononuclear cell infiltration, white matter demyelination, gliosis and intranuclear and intracytoplasmic eosinophilic inclusions in neuronal and glial cells in the temporal, parietal and occipital cortex as well as in the thalamus. 

Slit Ventricle Syndrome

• Slit ventricle syndrome occurs in minority of patients who have been shunted.
• "Slit ventricle" refers to finding of very small ("slit-like") ventricles on CT or MRI indicating excessive drainage.

Diagnostic criteria: 
An association of  clinical signs of headache, vomiting with signs of  slit like ventricles on CT or MRI. 

• Headache may be intermittent, often postural occurring when standing up and resolving when the patient lies down. Vomiting can be related to visual or auditory disturbance, drowsiness.
• Symptoms usually present years after shunt placement or shunt revision.
• Severe form of slit ventricle syndrome occurs in children. The absence of cerebrospinal fluid (CSF) within the ventricles combined with a growing brain leads to situation in which "the brain is too big for the skull." The intracranial pressure (brain pressure) can be very high. Adults can develop a milder form of slit ventricle syndrome. 
• The diagnosis of slit ventricle syndrome can be difficult and the condition is often misdiagnosed or the diagnosis delayed. The finding of small ventricles in a shunted patient can be misinterpreted as a properly working shunt. Most patients with small ventricles on CT or MRI may not have the slit ventricle syndrome clinically. Patients must be symptomatic to call Slit ventricle Syndrome. 
• Typically, the shunt is nearly blocked but still barely flowing.

A case of  Vp shunt done for Post TB Meningitis Hydrocephalus. Now came for follow up with new onset of headache and nausea. CT Brain plain shows right parietal Vp shunt with collapsed lateral ventricles. Possibility of "Slit Ventricle Syndrome" considered clinically and patient re admitted for further management.  

Treatment: 
The management of slit ventricle syndrome is difficult and challenging.
In general, a neurosurgeon with expertise in the management of hydrocephalus is optimal.
Various treatment options have been proposed, and include:
1. Observation.
Usually limited to minimally symptomatic patients
2. Anti-migraine medicines.
3. Shunt revision.
 Change the ventricular catheter.
 Change the shunt valve.
 Add siphon controlling device (SCD).
 Programmable valve with or without SCD.
 Converting to a lumboperitoneal shunt.
4. Temporarily blocking the flow of the shunt (via "externalization" of the shunt) in order to expand the ventricles.
This should be done with ICP monitoring due to the risk of coma.
Many patients have aqueductal stenosis, and therefore are candidates for endoscopic third ventriculostomy (ETV).
In some cases, a special shunt configuration draining both the ventricles and the cisterns (space around the brain) can equalize the inner and outer brain pressures, thus reducing the chance of producing slit ventricles again. This type of shunt is called a ventriculocisternoperitoneal shunt.
5. Subtemporal decompression.
This procedure is rarely performed because improvements, if any, are typically short-lived.