Abstract
Purpose
To report a case of external ophthalmoplegia due to an uncommon form of amyloidosis exclusively affecting the lateral rectus muscle, and to discuss the clinical manifestation, diagnostic challenges, and management pitfalls of isolated amyloidosis in the extraocular muscle.
Observations
A 64-year-old woman presented with diplopia in her left gaze lasting for six months. She had orthophoria in the primary position and abduction limitation in the left eye. Routine laboratory examinations were unremarkable. Orbital magnetic resonance imaging showed fusiform enlargement of the left lateral rectus muscle, without tendon involvement. Extraocular muscle biopsy was recommended to make a diagnosis, which revealed amyloid deposition in the lateral rectus muscle. A systemic work-up showed no evidence of systemic amyloidosis. Therefore, a diagnosis of primary isolated amyloidosis was made. Orthophoria in the primary position and diplopia in the lateral gaze persisted at the six-month follow-up.
Conclusions and importance
Atypical extraocular muscle enlargement should alert clinicians to the need for tissue biopsy to identify uncommon etiologies, such as amyloidosis. There are no pathognomonic or radiological features to distinguish localized from systemic amyloidosis. Therefore, if amyloidosis of the extraocular muscles is diagnosed, a systemic work-up is needed to rule out systemic amyloidosis, which is potentially life-threatening.
1
Introduction
Amyloidosis is a condition that occurs when an abnormal amyloid protein builds up in the organs and tissues and interferes with their normal functions. This condition is classified into systemic or localized types, both of which are typically progressive and sometimes life-threatening. Except for hereditary corneal dystrophies, the occurrence of amyloid deposits in the eyes and their adnexal structures is rare, although it is known to develop in systemic or localized amyloidosis. Orbital amyloidosis was reportedly seen in 0.3% of cases of orbital diseases, with the deposition being observed in the periorbital fat, extraocular muscles, and lacrimal glands. However, primary localized amyloidosis in the extraocular muscle without systemic involvement is extremely rare, and only a few cases have been reported. Herein, we report a case of external ophthalmoplegia caused by isolated amyloidosis in the external ocular muscle and discuss the diagnostic challenges and management pitfalls.
2
Case report
A 64-year-old woman presented to Asahikawa Medical University Hospital with a complaint of diplopia lasting for the past six months. She noticed the diplopia only in the left gaze. The external ocular examination was unremarkable, without any signs of eyelid swelling, proptosis, or ptosis. She was otherwise healthy, except for hyperlipidemia, which had been adequately treated with lipid-lowering medication. Her best-corrected visual acuity was 20/16 in both eyes, and orthophoria was present in the primary position. Ocular motility tests revealed abduction limitation in the left eye ( Fig. 1 ). The pupils were equal, round, and reactive to light, and no evidence of internal ophthalmoplegia was detected. The anterior segment and fundus examinations were unremarkable.
Magnetic resonance imaging (MRI) showed fusiform enlargement of the left lateral rectus muscle with tendon sparing. The enlarged lateral rectus did not show high signal intensity on short-tau inversion-recovery (STIR) MRI or on gadolinium enhancement ( Fig. 2 ). Laboratory work-up revealed that her thyroid-stimulating hormone (TSH) level was slightly elevated (5.79 μIU/ml), but free triiodothyronine, free thyroxine, and TSH receptor antibody levels were normal. Serum C-reactive protein, IgG4, soluble interleukin-2 receptor, angiotensin-converting enzyme, and lysozyme levels were also within the normal ranges.
To establish a diagnosis, a left lateral rectus muscle biopsy was performed. During surgery, the left lateral rectus was weak on active force generation testing, and the forced duction test (FDT) was negative. A left lateral limbal-conjunctival incision was made, and the lateral rectus muscle was held with a squint hook. The upper edge of the muscle was then resected to a width of 1–1.5 mm and a length of 10 mm. The muscle and the tendon appeared yellowish and could be torn easily using forceps ( Fig. 3 ). Histopathology revealed atrophy and focal loss of muscle fibers; in addition, deposition of eosinophilic amorphous material was widely observed on hematoxylin and eosin staining ( Fig. 4 A). The eosinophilic material was positive for direct fast scarlet staining ( Fig. 4 B) with the amyloid-specific birefringence on polarized microscopy. A systemic work-up was conducted, including serum and urine tests, electrocardiogram, echocardiography, immuno‐electrophoresis, and biopsy of subcutaneous fat from the abdomen, which revealed no evidence of systemic amyloidosis other than a slight increase in the kappa light chain (20.6 mg/L) in the serum. Therefore, a diagnosis of primary isolated amyloidosis was made.
The patient was followed up for six months after the biopsy. Her alignment remained excellent in the primary position and her diplopia in the left gaze persisted, but was not bothersome.
3
Discussion
Primary localized amyloidosis in the extraocular muscle is very rare; only 13 such cases without systemic involvement have been reported over the last four decades. In only nine of these was the amyloidosis confined to the extraocular muscles ( Table 1 ), , , , , , whereas the other four cases also exhibited involvement of the soft tissue mass surrounding the enlarged muscles. , , , The nine patients mentioned above ranged from 20 to 80 years old, the enlarged muscles ranged from single to multiple without predilection, and the affected side was unilateral or bilateral. Seven of the nine patients were women; this predominance of affected females is consistent with that of previous reviews of orbital amyloidosis (62.5–65.1%). ,
