Strabismus - Fundamentals of Clinical Ophthalmology.pdf

STRABISMUS
clinician should be alerted particularly if there is
an intermittent variability of muscle tone. Ocular
involvement is present in 90% of cases and is the
presenting feature in over half the cases. Ocular
myasthenia usually becomes generalised, within
2 years of onset, but remains restricted to the
ocular muscles in about 30% of patients. By
generalised is meant beyond the ocular muscles.
However, it needs to be recognised that this
extension beyond the ocular muscles may extend
only to surrounding muscles of the face and not
involve the trunk. The diagnosis may involve
several tests (see Box 5.1).
Myositis
Idiopathic orbital myositis may extend to
involve a posterior scleritis or anteriorly to
involve the lacrimal gland. It is usually painful
to pressure and during attempts to move the eye
in the direction of the muscle’s action, unlike
thyroid eye disease. It is unilateral, although it
may be bilateral in 25% of cases (more
frequently in women). Distinguishing features of
myositis on CT and MRI include involvement of
both muscle and tendon, in contrast to thyroid
eye disease, where the muscle belly is
predominantly involved.
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Box 5.1 Tests for myasthenia gravis
• Muscle fatigue test – patient looks up for
30 s to demonstrate worsening of ptosis.
• Dark room test – if ptosis resolves,
diagnosis confirmed.
• Cogan’s twitch sign of overshoot and
twitch with straight gaze after downgaze
for several minutes is diagnostic.
• Tensilon test – inject 0·2 cc edrophonium
chloride. If improvement occurs, this
confirms a diagnosis of myasthenia. If
no response, inject further until a total of
0·8 cc has been administered. Objective
improvement in the muscles being
observed confirms a diagnosis of myasthenia.
Intravenous atropine should be
kept available for adverse response such
as abdominal pain and gastrointestinal
disturbance. Neostigmine in children
pretreated with atropine allows more time
for assessment.
• Antibodies to acetylcholine receptors
are present in patients with generalised
myasthenia and in most patients with
ocular myasthenia.
• CT or MRI of the mediastinum should
be performed to exclude thymoma,
which is more common in children, but
may be present in 20–30% of adults,
particularly males.
Neurological causes of adult strabismus
Third nerve palsy
Disorders of the third nerve can occur
anywhere from the midbrain to the orbit at
various levels. Its relationship to the tentorial
edge as it crosses the subarachnoid space makes
it vulnerable to damage from raised intracranial
pressure, causing uncal herniation, and also
from hydrocephalus and trauma. Palsy of the
superior division of the third nerve involves
ptosis, an inability to elevate the eye with
involvement of the levator muscle and the
superior rectus. With injury to the inferior
division of the third nerve, there is inability to
adduct the eye or to look inferiorly and the pupil
can be involved. Pupillomotor fibres run on the
superior aspect of the third nerve and ultimately
reach the parasympathetic supply to the eye
through the inferior division and its branch to the
inferior oblique muscle. Aberrant regeneration
occurs in the third nerve particularly after a
compressive lesion, such as an aneurysm of
posterior cerebral or intracranial portion of
internal carotid artery or pituitary tumour or
following trauma. Third nerve palsy of sudden
onset with pupil involvement requires an
MRI. If aneurysm is suspected, neurological
consultation and angiography is important.
Presentation can occur with intermittent vertical
diplopia and pupil dilatation as the only sign. If
the pupil remains spared, MRI can be deferred,