Ophthalmology - Squint and Neuro-ophthalmology Flashcards
What is a squint?
The true definition of a squint (or strabismus) is that one of the eyes is not directed towards the object under scrutiny. Note that if the eyes converge for close work, this does not indicate a squint.
Patients often use the word to describe narrowing of the gap between the upper and lower eyelids, usually carried out by patients to create a pinhole effect. This reduces the consequences of any refractive error and improves the clarity of the image.
Why is a squint important?
A squint:
1) may show impaired visual acuity
2) may itself cause ambylopia in a child
3) may be a sign of a life threatening condition
1) The eyes are kept straight by the drive to keep the image of the object being viewed in the centre of the macular area, where highest defintion and colour vision are located. The tone in the extraocular muscles is constantly being readjusted to maintain this. If the vision is impaired in one or both eyes, this constant readjustment cannot occur and one eye may wander. This could be caused by very treatable diseases, such as cataract or refractive errors.
2) Double vision due to misalignment of the eyes, or congenital cataracts may lead to suppression of vision in the affected eye and the visual pathways then fail to develop properly. This leads to ambylopia of the eye.
3) Squint is a common presenting complaint of a child with retinoblastoma. A squint can also be caused by a 6th nerve palsy resulting from a tumour causing raised ICP. Patients with myasthenia may also present with a squint and diplopia.
What features in the history are important in squints?
A family history of a squint is a strong risk factor in the development of a squint. Children with disorders of the central nervous system such as cerebral palsy have a higher incidence of squint.
Squint is more common in preterm infants. Problems during delivery and delayed development also increase the likelihood of squint. The parents visual problems should be ascertained, particularly large refractive errors.
The earlier the age of onset, the more likely the squint will need an operation. A constant squint has a worse visual prognosis than one that is intermittent.
What is the approach to examining a squint?
1) Check the visual acuity
2) Look at the position of the patients eyes - large squints will be obvious. Wide epicanthic folds may give the impression of squint, but children with wide epicanthic folds may still have squints
3) Look at the corneal reflections of a bright light held in front of the eyes - note the position of the reflections, they should be symmetrical (this test gives a rough estimate of the angle of deviation)
4) Cover test
5) Test eye movements in all directions
6) Examine the pupil with dilating and relaxing agents
Why is it important to check the visual acuity when examining a squint?
If the visual acuity does not correct with glasses or pinhole, occular disease or ambylopia must be suspected. This is particularly important in children, as the ambylopia or ocular problems must be treated immediately if sight is to be preserved.
Visual acuity in infants is difficult to assess. A history from the parents is useful to find out whether the baby looks at them and at objects. If the sight is poor in one eye, covering the good eye may make the child try to push it away.
What is the cover test?
This is technique used to examine a squint. Two types of cover test help to reveal a squint, especially if it is small and the examiner is unsure about the position of the corneal reflections.
(i) In the cover and uncover test, one eye is covered and the other is observed. If the uncovered eye moves to fix on the object there is a squint that is present all the time - a MANIFEST squint. The test should then be carried out on the other eye. A problem arises when the vision in the squinting eye is reduced, and the eye may not be able to take up fixation. This emphasises the need to test any patient with a squint. If the cover and uncover test is normal (no manifest squint) then the alternate cover test should be performed.
(ii) In the alternate cover test, the occluder is moved to and fro between the eyes. If the eye that has been uncovered moves, then there is a latent squint.
Why is it important to examine an eye with a pupil dilating agent (mydriatic) and a ciliary muscle relaxing agent (cycloplegic)?
Dilating pupils allows you to check for retinal disease, such as retinoblastoma and the cycloplegic eye allows a check for any refractive error. Adequate examination of the peripheral fundus and refraction require dilation of the pupil and special equipment. Cataracts and other opacities in the media, and the white reflex suggestive of retinoblastoma may be checked without dilating the pupil, and looking for the red reflex.
How are paralytic squints managed?
Paralytic squints usually occur in adults. Underlying conditions such as raised ICP, compressive lesions, and diseases such as diabetes, hypertension and myasthenia and thyroid eye disease should be excluded.
