Microbial keratitis (Acanthamoeba sp.)

The CMGs are guidelines on the diagnosis and management of a range of common and rare, but important, eye conditions that present with varying frequency in primary and first contact care.

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Microbial keratitis (Acanthamoeba)

 

 

 

 

 

 

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Aetiology

Acanthamoebae are ubiquitous free-living protozoans, present in:

  • well water, drains, soil, dust
  • may be present in domestic tap water (especially from storage tanks)

Can exist in two forms

  • motile, feeding and replicating form: trophozoite (most common form found in water and easily destroyed)
  • dormant form: cyst (highly resistant to disinfection, can survive for long periods in hostile environments)

Acanthamoeba keratitis is rare in the general population (estimated annual incidence: 1.4 per million per annum) but much commoner in contact lens wearers

In the UK, higher incidence in hard water districts and where bathrooms are supplied by tank-stored water

In 10% of cases there is associated scleritis. Acanthamoeba sclerokeratitis is associated with a poor clinical outcome

Predisposing factors

Contact lens wear is associated with >90% cases of Acanthamoeba keratitis

  • majority are soft lenses (particularly reusable or extended wear)
  • inadequate disinfection
  • use of non-sterile solutions
  • tap water rinsing of lenses and/or storage case
  • contamination of storage case with bacteria and fungi (± biofilm) which provide substrate for Acanthamoebae
  • exposure to shower, pool, or hot tub water

Agricultural injuries

Symptoms

  • pain (may be severe and out of proportion to degree of ocular inflammation; may also be painless in the early stages)
  • visual loss
  • redness
  • epiphorapPhotophobia

Can be bilateral

May be a long history and condition may be misdiagnosed as herpetic, bacterial or fungal keratitis

NB in earliest stages, pain may be minimal

Signs

Early signs

  • epithelial or subepithelial infiltrates
  • pseudodendrites
  • radial keratoneuritis (infiltrates along corneal nerves)
  • recurrent breakdown of the corneal epithelium

Later signs

  • deep inflammation of the cornea consisting of a central or paracentral ring-shaped or disciform infiltrate or abscess
  • stromal thinning
  • extension of inflammation into sclera
  • anterior chamber cells and flare
  • hypopyon

Differential diagnosis

Signs may masquerade as herpes simplex with temporary improvement on anti-herpetic treatment, further delaying diagnosis

Suspect any painful epitheliopathy that:

  • does not respond to normal treatment 
  • has known risk factor (e.g. contact lens wear or corneal trauma associated with soil or contaminated water)

(NB: Dendritic keratitis in a contact lens wearer should raise the index of suspicion of an Acanthamoeba infection)

Bacterial or fungal keratitis 

Concurrent bacterial or fungal infection can occur

Management by optometrist

Practitioners should recognise their limitations and where necessary seek further advice or refer the patient elsewhere

GRADE* Level of evidence and strength of recommendation always relates to the statement(s) immediately above

Non pharmacological

Cease contact lens wear immediately (both eyes)

(GRADE*: Level of evidence=low, Strength of recommendation=strong)
 

Pharmacological

None

Management category

A1: emergency (same day) referral to ophthalmologist without intervention. Acanthamoeba keratitis can be difficult to treat; therefore prompt, aggressive therapy is vital. Delay is associated with a poorer prognosis. Emphasise to the patient the urgency of the condition and instruct them to attend the local hospital eye department or hospital A & E the same day, explaining that you will leave a message so that they are expected. Telephone the department to explain what you have done, preferably leaving your message with a doctor or other health care professional. Advise patient to take lenses and lens case for possible culture

Possible management by ophthalmologist

Diagnosis will usually be confirmed by histology (corneal scrape) and culture or PCR. Cystic form can also be imaged by confocal microscopy

Intensive (day and night) topical medical treatment with either a biguanide or a diamidine or a combination of the two:

Biguanides: polyhexamethylene biguanide (PHMB), chlorhexidine

Diamines: propamidine (Brolene), dibromopropamidine (Brolene ointment), hexamidine

Continuous treatment may be necessary for weeks or months

(A recent systematic review identified a paucity of evidence to inform robust conclusions for treating AK in practice)

Systemic analgesia as necessary

Topical steroid to limit inflammation

Topical antibiotics as necessary for secondary bacterial infection

Penetrating keratoplasty if corneal irregularity, thinning and/or scarring is severe following complete control of infection

Evidence base

*GRADE: Grading of Recommendations Assessment, Development and Evaluation (www.gradeworkinggroup.org)
 

Sources of evidence

Alkharashi M, Lindsley K, Law HA, Sikder S. Medical interventions for acanthamoeba keratitis.Cochrane Database Syst Rev. 2015;2:CD010792

Carnt N, Stapleton F. Strategies for the prevention of contact lens-related Acanthamoeba keratitis: a review. Ophthalmic Physiol Opt. 2016;36(2):77-92

Carrijo-Carvalho LC, Sant'ana VP, Foronda AS, de Freitas D, de Souza Carvalho FR. Therapeutic agents and biocides for ocular infections by free-living amoebae of Acanthamoeba genus. Surv Ophthalmol. 2017;62(2):203-218

Iovieno A, Gore DM, Carnt N, Dart JK. Acanthamoeba Sclerokeratitis. Ophthalmology 2014;121:2340-7

Ross J, Roy SL, Mathers WD, Ritterband DC, Yoder JS, Ayers T, Shah RD, Samper ME, Shih CY, Schmitz A, Brown AC. Clinical characteristics of Acanthamoeba keratitis infections in 28 states, 2008 to 2011.  Cornea. 2014;33(2):161-8

Lay summary

Acanthamoeba is a protozoan (single-celled organism) that is very widespread throughout the environment, especially where there is standing water, and it may be present in inadequately treated tap water. It has two forms: the trophozoite, which is active, capable of feeding, moving and reproducing, and which is easily destroyed; and the cyst, which is dormant and difficult to destroy. Acanthamoeba can change between these two forms, depending on whether it is in a favourable or a hostile environment.

Acanthamoeba is normally harmless to humans, but if it is transferred to the eye on a contaminated contact lens it can infect the cornea (the clear window at the front of the eye). Such infections can be difficult to treat owing to the lack or non-availability of anti-amoebic drugs. It is far better to prevent the infection by the use of effective contact lens hygiene, in particular avoiding contact of the lens and lens case with tap water.

Patients with early Acanthamoeba keratitis usually complain of discomfort, redness and light sensitivity of the affected eye. In the later stages the eye can become very painful as the nerves and deeper parts of the cornea become affected.

An optometrist who suspects such an infection is advised to refer the patient as an emergency (same day) to the ophthalmologist, who will try to confirm the diagnosis and then prescribe special eye drops given day and night. Often the patient will be admitted to hospital. If there is much scarring of the cornea following the eventual elimination of the infection, and vision is badly affected, a corneal transplant may be recommended.

Microbial keratitis (Acanthamoeba sp.)
Version 11
Date of search 20.10.17
Date of revision 29.03.18
Date of publication 10.05.2018
Date for review 19.10.19
© College of Optometrists

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