The Orbit and Adnexa
The adnexa refers to the accessory structures of the eye: the eyelids, conjunctiva, lacrimal system, and orbit. These structures protect the globe, maintain the tear film, and enable eye movement. As a CPOA, you frequently assess and interact with these structures during patient intake, slit lamp exams, and procedures involving drops or probing.
The Bony Orbit
The orbit is a pyramid-shaped bony cavity that protects the eyeball and contains fat, muscles, nerves, and vessels. It is formed by seven bones:
- Frontal (roof)
- Zygomatic (lateral wall)
- Maxillary (floor)
- Lacrimal (medial wall)
- Ethmoid (medial wall)
- Sphenoid (posterior wall/apex)
- Palatine (small posterior contribution)
💡 Clinical Tip: The medial wall is very thin (the lamina papyracea of the ethmoid bone). It is the most common site of blowout fractures from blunt orbital trauma. The floor (maxillary bone) is the second most common fracture site. Patients with blowout fractures may have enophthalmos, diplopia on upgaze (inferior rectus entrapment), or infraorbital anesthesia.
Orbital Openings
The optic canal transmits the optic nerve (CN II) and ophthalmic artery. The superior orbital fissure transmits cranial nerves III, IV, V1, and VI (plus the superior ophthalmic vein). The inferior orbital fissure carries the infraorbital nerve (a branch of V2).
Eyelid Anatomy
The upper and lower eyelids protect the eye and distribute the tear film via blinking. The eyelid consists of:
- Skin (outermost, thinnest skin in the body)
- Orbicularis oculi muscle (closes the eyelid; innervated by CN VII -- facial nerve)
- Tarsal plate (dense fibrous skeleton providing structural rigidity; contains Meibomian glands)
- Conjunctiva (innermost layer lining the eyelid inner surface)
The upper eyelid is elevated by the levator palpebrae superioris (innervated by CN III) and the Muller's muscle (sympathetic innervation). Ptosis (drooping eyelid) results from dysfunction of either structure.
| Muscle | Function | Innervation |
|---|---|---|
| Orbicularis oculi | Closes eyelid | CN VII (facial) |
| Levator palpebrae | Opens upper eyelid | CN III (oculomotor) |
| Muller's muscle | Assists eyelid opening | Sympathetic |
Eyelid Margin Structures
- Meibomian glands -- in the tarsal plate, secreting the lipid layer of the tear film. Blocked glands cause chalazia.
- Glands of Zeis and Moll -- modified sebaceous and sweat glands at the lash base. Infections cause external hordeola (styes).
- Lash line (cilia) -- eyelashes that help trap debris
- Puncta -- small openings near the medial canthus that drain tears into the lacrimal system
The Conjunctiva
The conjunctiva is a thin, transparent mucous membrane that lines the inner surface of the eyelids (palpebral conjunctiva) and covers the anterior sclera up to the limbus (bulbar conjunctiva). The conjunctival fornix is the fold where palpebral and bulbar conjunctiva meet.
The conjunctiva contains goblet cells that produce the mucin layer of the tear film. Inflammation (conjunctivitis) causes redness, discharge, and chemosis (conjunctival swelling).
⚠️ Common Mistake: Bulbar conjunctival redness concentrated near the limbus (ciliary flush or perilimbal injection) suggests intraocular inflammation (iritis/uveitis), not conjunctivitis. Conjunctivitis redness is diffuse and typically more pronounced in the fornices.
The Lacrimal System
The lacrimal system produces and drains tears. It has two parts: secretory and excretory.
Secretory (Tear Production)
- Lacrimal gland -- in the superolateral orbit, produces the aqueous (watery) layer of tears
- Goblet cells -- in conjunctiva, produce the mucin layer
- Meibomian glands -- produce the lipid (oily) layer that prevents evaporation
Tear Film Layers
| Layer | Source | Function |
|---|---|---|
| Lipid (outer) | Meibomian glands | Prevents evaporation |
| Aqueous (middle) | Lacrimal gland | Oxygen, nutrients, antibodies |
| Mucin (inner) | Goblet cells | Allows tear spreading on epithelium |
Excretory (Tear Drainage)
Tears drain from the medial conjunctival surface through the upper and lower puncta, into canaliculi, into the lacrimal sac, down the nasolacrimal duct, and into the nasal cavity (inferior meatus). This is why eye drops can cause systemic absorption -- they drain into the nose and are absorbed through nasal mucosa. Punctal occlusion (finger pressure at the medial canthus) after drop instillation reduces systemic absorption.
Extraocular Fat and Septum
Orbital fat cushions the globe. The orbital septum is a fibrous membrane running from the orbital rim to the eyelid, dividing preseptal (periorbital) from postseptal (orbital) tissues. This is critical clinically: preseptal (periorbital) cellulitis can be managed differently from orbital cellulitis (postseptal), which threatens vision and the CNS.
Key Takeaways
- The orbit is formed by 7 bones; medial wall (ethmoid) is most fragile and prone to blowout fracture
- The levator (CN III) and Muller's muscle (sympathetic) open the upper eyelid; orbicularis (CN VII) closes it
- Meibomian glands produce the tear film lipid layer; blockage causes chalazia
- Tears drain through puncta, canaliculi, lacrimal sac, and nasolacrimal duct into the nose
- Punctal occlusion after drop instillation reduces systemic absorption
- Perilimbal injection suggests uveitis; diffuse injection suggests conjunctivitis