Introduction

Fontanelles are gaps covered by a fibrous membrane that lie between the cranial bones at the intersection of the cranial sutures, which correspond to the junctions between the cranial bones.¹ They serve as the major sites of bone expansion during postnatal cranial growth.¹ The bregmatic fontanelle is located between paired frontal and parietal bones. In many breeds, incomplete ossification at this level is considered a normal feature at birth, and it closes in a few days or weeks. However, in toy breeds like the Chihuahua, this bregmatic fontanelle may persist in adult dogs.²

Brain herniation refers to a shift of brain structures from their normal position.³ Cerebral herniation is a potentially life-threatening condition. It may cause brain pressure necrosis or compression of cranial nerves and vessels. Compression of vessels can cause hemorrhage or ischemia. It may also obstruct the normal circulation of cerebrospinal fluid, resulting in hydrocephalus.⁴ This case report is the first case of surgical management of a traumatic brain herniation through the bregmatic fontanelle in a dog.

Case Report

A 1 yr old, 2.4 kg, female Chihuahua was presented for a traumatic skull injury. The dog had been attacked 2 days before by another dog. A focal subcutaneous swelling at the level of the skull was immediately observed following the trauma. On physical examination, the dog had evidence of a mass effect on the dorsal aspect of the skull. There was no evidence of an open wound at this level. The heart rate was 136 beats/min with a normal respiratory rate and temperature. The systolic blood pressure was measured at 140 mm Hg with Doppler. On neurologic examination, the dog had slightly depressed mentation. No cranial nerve deficits were observed. The dog was ambulatory with normal spinal reflexes. The modified Glasgow Coma Score assigned was 18. Serum biochemistry and complete blood count were within normal limits.

Computed tomography (CT) scan of the head and the cervical region was performed under general anesthesia to better assess the mass effect observed. Images were acquired using a 64-slice CT scanner^a with a 120 KVp and 185 mAs technique, before and after IV administration of iohexol^b at the dose of 600 mg iodine/kg. A single, sharply demarcated, persistent fontanelle was visible at the level of the frontoparietal and interfrontal sutures, measuring up to 1.2 cm in length and 1.1 cm in width. No evidence of traumatic bone fracture was associated with this congenital bone defect. At the level of the open fontanelle, there was a focal hypoattenuating swelling deforming the cutaneous surface over an area of about 1.7 cm in length, 2.3 cm in width, and 0.7 cm in height. Contrast injection revealed a continuity, based on blood vessels, between the right parietal lobe and the subcutaneous swelling described through the open fontanelle, consistent with a transcalvarial brain herniation (Figure 1). The congenital bone defect allowed the brain to expand with mild constriction giving the swollen herniated brain a “mushroom cap” appearance previously described in human literature.⁴ The brain parenchyma passing through the cranial defect remained hypoattenuating to normal brain tissue, and a poorly defined hypoattenuating area associated with a mild left-sided midline shift was also visible in the intracranial fronto-parietal region, suggestive of associated vasogenic edema. A mild caudal transtentorial herniation and a mild caudal cerebellar herniation were visible. These findings raised the suspicion of intracranial hypertension, although a congenital origin remained possible and could not be ruled out. Subcutaneous swelling of the face was also noted, particularly at the level of the frontal bone, indicating extensive subcutaneous edema. Multiple congenital anomalies related to the patient’s breed were also visible, such as aplasia of frontal sinuses, domed skull, occipital dysplasia, and a partial empty sella. There was moderate dilatation of the ventricular system, including dilation of the caudodorsal recess of the third ventricle, consistent with supracollicular fluid accumulation, probably congenital in origin.

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A surgical treatment was elected. The dog was placed in sternal recumbency with the head elevated with a towel. An incision centered over the hernia was performed. The subcutaneous tissue was dissected and reflected away from the skull. The brain hernia was identified, but the membrane normally covering the fontanelle was not clearly visualized. The brain at this level was edematous and with an associated hematoma. The hernia was reduced by applying delicate pressure with a periosteal elevator. An absorbable hemostat (Surgicel^c) was then placed at the level of the defect. A precut hexagonal piece of polypropylene mesh (Prolene^d) was then used to cover the defect (Figure 2). The mesh was fixed to the fascia of temporalis muscle using simple interrupted sutures of 3-0 polydioxanone (PDS II^e). The subcutaneous layer was closed using 3-0 polydioxanone in a simple continuous pattern and the skin was closed in an intradermal pattern with 4-0 polyglecaprone (Monocryl^f). A tie-over bandage was then placed to cover the surgical wound.

