Monday 6 August 2012

6th CN Palsy in Apical Petrositis MRI

A 15 y o male advised MRI with a clinical diagnosis of left Lateral Rectus - 6th CN Palsy.
MRI Post contrast SPGR T1w images at the level of Pons show:
E/o Left side Mastoiditis and Apical petrositis.
An abnormal soft tissue in the region of cisternal portion of left 6th CN where it is entering in Dorello’s canal explains involvement of left 6th CN. 
Note the normal 6th CN on right side. 
After MRI study is over despite of asking leading questions patient refused any kind of ear discharge or ear pain but said yes to mild occasional left facial pain. 
Imaging diagnosis: 6th CN involvement secondary to Apical petrositis. 

Apical petrositis
Involvement of petrous apex is a relatively rare complication that occurs when infectious otomastoiditis extends medially into the petrous apex usually via pneumatized air cells. Initially, intact petrous apex air cells are opacified with purulent exudate. With progressive infection, the epithelium is invaded and destroyed, and the supporting bony trabeculae and inner cortical margins undergo demineralization and resorption. Infection then spreads beyond the air cells to the adjacent marrow space of the petrous apex, essentially forming a localized osteomyelitis of the skull base.
Symptoms variable and depend on the stage of disease. Most patients present with severe otalgia and otorrhea with associated deep facial or retroorbital pain. Occasionally, patients present with the classic Gradenigo triad: otomastoiditis, deep facial pain secondary to trigeminal neuropathy, and lateral rectus palsy and diplopia secondary to sixth cranial nerve palsy. The classic triad is explained by the unique relationship of the petrous apex to Dorello canal (Abducens, 6th CN) and Meckel cave (Trigeminal, 5th CN), may not be present every time as in this case.

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.