FIESTA imaging for problem-solving in early Duane's retraction syndrome.

Sir,

Fast imaging employing steady state acquisition (FIESTA) is a modern magnetic resonance imaging (MRI) sequence, which is a type of steady state free precession (SSFP) sequence. SSFP sequences are gradient echo sequences with a small flip angle and short relaxation times in which a steady state develops between the pulse repetitions.[1] It gives strong signal from fluid tissues while suppressing background tissue signal, improving contrast, and anatomic detail of small structures.

SSFP sequences are being increasingly used for evaluating the cisternal segments of cranial nerves due to their high resolution, submillimetric thickness, and the improved contrast between cranial nerves and cerebrospinal fluid (CSF),[1] with the cranial nerves seen as dark signals against an extremely bright background provided by the CSF. We demonstrate its role as a problem solving modality in a child with Duane's retraction syndrome who was only diagnosed with the aid of FIESTA imaging.

A two-and-a-half-year-old girl presented with a history of acquired esotropia at one year of age. Clinical examination revealed a left lateral rectus palsy and subtle left sided face turn. MRI was performed in view of the history of acquired lateral rectus palsy. The MRI showed normal bulk and signal intensity in all extraocular muscles including the left lateral rectus [Figure 1]. The brain parenchyma including brainstem was also unremarkable. FIESTA images were then taken, which showed aplasia of the abducens nerve on the left side [Figure 2]. The right abducens nerve was well visualized and showed normal signal intensity. The rest of the cranial nerves were also normal. Subsequent clinical examination picked up mild retraction on adduction with subtle widening on abduction suggesting unilateral Duane's retraction syndrome type I, which was not forthcoming in the first examination, probably due to lack of cooperation by the child.

Figure 1

MRI Axial T1-weighted, coronal T1-weighted and postcontrast axial T1-weighted images in a two-and-a-half-year-old girl with history of acquired left lateral rectus palsy show preservation of normal bulk and signal intensity in the bilateral lateral rectii. Postcontrast images show no abnormal enhancement in the muscles. The bilateral abducens nerves are not identifiable on these routine MRI sequences

Figure 2

MRI Axial fast imaging employing steady state acquisition (FIESTA) images in the same child show the right abducens nerve (arrow) traversing the prepontine cistern to enter the Dorello canal. The anterior inferior cerebellar artery is also seen (arrowhead) arising from the basilar artery. Note the absence of a similar abducens nerve on the left side, suggesting aplasia of the nerve. This aplasia would not have been identified without the higher contrast, resolution, and submillimetric slice thickness of the FIESTA sequences. Demonstration of the abducens nerve aplasia prompted repeat clinical examination, which picked up subtle abnormality in abduction and adduction, helping to make the diagnosis of Duane's retraction syndrome

Duane's retraction syndrome is a congenital syndrome, sometimes described as a congenital miswiring of the recti muscles,[2] which presents with impaired eye movements in adduction and abduction along with globe retraction and palpebral fissure narrowing. The condition is usually unilateral or rarely bilateral. Associated abducens nerve hypoplasia or aplasia and aberrant oculomotor nerve supply to lateral rectus muscle have all been variously described as part of their imaging spectrum.[3]

FIESTA imaging can be employed to differentiate between lateral rectus palsy and Duane's syndrome especially when history suggests an acquired onset and the characteristic clinical signs are not obvious during the early phase. Most cases of Duane's syndrome are diagnosed clinically by the age of 10 years. Neuroimaging is important in suspected acquired lateral rectus palsy to rule out an intracranial space occupying lesion. Routine MRI sequences will demonstrate the absence of a tumor but may not be able to demonstrate abducens nerve aplasia or hypoplasia in such a case as they may not visualize the nerve even in normal subjects. Recent studies have even reported visualization rates for abducens nerve to be as low as 43% on routine sequences, increasing to almost 100% employing FIESTA.[45] Awareness among ophthalmologists of this modern sequence will help them to specifically request FIESTA in similar cases, which may otherwise cause diagnostic difficulties.