Beware of the midline scalp lump

The scalp is a common site for benign skin lesions. When excision is contemplated, careful attention must be paid to those in the midline, especially in children.

CASE HISTORIES

Case 1

A boy aged 13 months was admitted to his local hospital for excision of a ‘sebaceous cyst’ from the back of his head. During the procedure the general surgical team noted a cerebrospinal fluid (CSF) leak, closed the wound and sent him across to the Birmingham Children's Hospital for further treatment. He arrived intubated and ventilated. A CT scan demonstrated a dermoid sinus extending through the occipital bone to the cervico-medullary junction (Figure 1). At operation the original transverse incision was opened to reveal a 0.5-1.0 cm defect in the occipital bone. All abnormal tissue was removed from this area. The deficit was closed with a pericranial patch reinforced with Tissel fibrin glue and muscle to achieve a watertight seal. The patient recovered well and the CSF leak did not recur.

Figure 1.

T2-weighted axial MR scan of the first patient

Case 2

An 18-month-old girl was taken to her local hospital with increasing drowsiness, vomiting and diarrhoea. There had been an episode of jerking movements associated with eye rolling. She had previously been developing normally, but her family doctor had been consulted about a small discharging scalp lesion on the back of her head. This had been treated with topical antibiotics intermittently for several months. On examination she was drowsy although when woken was very irritable and there was a suggestion of discomfort on moving her neck. She was otherwise neurologically intact. A discharging sinus was noted between the hairline and the external occipital protuberance. Her white cell count was 29.7×10⁹/L. CT scan showed dilatation of the lateral, third and fourth ventricles with meningeal enhancement and she was transferred to our department. An MRI scan then revealed a sinus and inflammatory mass extending through the suboccipital tissues into the posterior aspect of the spinal canal down to the underlying dorsal aspect of the cervico-medullary junction (Figure 2). The posterior elements of C1 and C2 were deficient, with bulging of thickened dura through the defect. There was abnormal enhancement surrounding the upper cervical cord, basally in the posterior fossa and extending into the 4th ventricle. Incidental findings were a deficient septum pellucidum and a closed lip schizencephaly in the region of the left central sulcus.

Figure 2.

T1-weighted sagittal MR scan of second patient, obtained after intravenous injection of gadolinium contrast agent

The patient had an external ventricular drain inserted. CSF from the drain contained 3200 white cells per μL and grew Staphylococcus aureus. She was treated with intravenous cefotaxime and rifampicin as well as intrathecal vancomycin. After twelve days the CSF had cleared sufficiently to allow us to proceed with excision of her dermoid sinus. Subcutaneous dissection was performed down to the dura, which was then opened above and below the sinus tract. This revealed an organised abscess containing hair. The entire subarachnoid space over the cerebellar tonsils and upper cervical cord was covered with white granulation tissue. This was debrided and washed out as thoroughly as possible. After a further fourteen days the external ventricular drain was converted to a ventriculoperitoneal shunt. She recovered well and went home after one month in hospital.

COMMENT

Congenital dermoid sinuses of the scalp are a form of craniospinal dysraphism. During the fourth week of gestational life the ectodermal neural plate begins to fold in on itself to form the neural tube, the cranial end closing by day 24 and the caudal end by day 26. Dermoid sinuses are the result of focal segmental adhesions between the superficial and neural ectoderm.¹ Thus, displaced cellular elements with cutaneous commitments become trapped between the dermis and the developing neural tube.² The result is a persisting tract lined by stratified squamous epithelium.

Dermoid sinuses may occur anywhere in the midline from the nasion to the coccyx but have not been reported in the region occupied by the superior sagittal sinus.³ They are most frequently found in the lumbosacral area. On the head the commonest location is the external occipital protuberance (85%) followed by the nasion (11%) and the posterior parietal area (5%).¹ In the cervical region they are very rare.⁴ Dermoid sinuses are often associated with dermoid or epidermoid cysts, which can occur at any point along the tract but are usually at the terminus. The depth to which they penetrate can vary. Those in the nasal area can reach the crista galli and in occipital lesions the fistula extends to the dura mater in 17% of cases, to the subdural space in 32%, to the cerebellum in 44% and to the fourth ventricle in 7%.² In the spine they may reach the conus medullaris or the central canal of the spinal cord.

In general, cases present in one of three ways: the lesion is picked up in a symptomless individual; there is ingress of microbes into the tract causing meningitis or abscess; or an intracranial or intraspinal cyst begins to produce mass effect. Infection is the most frequent mode of presentation.¹ The average age of presentation is reported as 2.5 years, with an excess incidence in females.¹

For more than two decades the occipital ‘sebaceous’ cyst has been recognized as a trap for the unwary.⁵ Any midline lesion of the scalp should raise suspicion of intracranial extension. Detailed imaging is required, followed by prompt referral for neurosurgical evaluation.