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Q1: What is the most likely diagnosis?
The most likely diagnosis is internuclear ophthalmoplegia (INO) of abduction or so-called posterior INO. In this case, the presence of cerebellar signs, neuroradiological findings, and cerebrospinal abnormalities with a history of transient upper respiratory tract infection suggests a diagnosis of acute encephalitis.
Q2: Give possible differential diagnoses of an abduction deficit
An abduction deficit can be caused by muscle disorders, neuromuscular junction, cranial nerves, and brainstem lesions (box 1). Although the diagnosis of sixth nerve palsy is often made without the consideration of other causes, abduction paresis less commonly arises from myasthenia gravis, Duane's syndrome, convergence spasm, congenital esotropia, and INO. The absence of esotropia when the eyes are fixated in the primary position makes the diagnosis of sixth nerve palsy unlikely. In Duane's syndrome, there is usually globe retraction and palpebral fissure narrowing on attempted adduction. Convergence spasm is characterised by intermittent and usually painful convergence and miosis. In INO, nystagmus is often seen in the abducting eye, possibly as a result of altered inhibitory input to the lateral rectus muscle. INO can be categorised into posterior and classic INO. The latter condition is characterised by an adduction deficit with intact abduction. Myasthenia gravis may present as an isolated abduction deficit. The key to the neuromuscular nature of an abduction deficit is ptosis and an end-of-day fatigability. A tensilon test should be undertaken if there is any suggestion of myasthenia gravis.
Box 1: Causes of abduction deficit
Muscle: myopathy and restrictive syndromes.
Neuromuscular junction: myasthenia gravis.
Cranial nerves: sixth nerve palsy.
Brainstem: internuclear ophthalmoplegia, Duane's syndrome.
Other causes: convergence spasm, congenital esotropia.
Q3: How would you manage a patient with an abduction deficit?
Based on the information above, it is obvious that the evaluation of a patient with abduction deficit demands a thorough description of the ocular movement and function to determine the diagnosis. Once the mechanism of an abduction deficit has been established, a complete neurological evaluation should be meticulously carried out to search for evidence of an intracranial lesion. In children and adolescents, central nervous system infection, brain tumour, and head injury account for the majority of abduction disorders. Therefore, these patients should have a full clinical examination, and be investigated and treated appropriately.
Horizontal gaze is mediated by the neurons in the region of the para-abducens nuclear group via the medial longitudinal fasciculus. The presence of lateral gaze palsy without cranial nerve, muscular, and neuromuscular involvement should indicate involvement of the brainstem internuclear pathways leading to INO.
The description of INO of abduction goes back to Lutz in 1923,1 although we have found no report of it in childhood. This patient's illness fulfilled the criteria for this condition in that she had an impaired abduction of the left eye and abduction nystagmus of the contralateral eye. In this case, insufficient vestibulo-ocular reflex and slow saccades in the paretic eye suggest a lesion of the paraabducence group. In addition, MRI revealed multiple lesions located in the lower pons at the level of the paramedian pontine reticular formation and abducens nucleus. In all levels of pontine lesions some degree of cerebellar involvement will usually be found. In this patient, cerebellar dysfunction was evident with ataxic gait, dysmetria, and slurred speech.
Conclusive laboratory diagnosis of the cause of infective encephalitis can be very difficult. In this patient, the clinical course, biochemical, neuroradiological, and electroencephalography findings with a history of transient upper respiratory tract infection suggested the diagnosis of acute encephalitis. However, serological evidence of common infective agents was absent.
Although, the pathogenesis of INO of abduction remains uncertain,2-5 we suggest that in our patient an immune mechanism due to a transient inflammatory process led to a lesion and dysfunction of the para-abducens region in the pons. In conclusion, with knowledge of the neurophysiology of the ocular movements, a careful evaluation of each patient with an abduction deficit should alert the examiner to the appropriate diagnostic work-up.
Internuclear ophthalmoplegia of abduction.