Sliding uncus syndrome

The sliding uncus syndrome is a type of syndrome observed in uncal herniation patients in which the patient remains fully conscious despite suffering from the herniation. A well known undisputed axiom in neurology and neurosurgery states that an oculomotor palsy (dilated pupil and external oculomotor ophthalmoplegia) due to an intracranial space occupying lesion has to be accompanied by some alteration in the level of consciousness. This has been challenged by the sliding uncus syndrome.
The Unilateral transtentorial (uncal) herniation is a common subtype of transtentorial herniation which usually occurs in expanding traumatic hematomas or any other supratentorial mass lesion, frequently in the middle fossa or the convexity, pushing the innermost part of the temporal lobe named the uncus and hippocampal gyrus over the edge of the tentorium, entrapping the third (oculomotor) cranial nerve and directly compressing the midbrain part of the brainstem. Once the Uncal herniation meets its CT criteria and is accompanied by a full oculomotor palsy; decreased level of consciousness, hyperventilation and contralateral weakness ensues.
Uncal herniation
Unilateral transtentorial (uncal) herniation is mostly associated with a supratentorial mass lesion compressing the uncus against the midbrain and squeezing it through the Tentorial hiatus. The oculomotor nerve is compressed against the free edge of the tentorium resulting in fixed and dilated pupil. The is a complex, consciousness activating, neuronal system that extends from the midpons to the thalamus. Decreased level of consciousness and coma is expected when there is an interruption to these synapsing fibers either from a destructive or a compressive lesion. In uncal herniation, pupillary dilation is expected to be accompanied by a decreased level of consciousness.
Symptoms and signs
A patient suffering from uncal herniation can usually complain only about a headache or unilateral weakness because as the herniation proceeds the patient loses consciousness and only signs of the herniation can be seen in a spectrum of worsening deficit according to the stage of herniation:
* Pupil: ipsilateral dilated pupil → external oculomotor ophthalmoplegia → contralateral dilated pupil → both midposition and fixed → dilate widely with hypoxia.
* Respiration: Normal → hyperventilation → shallow and rapid → slow, irregular rate and depth → apnea.
* Consciousness: Once external oculomotor ophthalmoplegia → comatose.
* Motor: Contralateral weakness → contralateral paralysis → bilateral decerebrate rigidity → flaccidity.
Sliding uncus syndrome
The sliding uncus syndrome was first identified by Dr. Miki Katzir on a patient who suffered from a massive chronic subdural haematoma with a radiologically evident uncal herniation, an 8 millimeter non reactive dilated pupil and external oculomotor ophthalmoplegia with no change in his consciousness level, nor hemiparesis or other signs.

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