Anatomy of the Nasolacrimal System

The purpose of the nasolacrimal system is to:
- Drain tears from the ocular surface to the lacrimal sac and, ultimately, the nasal cavity
- Blockage of the nasolacrimal system:
  - Can cause tears to flow over the eyelid and down the cheek:
    - This condition is epiphora
Structure and Function:
- Both the upper eyelid and the lower eyelid have a small opening on the surface of the eyelid margin near the medial canthus:
  - These are called puncta:
    - Each puncta leads to a drainage canal that eventually flows into the lacrimal sac and then the nasal cavity
- The drainage canal connecting the ocular surface to the nasal cavity consists of multiple parts:
  - Within the lower eyelid:
    - The punctum leads to a 2 mm long ampulla:
      - Which runs perpendicular to the eyelid margin
    - The ampulla turns 90 degrees medially:
      - Becoming the inferior canaliculus and travels 8 to 10 mm before reaching the common canaliculus
    - The upper canaliculus travels 2 mm superiorly in the eyelid before turning 90 degrees medially and moving 8 to 10 mm before connecting to the common canaliculus
  - The common canaliculus:
    - Drains into the lacrimal sac
  - Within the junction between the common canaliculus and the lacrimal sac:
    - Is the valve of Rosenmuller:
      - This apparatus is a one-way valve that prevents reflux from the lacrimal sac to the puncta
- The lacrimal sac drains:
  - Inferiorly to the nasolacrimal duct:
    - Which is bordered:
      - Medially by:
        Palatine bone and the inferior turbinate in the nose
      - Laterally by:
        Maxillary bone
  - The nasolacrimal duct:
    - Opens at the inferior meatus:
      - Located underneath the inferior nasal turbinate
  - The lacrimal sac is:
    - Approximately 10 to 15 mm in axial length and 13 to 20 mm in corneal length
  - The nasolacrimal duct is:
    - 12 to 18 mm long
  - The inferior nasal meatus is partially covered by a mucosal fold:
    - Known as the valve of Hasner
Embryology:
- The nasolacrimal duct:
  - Starts forming around five weeks of gestation
  - It starts out as a linear thickening of ectoderm:
    - Located in a groove between the nasal and maxillary prominences
  - This thickening:
    - Eventually separates into a solid cord and sinks into the surrounding mesenchyme
    - Over time the cord canalizes:
      - Forming the lacrimal sac and the beginning of the nasolacrimal duct
  - The nasolacrimal duct extends:
    - Intranasally until it exits under the inferior turbinate
  - The lacrimal sac extends caudally:
    - To complete the canalicular system
  - The inside of the canal breaks down and forms a lumen:
    - So that the nasolacrimal system is patent:
      - This process is generally complete by the time of birth
Blood Supply and Lymphatics:
- Blood supply to the nasolacrimal area of the face:
  - Is generally from the angular artery:
    - The angular artery is considered a branch of the facial artery:
      - However, some studies have shown that it can originate from the ophthalmic artery in some individuals
    - It terminates in anastomosis with the dorsal nasal branch of the ophthalmic artery
    - The angular artery and vein:
      - Appear alongside the nose near the medial orbit
    - A correlating angular vein drains this region
- The medial and lateral portions of the eyelids have different lymphatic drainage systems:
  - The medial one-third of the upper eyelid and the medial two-thirds of the lower eyelid:
    - Drain to the submandibular lymph nodes
  - The lateral two-thirds of the upper eyelid and the lateral one-third of the lower eyelid:
    - Drain to the pre-auricular lymph nodes
Nerves:
- Cranial nerve VII:
  - Supplies the motor innervation to the muscles of the face
- The movement of these muscles:
  - Aid in proper drainage of the tears through the nasolacrimal system:
    - By what is known as the lacrimal pump mechanism
- Cranial nerve III and cranial nerve VII:
  - Innervate the muscles that control the blinking of the eyelids:
    - This action is the primary driver of the lacrimal pump mechanism
- Irritation of the ocular surface:
  - Stimulates the ophthalmic branch of cranial nerve five:
    - Which begins the reflex tear arc pathway:
      - The efferent pathway involves cranial nerve VII and parasympathetic fibers
    - The role of the sympathetic nervous system in tear production:
      - Is not well understood
Muscles:
- The action of the orbicularis muscle and surrounding tissues:
  - Helps propel the flow of tears from the canaliculi to the nasolacrimal duct:
    - Via the lacrimal pump mechanism
References:
- Computed tomography dimensions of the lacrimal gland in normal Caucasian orbits., Tamboli DA,Harris MA,Hogg JP,Realini T,Sivak-Callcott JA,, Ophthalmic plastic and reconstructive surgery, 2011 Nov-Dec.
- An Unusual Case of Nasolacrimal Obstruction Caused by Foodstuffs., Matsumoto H,Matsumoto A,, Case reports in ophthalmology, 2015 Sep-Dec.
- Lacrimal Gland Volume Changes in Unilateral Primary Acquired Nasolacrimal Obstruction., Yazici A,Bulbul E,Yazici H,Sari E,Tiskaoglu N,Yanik B,Ermis S,, Investigative ophthalmology & visual science, 2015 Jul.
- Incidence of neoplasia in patients with unilateral epiphora., Bewes T,Sacks R,Sacks PL,Chin D,Mrad N,Wilcsek G,Tumuluri K,Harvey R,, The Journal of laryngology and otology, 2015 Jul.
- Ducasse A,Arndt C,Brugniart C,Larre I, [Lacrimal traumatology]. Journal francais d’ophtalmologie. 2016 Feb.
- Modified External Dacryocystorhinostomy in Primary Acquired Nasolacrimal Duct Obstruction., Sharma HR,Sharma AK,Sharma R,, Journal of clinical and diagnostic research : JCDR, 2015 Oct.