Gaze Abnormalities: Ipsilateral vs Contralateral Localization
Gaze abnormalities can be either ipsilateral or contralateral depending on the anatomical location of the lesion, with pontine lesions causing ipsilateral gaze palsy and supranuclear (cortical/hemispheric) lesions causing contralateral gaze palsy. 1
Anatomical Framework for Horizontal Gaze
The direction of gaze deviation depends critically on where the lesion occurs along the oculomotor pathway:
Pontine (Brainstem) Lesions → Ipsilateral Gaze Palsy
- Lesions affecting the paramedian pontine reticular formation (PPRF) or abducens nucleus cause ipsilateral conjugate gaze palsy - the eyes cannot look toward the side of the lesion 1, 2
- A 1 mm lesion within the paramedian pontine reticular formation causes paralysis of ipsilateral conjugate gaze 3
- The pontine reticular formation, abducens nucleus, and median longitudinal fasciculus play an important role in ipsilateral conjugate gaze physiology 3
Supranuclear (Cortical/Hemispheric) Lesions → Contralateral Gaze Palsy
- Cerebral hemisphere lesions cause contralateral gaze palsy - the eyes deviate away from the side of hemiparesis and toward the side of the lesion 1
- Oculomotor pathways originating within the cerebrum project through the diencephalon to the brainstem, with the direction of vector action above the oculomotor decussation being predominantly contraversive 3
Special Anatomical Considerations
Internuclear Ophthalmoplegia (INO)
- Lesions in the medial longitudinal fasciculus (MLF) cause contralateral adduction weakness with ipsilateral abduction nystagmus 1
- A 1 mm lesion within the median longitudinal fasciculus causes impairment of contralateral (disconjugate) gaze with paralysis of adduction of the ipsilateral eye 3
- Abnormalities in the rostral pons and midbrain result in contralateral hypotropia and head tilt, whereas abnormalities in the vestibular periphery, medulla, and more caudal pons result in ipsilateral hypotropia and head tilt 2, 1
Vertical Gaze Abnormalities
- Vertical gaze palsies require bilateral lesions and do not follow simple ipsilateral/contralateral rules 3
- Bilateral lesions within the rostral interstitial nucleus of the MLF result in isolated paralysis of downward gaze 3
- Bilateral lesions within the pretectum or posterior commissure result in paralysis of upward gaze 3
Clinical Localization Algorithm
To determine lesion location based on gaze abnormality:
If horizontal gaze palsy with eyes looking away from hemiparesis → Supranuclear (cortical) lesion contralateral to gaze deviation 1
If horizontal gaze palsy with eyes looking toward hemiparesis → Pontine lesion ipsilateral to gaze deviation 1, 3
If adduction weakness with contralateral abduction nystagmus → MLF lesion (INO) ipsilateral to adduction weakness 1, 3
If vertical gaze palsy → Bilateral midbrain/diencephalic pathology 3
Critical Pitfalls to Avoid
- Do not assume all gaze palsies are ipsilateral - the anatomical level determines directionality 1
- Do not miss INO by failing to test adduction separately - this represents MLF pathology with contralateral effects 1
- Do not delay high-resolution brainstem MRI when INO or vertical gaze abnormalities are present, as standard CT is insufficient 1, 4
- Always assess the vestibulo-ocular reflex (VOR) - supranuclear lesions preserve VOR while nuclear/infranuclear lesions affect all eye movements including VOR 5