If diplopia is a problem, one eye may need to be occluded temporarily, for example by a patch stuck to the patients glasses. Alternatively, temporary prisms can be stuck onto the glasses to eliminate diplopia. An operation on the ocular muscles may be indicated if the squint stabilises. If this is inappropriate or proves inadequate, permanent prisms may be incorporated into the glasses prescription. Botulinum toxin can also be used as part of the diagnostic or treatment process.
How can non paralytic squints be treated?
Non paralytic squints usually occur in children/ If the squint is caused by disease in the eye that is causing reduced vision and subsequent deviation of the eye (e.g. cataract) this needs to be treated.
Treatments for non paralytic squints include:
- spectacles
- occlusion
- orthoptic (visual exercises)
- surgery
- botulinum toxin
What are the 2 main indications for prescribing glasses to a child?
1) A child who is hypermetropic (longsighted) and has a convergent squint. If the child is hypermetropic, the ciliary muscle has to contract strongly during accommodation for the child to be able to focus on the near object. This excessive accommodation may cause overconvergence so that a squint occurs. This type of squint is called an accommodative convergent squint. The use of hypermetropic glasses relaxes the ciliary muscles and reduces the drive to converge
2) A child who has a refractive error, particularly if this is unilateral. Because of the refractive error the image on the retina will be indistinct. The visual pathways will then not develop properly (resulting in ambylopia). Children with a refractive error may not develop a squint until the vision is poor in one eye
At what age is occlusive therapy unlikely to correct a squint in a child?
After the age of about 7, occlusion therapy is unlikely to be helpful. Occlusion itself can also only be done for a short amount of time, as there is a danger of ambylopia in the good eye. The vision of the good eye must also be “blurred” with drops containing atropine.
After what age is ambylopia unlikely to be correctable?
The effectiveness of treatment in reversing ambylopia decreases as the child gets older. Once the child is about 8 or 9 years old the visual system is no longer flexible and ambylopia may not be reversed. However, the child may still need glasses to correct any refractive error.
A parent complains that her 2 year old infant has developed a turn in one eye. What is the differential diagnosis?
Non-paralytic squint
Paralytic squint
Retinoblastoma
“My child’s eye has a turn.” What issues in the history support the diagnosis?
Squint or strabismus occurs in about 3% of children. Most cases present between the ages of 3 months and 4 years. Most childhood squints are non paralytic, but paralytic squints may still occur. Exclusion of underlying ocular, orbital and/or systemic disease is of the utmost importance.
“My child’s eye has a turn.” What additional features in the history would you seek to support a particular diagnosis?
The antenatal, perinatal and neonatal history, including prematurity, birth weight, birth trauma and developmental milestones, is important. Ask about family history of refractive errors, ambylopia (“lazy eye”), squint, congenital cataracts or retinoblastoma. Is there any history of previous occlusion therapy of one eye, or squint surgery?Are there any associated neurological symptoms - headache, clumsiness, drowsiness or vomiting?
“My child’s eye has a turn”. What clinical examination would you perform and why?
Inspect the child for abnormal head posture, dysmoprhic features or hydrocephalus. Examination of the red reflex is important. It is reduced or absent in cataract or corneal opacification. There may be leukocoria (white pupil) in cataract, retinoblastoma, or other posterior segment pathology. Examine extraocular movements for limitations of eye movements indicative of palsy of a rectus muscle. In children with convergent squints, full aBduction of each eye must be demonstrated to ensure that a cranial nerve VI palsy is not present. Dilated fundoscopy is essential as conditions such as retinoblastoma may present with strabismus.
“My child’s eye has a turn.” What investigations would you perform and why?
Cycloplegic refraction is mandatory. Ultrasound examination of the posterior segment is performed in cases of media opacity. In some cases, CT/MRI of the brain, eye and orbit may be indicated.
“My child’s eye has a turn.” What treatment options are appropriate?
There is a very close relationship between visual function and ocular alignment in children, and many children with squint develop ambylopia.
- Conservative management: detection and treatment of refractive errors and ambylopia (occlusion therapy of the “good eye”) are important
- Surgical treatment: ocular realignment by means of strabismus surgery can then be carried out if necessary
- Retinoblastoma may be treated with chemoreduction and focal therapy (laser photocoagulation and cryotherapy). In some cases, external beam radiotherapy may be needed.
