Thoracic intramedullary schwannoma

Spinal schwannomas account for 30% of all spinal tumours and are less common in patients not known to have neurofibromatosis [5,12]. They are rarely intramedullary in location; since 1932 only 38 have been reported in the literature. The majority are located in the cervical spinal cord and only six have been located entirely in the thoracic spinal cord. A further three had some thoracic involvement, with their bulk in the adjacent spinal cord. We report a rare case of intramedullary schwannoma of the thoracic spinal region before proceeding to review the existing literature, together with theories that might explain the origin of these unusual lesions. A 51-year-old female suffered a sensation of rising numbness and coldness for 2 years that had originally started in the right foot. At the time of referral this had reached the T6 dermatome and was associated with hyperaesthaesia. She had also noticed increased weakness of the right leg for 18 months that had worsened in the 6 weeks prior to admission such that she could no longer walk unaided. There were no symptoms of upper limb or sphincter disturbance. There was no evidence of neurofibromatosis and neurological examination revealed normal upper limbs and a sensory level at D6 below which all modalities were reduced. The right lower limb exhibited increased tone, hyperreflexia and extensor plantar response, with reduced sensation in all modalities. The left lower limb showed hyperreflexia but was otherwise normal. Gadolinium-enhanced magnetic resonance imaging (MRI) of the spinal cord showed an irregularly shaped contrast-enhancing intramedullary mass extending from D3 to the superior border of D8, with an associated syrinx. The axial image shows severe compression of the spinal cord by the tumour. 1, 2 show sagittal and axial images, respectively. Gadolinium-enhanced T1 sagittal MRI. Gadolinium-enhanced axial MRI. A three-segment thoracic laminectomy revealed a tense dura due to swelling of the cord. A midline myelotomy was made through which white liquid matter started to extrude. An anteriorly located, deeply situated, dark brown gritty and vascular tumour was splitting the cord into two halves and could be separated from it. There was no connection with either the dura or spinal nerve roots. The tumour was completely excised. Microscopic examination revealed a typical schwannoma featuring partial encapsulation, areas comprising compact fascicles of spindle cells (Antoni A) alongside loosely textured (Antoni B) zones, primitive Verocay bodies and vascular hyalination (Figure 3). There was close proximity to an artery (Figure 4). Immunocytochemically, the tumour stained positively for S-100, and negatively for GFAP, EMA and CD34. Schwannoma featuring partial encapsulation, Antoni A and B zones, primitive Verocay bodies and vascular hyalination. Close relation of the tumour to an artery is shown. Haematoxylin–eosin (H&E); original magnification × 50. Post-operatively, the sensory level at D6 remained but the patient had no voluntary movement of either leg and was unable to open her bowels or bladder. Three weeks after operation, sensation began to return to the right leg and she recovered voluntary control of her toes on that side. Six months later, the right leg had regained grade 3 power. However, only toe movement was possible on the left. She continues with extensive rehabilitation at a specialist unit. Extramedullary schwannoma is relatively common and accounts for around 30% of all spinal neoplasms; intramedullary lesions, by contrast, account for less than 1% of all schwannomas [18], with only 38 cases reported since 1932. However, the incidence might be underestimated in view of the increased number of reported cases in recent decades (e.g. 17 within the 1970s). Whether the shift is due to advances in neuroradiology, anaesthaesia and neurosurgical technique or a biological phenomenon is unresolved [2]. The sex ratio of intramedullary schwannoma is equal (19 females and 20 males, including the present case). Topographically, intramedullary schwannomas decline in frequency on descending the neuraxis. In the present historical review 23 out of 39 were cervical, 10 thoracic and three lumbar. The remaining three were located at the cervicomedullary junction or pons and justify inclusion for their midline location and continuity with the spinal cord proper [1,11,12]. Regarding cross-sectional tumour location, 22 were located dorsally while the remainder were unspecified. The present case and that of Rout et al. [13], connected to the anterior spinal artery, therefore constitute the only documented instances of anteriorly located tumours, notwithstanding the ventrolateral example of Ross et al. [12]. Clinical presentation encompasses a wide range of symptoms including dysaesthesiae, weakness, and paresis and sphincter disturbance. However, patients commonly seek help because of pain. Although CT may be diagnostic [3]. MRI is now the examination of choice for demonstrating intramedullary schwannomas, which possess low or mixed signal on T1-sequences and high signal on T2-sequences. Nonetheless, they might still evade detection [4]. Twenty-one patients had total excision and 13 had subtotal excision of their tumours, while other diagnoses were made at post-mortem. Lack of long-term reported follow-up makes it difficult to comment on recurrence. However, the patient of Ross et al. [12] underwent subtotal excision and adjuvant radiotherapy with no recurrence at 6 months. In the case reported by Prakash et al. [10] radiotherapy was not given following subtotal excision and no signs of recurrence were found at 1 year. The origin of intramedullary schwannoma is problematic because schwann cells are not normal central nervous system (CNS) residents. Several hypotheses have been entertained. The first holds that schwann cells might have entered the spinal cord in reaction to chronic disease or trauma [12]. Antecedent trauma was not identified in our patient and the general lack of association between spinal trauma and intramedullary schwannoma argues against a strong aetiological link. The second is that the tumour arises from aberrantly located peripheral nerves that retain their original schwann cell investment [6]. Third, Ross et al. [12] quote Kernohan as proposing that intramedullary schwannoma might derive from nerve plexi surrounding blood vessels. Indeed, Rout et al. [13] described a case where the capsule was supplied by, and in Marchese and McDonald's case [7] came very close to, the anterior spinal artery. The present case also had a close vascular association. Nonetheless, the vascular plexus hypothesis on its own fails to account for the cervical and dorsal predominance that emerges from the present review. Fourth, Mason and Keigher [8] proposed that schwannomas arose where dorsal roots joined the pia matter, a so-called critical area. This seems plausible in the case of Solomon et al. [15], in which the tumour occurred near the C1 root entry zone, and in the case of Wood et al. [17], where microscopic continuity was observed. However, the apparent predilection for the cervical spinal cord is not explained [7,9,13] and as four roots enter at each segment we might again expect an even distribution. Furthermore, their presence in the brain and brainstem is also not explained. There might, however, be a different aetiology for each, as suggested by the wide difference in average age of each group, 22 years for intracranial lesions and 44 years for extracranial lesions [11]. Finally, Russell and Rubinstein [14] discuss the conversion of pia cells to schwann cells, with similarity between the two cell lineages. During development, neural crest cells could form ectopic rests that subsequently give rise to schwannomas and this could explain their ubiquity in the spinal cord and brain, although yet again failing to account for their cervical predilection. In the light of the present data and review, the conclusion must be drawn that the origin of these tumours is still obscure. In summary, it seems that total excision is the treatment of choice for these rare lesions, although long-term follow-up data have not been published so that it is not known whether subtotal excision is sufficient. Certainly, when considering surgery for intramedullary tumours, it is always worth considering the possibility of benign lesions such as schwannoma that might be curable if excised.