Diplopia means double vision and is the simultaneous perception of two images when observing a single object with both eyes open. These images can have a horizontal, vertical or oblique disposition. There are 2 types of diplopia:
- Monocular diplopia: secondary to ocular globe pathologies.
- Binocular diplopia: secondary to ocular misalignment due to impaired extraocular muscle function (ophtalmoplegia or ophtalmoparesia). It is caused by lesions located inside the orbital cavity or along the course of oculomotor (CN III), trochlear (CN IV) and abducens (CN VI) cranial nerves.
These cranial nerves provide motor innervation to the extraocular muscles (EOM) and each of them follow a particular pathway. They originate at the brainstem and traverse the following key anatomic locations (distal to proximal)
- Superior orbital fissure
- Cavernous sinus
- Cisternal/Subarachnoid space
The clinical presentation of diplopia varies depending on the site of the lesion from the cerebral cortex to the ocular globe.
- Cerebral cortex = frontal eye field and parieto-occipito-temporal cortex
- Brainstem
- vertical and horizontal gaze centers and CN III, IV, VI nucleus
- connection fibers between CN III/VI (conjugated lateral gaze) = paramedian pontine reticular formation (PPRF) and medial longitudinal fasciculus (MLF).
- Cranial nerves trajectory = subarachnoid space, cavernous sinus, superior orbital fissure and intraorbital space.
It is important to note that lesions only affecting the cerebral cortex present eyeball deviation (strabismus) without diplopia and lesions affecting the eyeball manifest as monocular diplopia.
EXTRAOCULAR MUSCLES
There are six extraocular muscles that execute specific eyeball movements and they are innervated by the following cranial nerves:
- CN VI = Lateral rectus muscle (LR) (abduction)
- CN IV = Superior oblique muscle (SO) (depression and abduction)
- CN III =
- Medial rectus muscle (MR) (adduction)
- Inferior oblique muscle (IO)(elevation and abduction)
- Superior rectus muscle (SR) (elevation)
- Inferior rectus muscle (IR) (depression)
The CN III provides somatic and visceral motor (parasympathetic) innervation to other muscles that play crucial roles in ocular function:
-
- Levator palpebrae muscle (lifts the eyelid)
- Sphincter pupillae muscle (miosis)
- Ciliary muscle (lens accomodation)
CN III lesions manifest as vertical and oblique diplopia (ophtalmoparesis of sphincter pupillae, SR, IR, MR, IO) + ptosis (ipsilateral levator pallpebrae).
CN IV lesions manifest as vertical diplopia affecting downward gaze and side gaze contralateral to the lesion (ophtalmoparesis of SO) + head deviation towards the opposite side of the lesion.
CN VI lesions manifest as horizontal diplopia affecting lateral gaze ipsilateral to the lesion (ophtalmoparesis of LR) + esotropia (nasal deviation of the eye).
NEURAL PATHWAYS
In order to avoid missing out on lesions involving the CN III, IV and VI, it is imperative as radiologists to have a detail-oriented knowledge of the anatomic references along these three cranial nerves pathways.
- Superior orbital fissure: cleft between greater and lesser sphenoid wing that communicates the cavernous sinus with the orbital apex. It contains fat and the following structures (from superior to inferior):
-
- CN III superior and inferior division
- CN IV
- Ophtalmic nerve branches (trigeminal nerve): lacrimal, nasociliary and frontal nerves
- CN VI
- Opthalmic vein
- Sympathetic nerve fibres
- Cavernous sinus: venous cavity located lateral to sella turcica, limited by the temporal and sphenoid bone.
- Content from superior to inferior
- CN III
- CN IV
- Ophtalmic division of trigeminal nerve
- Maxillary division of trigeminal nerve
- Content from medial to lateral
- Internal carotid artery (ICA)
- CN VI
1- Oculomotor cranial nerve (CN III).
- Nuclear portion = originates in the mesencephalic tegmentum (at the level of the superior colliculi). Located anterior to the cerebral aqueduct and Edinger Westphal nucleus with the medial longitudinal fasciculus as its lateral limit.
- Midbrain course = traverses the tegmentum anteriorly passing through the red nucleus and cerebral peduncle, emerging over the medial interpeduncular cistern.
- Subarachnoid portion = continues towards the prepontine cistern passing below the posterior communicating artery (PCoA), between the superior cerebellar artery (SCA) and posterior cerebral artery (PCA).
- Cavernous sinus portion = penetrates the dura and follows a cranial trajectory, placed at the superior lateral wall of the sinus (near the sella turcica)
- Orbital portion = exits via the superior orbital fissure and enters the orbit into the annulus of Zinn where it divides into superior and inferior components, innervating the MR, IO, SR and IR muscles.
2- Trochlear cranial nerve (CN IV).
- Nuclear portion = arises in mesencephalic tegmentum (at the level of the inferior colliculus), right below the CN III nucleus.
- Midbrain course = the only cranial nerve that decussates and arises from the contralateral dorsal midbrain.
- Subarachnoid portion = it enframes the midbrain anterolaterally over the superior cerebellar peduncule towards the prepontine cistern (next to the PCA) arriving unto the free edge of the tentorium cerebelli.
- Cavernous sinus portion = located at the lateral wall under CN III.
- Orbital portion = traverses the superior orbital fissure and innervates the SO muscle.
3- Abducens cranial nerve (CN VI)
- Nuclear portion = located in the pontine tegmentum, anterior to the facial colliculus of the fourth ventricle, near the facial nerve nucleus.
- Pontine course = follows an anterior trajectory and emerges at the junction between pons and medullary piramids.
- Subarachnoid portion = travels through the prepontine cistern, penetrating the dura towards the petroclival confluence (Dorello canal) extending over the petrous apex. This cranial nerve has the longest subarachnoid segment which makes it very vulnerable to rises in intracranial pressure.
- Cavernous sinus portion = most medial location of all the cranial nerves (lateral to the ICA).
- Orbital portion = passes through the superior orbital fissure and innervates the LR msucle.