A 60 year old patient presents with intermittent loss of vision in one lasting a few seconds, which then returns to normal. What is the likely diagnosis?
Amaurosis fugax (transient loss of vision)
What is amaurosis fugax?
This is reversible loss of vision lasting less than 24 hours. It is a subtype of TIA. The blindness or partial blindness usually lasts less than 10 minutes
What causes amaurosis fugax?
- thromboembolism
- vasospasm
- blood hyperviscosity
- vasculitis involving blood vessels that supply the visual pathway
- intermittent optic nerve compression with gaze-evoked amaurosis
The most common cause of amaurosis fugax is is atherosclerosis of the internal carotid or vertebrobasilar system. Amaurosis fugax resulting from involvement of the internal carotid artery system is unilateral.
What are the important features of a history of amaurosis fugax that support the diagnosis?
Visual loss lasting more than 10 minutes or not returning completely should alert one to the possibility of a retinal vascular occlusion. Ask about symptoms of giant cell arteritis such as headache, scalp tenderness, jaw claudication, anorexia and weight loss.
What investigations are most useful for a patient with amaurosis fugax?
An ESR should be requested to exclude possible vasculitis associated with giant cell arteritis, SLE or other autoimmune disorder. Carotid Doppler ultrasonography will determine the extent of carotid artery stenosis.
What treatment options are appropriate for amaurosis fugax?
After a single TIA, a patient has a stroke risk of 5% per annum and death of 5% per annum.
1) Medical treatment - patients with carotid artery stenosis may be treated with antiplatelet drugs such as clopidogrel. Anticoagulation with warfarin may be considered if a cardiac source of recurrent emboli is identified or if the symptoms persist despite antiplatelet drugs
2) Carotid endarterectomy - recommend if patient has suffered stroke or TIA in the carotid territory and are not severely disabled
- should only be considered if carotid stenosis > 70% according ECST* criteria or > 50% according to NASCET** criteria
A patient presents with intermittent loss of vision in both eyes and then her vision clears. What differentials would you consider?
Migraine
Increased ICP
Pseudotumour cerebri (idiopathic intracranial hypertension)
Amaurosis fugax
A patient presents with intermittent loss of vision in both eyes and then her vision clears. What issues in the history would support the diagnosis?
Migraine is a common cause of visual disturbance in young patients.
External compression of blood vessels supplying the visual pathway may also cause transient loss of vision. In patients with papilloedema, pressure on blood vessels within the swollen optic nerve head may cause transient loss of vision in one or both eyes.
Amaurosis fugax resulting from involvement of the vertebrobasilar system may produce binocular visual obscuration. Episodes usually last less than 1 minute.
What are the features of migraine that may help to identify it as a cause of intermittent visual loss?
The transient visual loss is usually associated with fortification spectra or scintillating scotomata (coloured zig zag lines oscillating in brightness) usually lasting 15-20 mins but can last longer. If so, the diagnosis of migraine is likely.
A personal history or family history of migraine is helpful. In patients with migraine, ophthalmic examination and pupillary reflexes are usually normal.
What are the features of headache associated with raised ICP?
Raised ICP often causes a headache that is worse on waking, and exacerbated by bending, straining and coughing. Establish whether there is any history of head trauma and whether the headache follows the visual disturbance.
What are the features of pseudotumour cerebri that can help establish the diagnosis in patients with intermittent loss of vision?
Drug history!
Tetracycline, steroid use or withdrawel, nalidixic acid, nitrofurantoin, danazol, ciclosporin and vitamin A in excess are all associated with pseudotumour cerebri, as is obesity.
A patient presents with intermittent loss of vision in both eyes and then her vision clears. What investigations would be most useful and why?
Request FBC and ESR to exclude anaemia and possible autoimmune disorder or hyperviscosity syndrome. Visual field analysis may detect an enlarged blind spot.
Neuroimaging may be indicated to exclude a SOL, hydrocephalus or dural sinus thrombosis.
In patients with pseudotumour cerebri, the CSF opening pressure is often >200mmHg but the CSF composition is normal. Lumbar puncture should always be performed with extreme caution in patients with papilloedema.
