Approach
Night blindness can be caused by many conditions. It is important to establish the difference between blurred vision made worse by low light levels (e.g., due to cataracts) and true night blindness (i.e., loss of vision in a dark environment). A systemic approach to the history and examination can help narrow the differential diagnosis. Seek the opinion of an ophthalmic specialist because slit-lamp biomicroscopy and retinal electrophysiologic testing are often essential for diagnosis.
History
Note the onset and progression of symptoms, and specifically enquire about reduced vision during the daytime or any problems distinguishing color. A clear ophthalmic history can exclude conditions responsible for blurred vision at low light levels, such as previous refractive surgery.[23]
Past medical history should include any previous history of gastric or abdominal surgery, alcohol intake, and symptoms of intestinal malabsorption, such as diarrhea or steatorrhea (i.e., fatty stools). Also, take care to exclude a history of cancer treatment, especially malignant melanoma.
In the drug history, establish if the patient has taken any medications that can affect vitamin A metabolism, such as retinoic acid derivatives.[13]
Given that many conditions responsible for night blindness are inherited, always take a detailed family history. Referral to a clinical genetics service may help to establish the inheritance pattern where it is unclear. Establishing the mode of inheritance may give an indication of the prognosis.[3] Newer techniques, such as next-generation-sequencing, provide a much faster and more detailed analysis of DNA/RNA sequencing and mutation detection. Next-generation-sequencing can also be used, at the geneticist’s discretion, for more accurate testing of specific mutations associated with hereditary conditions responsible for night blindness.[24]
If the patient currently drives and you identify any safety concerns (e.g., exam indicates that minimum driving standards are not met), consider asking the patient to stop.
Opthalmic exam
Document the results of any ophthalmic exam, including the patient's visual acuity, pupillary responses, confrontational visual fields, and direct ophthalmoscopy, at the initial presentation. Take care to exclude any conditions that may cause blurred vision and that are made worse by low light levels (e.g., cataracts, myopia, glaucoma).
Refer for a specialist ophthalmic opinion, as slit-biomicroscopy and indirect ophthalmoscopy are essential parts of the exam. The patient may also require specialist referral for further investigations, such as electroretinography (ERG), to determine the underlying cause.
Common ocular manifestations of vitamin A deficiency include conjunctival xerosis, bitot spots, corneal ulceration, and keratomalacia, with the most severe forms of keratomalacia including corneal edema and corneal melt. The xerophthalmic fundus may present with small, white, deep retinal lesions throughout the posterior pole.[25]
Include a general medical exam to exclude systemic causes of night blindness, or to document associated findings if you suspect an inherited retinal condition.
Investigations
Investigations should be carried out in a specialist ophthalmic unit. ERG is a first-line investigation that can help narrow the differential diagnosis and guide referral for further investigation. Measurement of serum vitamin A may be indicated if the patient has a known risk factor for deficiency, or if ERG indicates deficiency as a possible underlying cause for night blindness. The pattern of electrical activity produced in the retina may indicate the cells affected by the underlying disease process. ERG shows the classic changes of vitamin A deficiency as loss of, or reduced, a- or b-wave amplitudes.[26] Congenital stationary night blindness can cause an electronegative ERG pattern.[27]
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