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The dog recovered uneventfully from anesthesia. Postoperatively, pain management, IV fluids, and antibiotic therapy were continued. The neurologic examination was unremarkable the day after the surgery. The dog was discharged 2 days after the procedure. Prednisolone^g was prescribed for 5 days (0.5 mg/kg per os q 24 hr) and amoxicillin-clavulanic acid^h for 10 days (20mg/kg per os q 12 hr).

The tie-over bandage was removed 1 wk after surgery by the primary veterinarian. Clinical follow-up at 6 wk revealed no concerns or complications. At a 6 mo follow-up phone call with the owner, it was reported that the dog continued to be neurologically appropriate.

Discussion

Five major types of brain herniation are described in the literature. It includes subfalcine herniation, rostral transtentorial, caudal transtentorial herniation, herniation at the foramen magnum, and transcalvarial hernia.⁵ In transcalvarial hernia, the brain herniates through a skull defect (most often following a fracture or at the level of a craniectomy site). Our patient’s hernia is a type of transcalvarial herniation and was first explained by a rupture of the membrane located at the level of the bregmatic fontanelle. Then the edematous brain exited through the defect, corresponding to the path of least resistance.

Clinical signs classically associated with brain herniation include altered consciousness accompanied by changes in pupil size and responsiveness to light (in the case of caudal transtentorial herniation) and altered cardiorespiratory patterns (in the case of foramen magnum herniation).³ In our patient, only a slightly depressed mentation was observed. The lack of major clinical signs in our patient may have been due to three reasons: The volume of the herniated brain was small. The location of the hernia involved the parietal lobe, which is primarily responsible for processing sensory input. Finally, the hernia could have attenuated the increase in intracranial pressure following the trauma, which could have prevented the worsening of the transtentorial hernia and the hernia of the foramen magnum.

Our patient prognosis can be refined using the modified Glasgow Coma Score.⁶ The score was 18 on presentation, suggesting a good prognosis.

The presence of cerebral parenchyma just beneath the skin requires surgical management to avoid further brain damage. When the defect in the skull is small, the brain may herniate with a “mushroom cap” appearance as was seen in our patient. This constriction can cause compression of cortical veins, leading to venous infarction and contusion of the brain.⁴ If the defect is small compared to the volume of herniated brain tissue, the defect may need to be widened before the hernia is reduced. In our case, the reduction was relatively easy and did not require enlargement of the fontanelle.

The Chihuahua is a breed known for the frequent presence of a persistent bregmatic fontanelle.² It is generally clinically irrelevant.¹ Kiviranta et al¹ studied the prevalence of bregmatic fontanelle in Chihuahuas and whether it is accompanied by other persistent fontanelles. They concluded that, in this group of Chihuahuas (n = 50), 92% had had either one or several persistent fontanelles. No other persistent fontanelle was observed on CT in our patient.

The use of various surgical materials has been described for skull defect reconstruction. These materials include autologous bone graft, calvarial allografts, acrylic cranioplasty (particularly polymethylmethacylate), and metallic mesh.⁷ Given the size of the bone defect, the authors deemed that an extremely rigid material was not necessary. This was the main reason of the selection of the polypropylene mesh. The piece of precut mesh used was relatively large compared to the defect, but this made it easier to secure into the fascia of the temporalis muscle.

CT has been described as the method of choice for patients with traumatic brain injury.⁸ It permits evaluation of bony structures and is a good modality to assess for acute hemorrhage.⁸ Moreover it is faster than performing MRI, which is an advantage when a shorter anesthesia is required for neurologic or unstable patients. However, MRI provides a better assessment of the brain parenchyma and can give valuable information with regard to prognosis.⁹ In our case, CT was the only imaging modality available.

The Chihuahua has been described as a breed with an above-average aggression score toward humans and other dogs.¹⁰ This breed tendency, along with the delayed closure of the bregmatic fontanelle, may explain why this specific type of herniation occurred in our patient.

Conclusion

This case report is the first to describe the advanced imaging findings and successful surgical management of a traumatic brain herniation through the bregmatic fontanelle in a dog. It highlights the possibility of brain herniation in small breed dogs with persistent fontanelles and without skull fracture.