Normally, the eyes move in synchrony together. Both eyes look right together, up together, down together and left together. At rest both eyes are maintained in the center position. This is called conjugate gaze. The purpose of conjugate gaze is to form images on the retina in a consistent fashion. The visual system transfers those images into signals that are transmitted via the retina, optic nerves, optic chiasm, optic tract, lateral geniculate nuclei and the optic radiation to the occipital cortex. The signals are processed into images by the primary visual cortex and its connections. Therefore using both eyes, binocular vision, allows us to create a single representation of one structure from two images. This allows us to process depth and distance.
Diplopia (double vision) occurs when there is a problem forming the images in the correct area on the retina. This can occur due to the eyes not moving in synchrony; disconjugate gaze. In this situation the brain receives two signals that are not in synchrony and interprets them as two separate objects. You can imagine that if the patient closed one eye the diplopia would disappear because there is only one image. This is true and it is called binocular diplopia. In binocular diplopia, the diplopia disappears regardless of whether the affected eye or the unaffected eye is closed. Much less commonly diplopia occurs when two images are formed on the retina within a single eye; due to two refractive surfaces. In this situation, closing the affected eye will cause the diplopia to disappear but closing the other eye will not relieve the diplopia. This is called monocular diplopia.
Binocular diplopia, diplopia due to disconjugate gaze, may be subtle where the patient sees double but the eyes appear to be conjugate to the examiner. In other cases, the disconjugate gaze is obvious to the examiner. A discussion of the anatomy and physiology of control of eye movements is beyond the scope of this section. There are a few points worth mentioning nonetheless. There are two systems that control conjugate gaze; a system for horizontal gaze and a system for vertical gaze. These systems help move the eyes up and down and side to side. Impairment in these systems generally does not cause diplopia but causes gaze deviation. Separately, there is a system that keeps the eyes moving conjugately. This system includes the cranial nerves responsible for ocular movement: the oculomotor nerve CN III, the trochlear nerve CN IV, and the abucens nerve CN VI. Also included is the medial longitudinal faciculus that connects the oculomotor and abducens nerves to allow them to move together, as well as, other brainstem fibers that connect these nuclei. This system controls conjugate ocular movement by innervating the extraocular muscle.
Therefore the lesions responsible for diplopia affect the following anatomical structures:
- The midbrain and pons (connecting tracts and the cranial nerve nuclei exist here)
- The cranial nerves (the nuclei and the entire tract of the nerves) responsible for ocular movement: the oculomotor nerve CN III, the trochlear nerve CN IV, and the abucens nerve CN VI
- The cranial nerves along their course (in the brainstem, cisternal course in subarachoid space, in the cavernous sinus, in the superior orbital fissure)
- The medial longitudinal faciculus
- The neuromuscular junctions at the extra-ocular muscles
- The extra-ocular muscles
Clinical assessment:
The first step in clinical assessment is to determine if the patients complaint of “double vision” really represents diplopia. Ask if the image is partially or completely splitting into two images; both represent diplopia. Simple blurred vision or oscilopsia (the images appear to jiggle and move) are not specific features of diplopia and may indicate decrease acuity or nystagmus respectively. Also ask if the images are horizontally aligned (side by side) or slightly misaligned (one higher than the other). This might help determine horizontal diplopia as can occur with abducens nerve palsy versus vertical diplopia which has a broader differential.
Next determine whether the diplopia is binocular or monocular. Cover each eye sequentially and ask if the diplopia disappears. With binocular diplopia, the symptom disappears no matter which eye you cover. With monocular diplopia, the symptom only disappears when covering the affected eye. Examine the patient in resting neutral position at rest. Also examine visual pursuits with the patient following your finger from left to right and up and down in all 4 corners. Use the cover uncover test to detect subtle disconjugate gaze in the neutral position.
As you clinically assess the patient look for features (by history and examination) to help localize the lesion. Are there features of brainstem dysfunction (hemiparesis, ataxia, intension tremor, hemisensory loss)? are there features of an isolated cranial neuropathy (isolated oculomotor nerve palsy, isolated trochlear nerve palsy, isolated abducens nerve palsy)? Are there features of multiple cranial nerves involved, suggesting cavernous sinus syndrome or superior orbital fissure syndrome? Are there temporal features or limb features of myasthenia gravis or other neuromuscular junction disorders? is there fatiguability? Is the pattern different from above but restricted to one or more ocular muscles? is there proptosis that may suggest carotid-cavernous fistula, thyroid ophthalmopathy or orbital tumor?
