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Sir,

Uric acid is a product of purine metabolism. The deposition of such crystals is seen in cases of hyperuricaemia that can present as gout. It is usually idiopathic and caused by either an overproduction of uric acid (phosphoribosyl transferase deficiency) or reduction in renal urate excretion. It may also be due to secondary causes such as myeloproliferative disorders and blood dyscrasias. However, the deposits of such crystals in gout are usually in soft tissues of joints and the ear, although deposits in the eye1 and deposits causing spinal cord compression have been described.2 We describe the case of an orbital mass in a healthy man, which on histopathological analysis consisted of uric acid crystals.

Case report

A 41-year-old man presented with a 1-month history of an ache around the left eye associated with a left ptosis. On examination, the palpebral fissure measured 8 mm compared to 12 mm on the right. Visual acuity and ocular motility were unimpaired. There was loss of the upper lid sulcus and a soft mass palpable in the superolateral aspect of the orbit. This gentleman was fit and well with no serious past medical history or regular medications.

CT scan revealed a 2 cm superolateral mass extending into the frontal bone to lie in the medullary cavity of the skull. There was no intracranial extension. MR imaging was performed on a 1.5-T system (Philips Gyroscan ACS-NT) (Figure 1,Figure 2 and Figure 3). This showed a well-demarcated but lobulated area of heterogeneous high signal intensity on all sequences arising from the bone of the upper outer orbit wall.

Figure 1
figure 1

Coronal T1-weighted spin-echo (WSE) fat suppression scan. Lesion of left orbit demonstrating high signal intensity.

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Figure 3
figure 2

Axial T2-WSE scan.

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The patient consented for exploration and biopsy of the lesion with possible exenteration due to the possibility of neoplasia. Differential diagnosis considered were lacrimal gland carcinoma and dermoid.

Left frontal craniotomy, orbitotomy, and excision of the lesion were performed with a frozen section peroperatively. Histological examination revealed a granulomatous reaction to brown crystalline deposits. The crystals were rhomboid (Figure 4), with yellow to brown birefringence on polarised light (Figure 5). Multinucleated giant cells, lymphocytes, neutrophils, a few eosinophil and haemosiderin-laden macrophages were present. The crystals stain strongly positive with Gomori's silver stain, and electron probe analysis reveals the crystals to be of a carbon-rich organic composition, all consistant with crystals of uric acid.

Figure 4
figure 3

Haemotoxylin and eosin×400.

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Figure 5
figure 4

Polarising light×400.

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The patient made a full and rapid recovery with no neurological sequelae. The left ptosis improved and there were no visual problems.

Comment

MR findings of gouty tophi within the appendicular3 and axial4 skeleton have been reported as intermediate (isointense to muscle) and occasionally of low signal intensity on T1-weighted images.5 The findings on T2-weighted images have been more variable. The findings in our patient on T1-weighted imaging (high signal) are not consistent with this description. The signal intensity of this lesion was hyperintense on the STIR sequence. Although there is no description within the literature, this would suggest a high water content possibly due to the necrosis or inflammation. On T2-weighted images, there was also high signal intensity. This hyperintense signal intensity on T2W SE images may reflect the high protein content in the amorphous centre of the lesion, whereas the decreased signal intensity may indicate regions of fibrous tissue and crystals, haemosiderin deposition, or protein immobility. Although inconsistent, the majority of reports state a marked enhancement post-injection of i.v. gadolinium. Our patient did not show this.

This case has a very unusual and unexpected diagnosis. The patient was otherwise healthy, with no precipitating cause of hyperuricaemia. His serum urate and renal function were within the normal range and the case was discussed with the physicians.

To our knowledge there is no report in the literature of uric acid deposition presenting as a mass in the orbital region.

Figure 2
figure 5

Coronal T1-WSE (post-gadolinium) scan.

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