Friday 17 August 2012

Unilateral Abnormal Enhancement of 6th CN in Lateral Rectus Palsy

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A 14 yo male, relatives complaining about his sudden recent onset squint and trying to correlate this finding with a history of trauma. On clinical examination right lateral rectus palsy.
MRI advised with clinical diagnosis of right side 6th Cranial Nerve palsy.
MRI study of Brain routine sequences were normal including 3D FIESTA (Steady state free precession (SSFP) sequence.
Due to strong clinical suspicion, MRI study was repeated with thin axial sections of SPGR T1 sequence after injection of intra venous contrast.

Contrast enhanced SPGR T1 sequence show a faint abnormal enhancement along cisternal portion of right side Abducens nerve consistent with clinical diagnosis of 6th Nerve palsy - Post traumatic.
No abnormal enhancement in the region of left side 6th CN.

Abnormal Cranial Nerve Enhancement on MRI

MRI is a valuable tool in evaluation of cranial Nerves and detecting the diseases of the cranial nerves. Contrast enhanced MRI increases the ability of MRI to detect such abnormalities. Abnormal cranial nerve enhancement on MRI may sometimes be the only finding of an underlying disease.
Layers of connective tissue sheaths of cranial nerves are endoneurium, perineurium, and epineurium. The blood-nerve barrier of cranial nerves is formed by the combined actions of tight junctions in the endothelium of the endoneural capillaries and tight junctions in perineurium.
Various insults like neoplasm, autoimmune disease, inflammation, demyelination, ischemia, trauma, radiation can disrupt this blood-nerve barrier, allowing leakage and accumulation of intra venous contrast material with resultant enhancement along the cranial nerves.

References : MRI of Cranial Nerve Enhancement, Farhood Saremi, Mohammad Helmy, Sahar Farzin, Chi S. Zee and John L.

Normal Anatomy of Abducens Nerve (VI or 6th CN)

Divided into four portions:
1. Nuclear portion
2. Cisternal portion
3. Cavernous sinus portion
4. Orbital portion

The Nuclear or intra parenchymal portion is its nucleus in the caudal pons, the abducens nerve exits the brainstem at the pons-medulla junction.
Cisternal portion is the part of nerve after emerging from pons in prepontine cistern. It courses superiorly with the anterior inferior cerebellar artery anterior to it, and the pons posteriorly, pierce the dura at the medial most portion of the petrous apex, passing through the inferior petrosal sinus in Dorello's canal. It is its oblique course and relatively fixed anchor in Dorello's canal which makes it prone to stretching when raised ICP from any space occupying lesion.
Cavernous sinus portion is within the cavernous sinus, the abducens nerve is located inferolateral to the internal carotid artery, medial to the lateral wall of the sinus.
Orbital portion is after having entered the orbit through the tendinous ring. It supplies the lateral rectus. Damage to the abducens nerve results in lateral rectus palsy, a tendency for the eye to deviate medially, may result in double vision.

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