How Common Can a Pontine Lesion Cause Locked-In Syndrome
Locked-in syndrome is a well-recognized but relatively uncommon complication of pontine lesions, occurring most frequently with ventral pontine infarction from basilar artery occlusion, though the exact incidence rate among all pontine lesions is not precisely quantified in the literature. 1
Anatomical Basis and Mechanism
The pons contains critical structures whose disruption leads to locked-in syndrome 1:
- Ventral pons contains the corticospinal, corticobulbar, and corticopontine tracts that when bilaterally damaged produce the characteristic quadriplegia and anarthria 1
- Dorsal pontine tegmentum contains cranial nerve nuclei (CN V-VIII) and white matter tracts; lesions here typically spare consciousness but affect horizontal gaze 1
- Preserved vertical eye movements occur because the midbrain structures controlling vertical gaze remain intact 2
Primary Etiologies and Their Relative Frequencies
Vascular causes dominate the etiology of locked-in syndrome:
- Ischemic stroke accounts for 86.4% of locked-in syndrome cases, typically from basilar artery occlusion causing ventral pontine infarction 2
- Traumatic brain injury represents only 13.6% of locked-in syndrome cases 2
- Ischemic and hemorrhagic infarcts are the most frequent cause of acute brainstem syndromes overall, with the pons being the most frequently affected brainstem structure 1
Non-vascular pontine lesions rarely cause locked-in syndrome but include: 1
- Demyelinating disease
- Encephalitis
- Neoplasms
- Central pontine myelinolysis
- Neurodegenerative disorders
- Syringobulbia
Clinical Recognition and Diagnostic Timing
The diagnosis of locked-in syndrome is frequently delayed:
- Mean time to diagnosis is 78.76 days (approximately 2.5 months) after onset 2
- This delay reflects the challenge of recognizing preserved consciousness in a completely paralyzed patient 2
- Initial reduced level of consciousness occurs in 80% of patients (16 of 20 in one series), lasting a mean of 3 days (range 1-15 days) 3
Mortality and Prognosis
Locked-in syndrome from pontine lesions carries extremely high mortality:
- Overall mortality in the acute phase is 75%, with median survival time of 42 days 3
- 25% die within the first 10 days from stroke progression or cardiac arrest 3
- Later deaths result primarily from pulmonary infections and sepsis 3
- More extensive parenchymal brainstem lesions show statistically significant association with higher mortality 3
Critical Diagnostic Pitfalls
Several factors contribute to missed or delayed diagnosis:
- Locked-in syndrome is listed among specific pontine syndromes alongside Millard-Gubler syndrome, Foville syndrome, and facial colliculus syndrome, indicating it represents one of several possible presentations rather than the most common 1
- Small single brainstem lesions can produce severe mixed neurologic deficits, but the specific pattern determines whether locked-in syndrome develops 1
- Bilateral ventral pontine involvement is required; unilateral lesions produce different syndromes 2, 4
Distinguishing Features from Other Pontine Syndromes
Location within the pons determines the clinical syndrome:
- Ventral pontine lesions → locked-in syndrome (when bilateral) 2, 3
- Dorsal pontine tegmentum lesions → cranial nerve palsies (CN V-VIII) with horizontal gaze abnormalities 1
- Pontine nucleus lesions → complete horizontal gaze deficit with ipsilateral facial paralysis 1
- Lesions through corticospinal tracts → contralateral hemiparesis 1
Practical Clinical Implications
When evaluating pontine lesions, consider:
- Not all pontine strokes cause locked-in syndrome; the specific location and bilaterality determine the syndrome 1
- Basilar artery occlusion with bilateral ventral pontine infarction represents the highest-risk scenario 2, 3
- Preserved consciousness with complete paralysis should immediately raise suspicion for locked-in syndrome 2
- Vertical eye movements and blinking are typically the only preserved voluntary movements 2