Keratoconus – 圓錐角膜

What is keratoconus?

Keratoconus is an eye condition in which the normal, round-shaped cornea becomes thin and bulges outward to form a cone shape (changing from the shape of an orange to the shape of a lemon). Keratoconus usually affects both eyes, but it may affect one eye more than the other initially. It usually progresses slowly over several years, but it can also worsen suddenly. Keratoconus can manifest at different ages, from childhood up to the age of about 40 years.

What symptoms does keratoconus cause?

The signs and symptoms of keratoconus vary with the progression of the disease. Early symptoms include blurring of vision. Halos (rings seen around lights), difficulty with night vision and glare. A patient with keratoconus may notice frequent changes in their spectacle prescription especially the degree of astigmatism. Rarely, patients may experience sudden clouding of vision (hydrops).

What are the causes of keratoconus?

The exact cause of keratoconus is not clearly understood. This condition has an inherent genetic component, and it can certainly be passed on from one generation to the next. It is thought to be due to weakening of collagen fibres, which maintain the shape of the cornea. There is a strong association between keratoconus and excessive eye rubbing, and hence people with allergic eye disease are strongly advised to stop eye rubbing. Also, keratoconus is more commonly seen in patients with certain medical conditions, such as Down’s syndrome, Ehlers-Danlos syndrome and atopic dermatitis.

What is the best treatment for keratoconus?

When you present with any of the above symptoms, you should consult a corneal specialist (an ophthalmologist with subspecialty interest in corneal diseases), who will review your medical history, conduct a thorough eye examination, and order a few diagnostic tests to measure the corneal curve to evaluate the projection and shape of your eyes.

The current treatment for keratoconus emphasizes stabilising the disease and improving the vision. It is essential to recognise that the treatment of keratoconus should be personalised for individual patient. Your ophthalmologist should be able to suggest appropriate treatment options based on the severity and progression of your condition, your work and lifestyle.

The mainstay of treatment for keratoconus is to stop eye rubbing. As mentioned, it has been shown that excessive eye rubbing is strongly associated with the progression of keratoconus. Patients with keratoconus must stop rubbing their eyes at all time, and they should receive appropriate medical therapy for any coexisting allergic diseases. For visual improvement, glasses and soft contact lenses should be prescribed in the first instance. However, specialised contact lenses (rigid gas permeable or semi-scleral contact lenses) are usually needed in the majority of cases to neutralise the irregular corneal astigmatism.

What is corneal collagen crosslinking?

In recent years, corneal collagen cross linking has been utilised to halt the progression of the disease by stiffening the collagen fibres within the cornea. In addition, this procedure may help to improve patients’ vision, although this beneficial effect is not always predictable.

The primary aim of this procedure is to allow eye drops that are rich in vitamin B2 (riboflavin) to diffuse into the cornea. After adequate absorption of the eye drops, the patient is positioned with the UV light at a short distance from the cornea for 30 minutes. A bandage contact lens will then be inserted and the eye will be patched for 24 hours.

The full effect of corneal collagen cross linking may take 6 months or longer to settle, and not all patients with keratoconus are suitable to undergo this procedure. Your ophthalmologist will evaluate your condition before discussing the best treatment options for you.

What other surgical treatments are available?

A corneal transplant may be required in extreme or end-stage keratoconus. Traditionally, a Penetrating Keratoplasty (PK) will be performed in which the full thickness of the patient’s cornea will be replaced by a donor corneal graft. Although the success rate of PK is very high, the postoperative recovery time is relatively long and sometimes it may take years to achieve functional vision in the operated eye. Also, there is a higher risk of graft rejection compared with other types of corneal transplantation. The latest surgical treatment of choice for keratoconus is Deep Anterior Lamellar Keratoplasty (DALK). Instead of replacing the full thickness of the cornea, only the top 80% of the cornea is replaced. The bottom two layers (Descemet’s membrane and endothelium) of the patient’s cornea are preserved as these layers are not affected by the disease process. This technique may shorten the recovery time and reduce the risk of graft rejection.