The “Black Turbinate” Sign: An Early MR Imaging Finding of Nasal Mucormycosis

Discussion

Mucormycosis affects patients who are immunocompromised and is caused by fungi in the order Mucorales, including Mucor, Rhizopus, and Absidia species.² The spores invade the nasal mucosa and are not phagocytized as in an immunocompetent individual which then germinate, forming angioinvasive hyphae that cause infarction of the involved tissue, resulting in a “dry” gangrene appearance. This devitalized mucosa appears on contrast-enhanced MR imaging as contiguous foci of nonenhancing tissue. Invasion occurs with spread to the maxillary sinuses and subsequent orbital spread through the ethmoid sinuses. Additionally, there are several routes for intracranial extension, including the ophthalmic arteries and cribriform plate into the frontal lobes, or through the cavernous sinus via the orbital apex.^1–3 Extension of the disease leads to vascular thrombosis, including the cavernous sinus and other cerebral vessels due to fibrin reaction and Mucor thrombus formation. Subsequent infarctions lead to eschar formation or cerebral infarctions and, eventually in some cases, to coma or death.³

The morbidity and mortality rate of mucormycosis is high, and death can occur within as little as 2 weeks. Blindness and cranial nerve palsies are major sequelae of the disease because it is aggressively invasive.⁴ Thus, early detection and treatment are important factors in the resulting disposition of the patient with a substantial increase in mortality once the cranium has been penetrated. Patients treated with amphotericin B who undergo wide debridement have an overall higher survival rate.⁵ Diseased tissue is prone to necrosis and devitalization, which in turn leads to a lack of enhancement seen on MR imaging, “the black turbinate sign,” a feature that can aid in earlier detection of the disease.

Imaging characteristics of mucormycosis infection generally demonstrate a rim of soft-tissue thickness along the paranasal sinuses. Sinus opacification, air-fluid concentration, and obliteration of the nasopharyngeal tissue planes are features of sinonasal mucormycosis infection. On MR imaging, variable intensity within the sinuses on T1- and T2-weighted images is usually seen. Fungal elements themselves may cause a low signal intensity on T2 sequences.⁶ Furthermore, slightly decreased T2 signal intensity with rhinocerebral mucormycosis can be due to the involved mucosa itself (Fig 2). DWI sequences may aid in the diagnosis as demonstrated in Figs 3 and 7, showing an increased signal intensity of the affected sinus. The infarcted mucosa may lead to a restriction of diffusion. An article by Tung and Rogg⁷ reported that mycotic invasion and infarction of cerebral vessels could lead to cytotoxic edema and cause a restriction of diffusion. While the application to sinus mucosa is as yet unclear, the findings demonstrated in the above 2 cases may be examples of a parallel situation.

In general, the MR imaging findings of sinusitis usually demonstrate enhancement of the peripheral mucosa on T1-weighted contrast-enhanced sequences, in contrast to these 2 cases of documented sinonasal mucormycosis.⁸ Rhinocerebral mucormycosis should be a strong consideration when there is a lack of enhancement of the mucosa, given its angioinvasive nature, because the hyphae likely invade smaller vessels supplying the mucosa. A similar finding has been reported in an article by Horger et al,¹ in which a case of rhinocerebral mucormycosis was described with areas of nonenhancement.

Of further interest concerning the case presentations of both patients are the presenting symptoms of unilateral cranial nerve involvement. The first patient had unilateral facial numbness of the affected left side while the second had right-sided facial numbness, ear pain, and proptosis. This constellation of findings is consistent with Garcin syndrome. A case report of Garcin syndrome was presented in a Japanese neurology journal in 2004. A 64-year-old patient with alcoholism and unilateral cranial nerve involvement was found to have rhinocerebral mucormycosis after presenting to the emergency department with right-sided proptosis, facial numbness, and periorbital edema.⁹ The disease is usually associated with infiltrating tumor; however, as in the case cited from 2004 and in the 2 case presentations discussed above, rhinocerebral mucormycosis should be considered in patients who are immunocompromised or have diabetes, presenting with multiple unilateral cranial nerve deficiencies.