What is the most likely site of the lesion in a patient with decerebrate posturing on the right side, third cranial nerve palsy, and Cheyne-Stokes breathing pattern?

Left Midbrain Lesion

The most likely site of the lesion is the left midbrain, based on the combination of ipsilateral (left) third cranial nerve palsy with contralateral (right) decerebrate posturing.

Clinical Localization

Third Cranial Nerve Palsy Localizes to Left Midbrain

• The left eye looking downward and laterally with unreactive pupil indicates a complete left third nerve palsy, affecting both the motor and parasympathetic fibers 1.
• The third nerve nucleus lies near the midline of the midbrain, and fascicular lesions at this level produce ipsilateral third nerve dysfunction 1, 2.
• Lesions involving the cerebral peduncle (which runs through the midbrain) cause ipsilateral hemiplegia or hemiparesis, but in this case the motor findings are contralateral, indicating the lesion affects the midbrain before motor fibers have crossed 1, 2.

Decerebrate Posturing Confirms Midbrain Pathology

• Decerebrate rigidity in humans results specifically from midbrain lesions and is manifested by exaggerated extensor posture 3.
• The presence of midbrain lesions correlates significantly with decerebrate posturing, with a particularly strong association when midbrain lesions occur without pontine involvement 4.
• Right-sided decerebrate posturing with normal left-sided withdrawal indicates the lesion is affecting descending motor pathways on the left side of the midbrain, producing contralateral motor dysfunction 4.

Cheyne-Stokes Breathing Pattern

• Cheyne-Stokes respiration frequently accompanies severe cerebrovascular accidents and can be associated with midbrain dysfunction and elevated intracranial pressure 5.
• This breathing pattern supports but does not specifically localize the lesion, as it can occur with bilateral hemispheric or upper brainstem pathology 5.

Why Not Other Locations?

Excluding Pontine Lesions

• Pontine lesions typically affect cranial nerves V, VI, VII, and VIII, not the third nerve which exits at the midbrain level 1.
• The preserved bilateral gag reflexes (cranial nerves IX and X) argue against lower pontine or medullary involvement 1.

Excluding Thalamic Lesions

• Thalamic lesions do not directly affect cranial nerve nuclei or fascicles, as the third nerve nucleus is located in the midbrain, not the thalamus 1, 2.
• While thalamic hemorrhages can cause contralateral conjugate eye deviation through mass effect on the midbrain, the specific pattern of isolated third nerve palsy points to direct midbrain involvement 6.

Lateralization to Left Side

• The ipsilateral third nerve palsy (left eye affected) with contralateral motor findings (right decerebrate posturing) is the classic pattern of a unilateral midbrain lesion 1, 2.
• This represents a Weber syndrome variant, where the midbrain lesion affects the third nerve fascicle and the cerebral peduncle, producing ipsilateral third nerve palsy with contralateral hemiparesis 1, 2.

Clinical Pitfall

• Do not be misled by the contralateral motor findings—midbrain lesions characteristically produce ipsilateral cranial nerve deficits with contralateral motor deficits because the corticospinal tracts have not yet crossed at this level 1, 2.
• The combination of pupil-involving third nerve palsy with altered consciousness in this acute setting requires urgent neuroimaging (MRI with gadolinium and MRA or CTA) to rule out vascular compression from aneurysm or acute midbrain stroke 1.