Persistent hypophyseal canal

Persistent hypophyseal canal, also known as the craniopharyngeal canal when larger than 1.5 mm in diameter, is a rare congenital defect characterised by a communication through the central skull base between the nasopharynx and the pituitary fossa. 

Terminology

There are a number of terms which refer to this abnormality and some authors distinguish between canals based on whether they are larger or smaller than 1.5 mm in diameter. To avoid confusion:

• a canal less than 1.5 mm in diameter
  • persistent hypophyseal canal
• a canal greater than 1.5 mm in diameter
  • craniopharyngeal canal
  • large craniopharyngeal canal
  • transphenoidal canal

The remainder of this article will refer to the defect as a persistent hypophyseal canal, keeping in mind that when large, alternative terms are preferable. 

Additionally, the larger defects are possibly more closely related to transsphenoidal meningoencephaloceles than to persistent hypophyseal canals ^2,4. 

Epidemiology

The prevalence of persistent hypophyseal canals has been reported to be as high as 0.42% ¹.  

Clinical presentation

In most cases, individuals with a persistent canal are asymptomatic. Possible presentations include ^1-3:  

• CSF rhinorrhea and intracranial hypotension
• pituitary gland prolapsing into nasopharynx ²
• pitutiary mass presenting as a nasopharyngeal mass

In addition, many craniofacial anomalies have been reported, especially with the larger defects, including ⁴:

• optic apparatus 
  • hypertelorism
  • abnormal optic tracts
  • malformed globes
• midfacial cleft and cleft lip and palate
• agenesis of the corpus callosum

Pathology / embryology

The exact cause of these defects is uncertain, as is whether or not they all share the same underlying aetiology. Two theories have been proposed: 

1. persistence of Rathke's pouch
2. persistence of a vascular channel

Radiographic features

Persistent hypophyseal canals, as well as the larger craniopharyngeal canals, are found in the midline, oriented vertically and extending form the floor of the pituitary fossa to the posterior wall of the nasopharynx, passing behind the sphenoid sinus.

CT

They are best appreciated on thin slice bone filter CT, and appear as a sharply circumscribed corticated canal. 

Differential diagnosis

• spheno-occipital synchondrosis: more inferior and horizontal
• transclival canal: off-midline, contains a vessel (the transclival artery)
• neurenteric cyst