Sclera

The sclera (plural: scleras or sclerae) is the fibrous, opaque white, coat of the eye. It functions to protect the intra-ocular contents. 

Summary

• location: posterior fifth-sixths of the eyeball
• blood supply: ciliary arteries
• innervation: ciliary nerves
• relations: anteriorly continuous with the cornea

Gross anatomy

Outermost coat of the 3-layered globe. Anteriorly continuous with the cornea. Posteriorly perforated by the optic nerve at the lamina cribrosa. 

Arterial supply

Anteriorly, the anterior ciliary arteries form a dense episcleral plexus. Posteriorly, supplied by branches of the short and long posterior ciliary arteries. 

Lymphatic drainage

The sclera is an alymphatic organ.

Innervation

Anteriorly supplied by the long ciliary nerves. Posteriorly supplied by the short ciliary nerves. 

Histology

The sclera may be divided into its three histological layers:

1. episclera 
   • outermost, loose connective tissue layer, connected to the Tenon's capsule 
   • rich vascular supply from anterior ciliary arteries 
2. scleral stroma 
   • dense fibrous tissue intermingled with fine elastic fibers
   • irregular arrangement of type I and III collagen responsible for its opacity
3. lamina fusca 
   • innermost layer with melanocytes 
   • weakly related to the choroid 

Radiographic features

Ultrasound and CT are not useful modalities. 

Anterior segment ocular coherence tomography (AS-OCT) 

• readily used in clinical practice to measure sclera thickness, monitor therapeutic response in scleritis treatment
• quantitative measurements obtained 

MRI

• useful in the diagnosis of posterior scleritis and episcleritis 
• T1-weighted sequences, sclera appears as the hypointense rim of the globe 

History and etymology

Medical Latin, from Greek sklera (menix) "the hard (membrane)," fem. of skleros "hard".

Related pathology

• scleral plaques
• episcleritis/scleritis: inflammation of the sclera
• staphyloma: full or partial thickness sclera defect lined by uvea