Causes of binocular diplopia:
Restrictive:
- Thyroid disease
- Myositis
- Muscle entrapment
- Infection (e.g. orbital abscess)
- Orbital pseudotumour
- Tumour
Neurogenic:
- Myasthenia gravis
- III nerve palsy
- IV nerve palsy
- VI nerve palsy
- Multiple ocular nerve palsies
- Internuclear ophtalmoplegia (INO)
Causes of Oculomotor nerve palsy (CN III): think of anatomy
Nuclear & fasciular:
- Tumours: Glioma
- Part of a brainstem stroke syndrome
Basilar area:
- Meningitis:
- Bacterial, Meningovascular syphilis
- TB meningitis
- Fungal meningitis
- Basilar aneurysms
- Posterior communicating artery PCOM aneurysm
- Temporal lobe herniation (uncal herniation)
Cavernous sinus area:
- Tumours: Intrasellar & extrasellar tumours e.g. pituitary, chordoma, meningioma, Nasopharymgeal tumours, craniopharygioma
- Internal Carotid artery aneurysms
- Cavernous sinus thrombosis
- Mucormycosis
Superior orbital fissure and Orbital apex area:
- Tumours: nasopharygeal, meningioma, hemangioma, glioma, sarcoma, Hand-Schuller-Christian disease, metastasis
- AVMs
- Tolosa-Hunt syndrome
- Pseudotumour of the orbit
Others:
- Idiopathic
- Vascular:
- Vasculopathy: diabetes mellitus, hypertension & atherosclerosis, giant cell arteritis
- Wegner’s granulomatosus
- Hodgkin’s disease, VZV, encephalitis, collagen vascular disease, Paget’s disease
- Trauma
Causes of trochlear nerve palsy (CN IV):
Nuclear & fasciular:
- Tumours: glioma, medulloblastoma
- Part of a Brainstem stroke syndrome
Basilar area:
- Meningitis:
- Bacterial, Meningovascular syphilis
- TB meningitis
- Fungal meningitis
- Basilar artery aneurysm
Cavernous sinus area:
- Internal Carotid artery aneurysm
- Cavernous sinus thrombosis
Superior orbital fissure and Orbital apex area:
- Tumours: nasopharygeal, meningioma, hemangioma, glioma, sarcoma, Hand-Schuller-Christian disease, metastasis
- AVMs
- Tolosa-Hunt syndrome
- Pseudotumour of the orbit
Others:
- Idiopathic
- Vasculopathy:
- Atheroma, Hypertension, Diabetes mellitus
- Giant cell arteritis
- Trauma
Causes of abducens nerve palsy (CN VI):
Nuclear & fasciular:
- Tumours: glioma
- Part of a Brainstem stroke syndrome
- Multiple sclerosis
Basilar area:
- Meningitis:
- Bacterial, Meningovascular syphilis
- TB meningitis
- Fungal meningitis
- Basilar artery aneurysm
Petrous tip area:
- Raised intracranial pressure ‘false localising sign’
- Hydrocephalus
- Mastoiditis
- Nasophareygeal tumours, paranasal sinus tumours
- Lateral sinus thrombosis
Cavernous sinus area:
- Internal Carotid artery aneurysm
- Cavernous sinus thrombosis
- Tumours: Intrasellar & extrasellar tumours e.g. pituitary, chordoma, meningioma, Nasopharymgeal tumours, craniopharygioma
Superior orbital fissure and Orbital apex area:
- Tumours: nasopharygeal, meningioma, hemangioma, glioma, sarcoma, Hand-Schuller-Christian disease, metastasis
- AVMs
- Tolosa-Hunt syndrome
- Pseudotumour of the orbit
Others:
- Idiopathic
- Vasculopathy:
- Atheroma, Hypertension, Diabetes mellitus
- Giant cell arteritis
- Wegner’s granulomatosus
Causes of Internuclear ophthalmoplegia (INO):
- Multiple sclerosis
- Brainstem infarct
- Brainstem glioma
- Brainstem hemorrhage
- Wernicke encephalopathy
Causes of cavernous sinus syndrome:
- Sepsis
- Tumor
- Internal carotid artery aneurysms
- Wegener’s granulomatosis
- Tolosa hunt syndrome
Causes of amblyopia:
- Strabismus
- Opacities of the optic axis:
- Ptosis
- Corneal opacities
- Cataract
- Anisometropia (unequal refractive errors in both eyes)
Causes proptosis:
- Grave’s Ophthalmopathy
- Orbital pseudotumour: the muscle tendons are spared
- Orbital cellulitis
- Neoplastic tumours: primary or secondary
- Carotid cavernous fistulas ‘auscultate for bruit or ask the patient if they can hear it’
Causes ptosis:
Neurogenic ptosis:
- Horner’s syndrome
- Oculomotor nerve palsy (unilateral or bilateral)
Myogenic ptosis:
- Myasthenia gravis (bilateral)
- Lambert Eaton myasthenic syndrome (bilateral)
- Chronic progressive external ophthalmoplegia ‘mitochondrial DNA mutations’ (bilateral)
- Oculopharyngeal muscular dystrophy (bilateral)
- Myotonic dystrophy (bilateral)
- Other myopathies (bilateral)
Mechanical ptosis:
- Saging tissue in elderly
- Infection etc.
- Aponeurotic ptosis: stretching or dehiscence of the tendon