The clinical diagnosis is backed up by investigative findings. Citrulline level in blood is decreased. Mitochondrial studies or NARP mtDNA evaluation plays a role in genetic diagnosis which can also be done prenatally.

A diagnosis of choroideremia can be made based on family history, symptoms, and the characteristic appearance of the fundus. However, choroideremia shares several clinical features with retinitis pigmentosa, a similar but broader group of retinal degenerative diseases, making a specific diagnosis difficult without genetic testing. Because of this choroideremia is often initially misdiagnosed as retinitis pigmentosa. A variety of different genetic testing techniques can be used to make a differential diagnosis.

An accurate diagnosis of retinitis pigmentosa relies on the documentation of the progressive loss photoreceptor cell function, confirmed by a combination of visual field and visual acuity tests, fundus and optical coherence imagery, and electroretinography (ERG),

Visual field and acuity tests measure and compare the size of the patient's field of vision and the clarity of their visual perception with the standard visual measurements associated with healthy 20/20 vision. Clinical diagnostic features indicative of retinitis pigmentosa include a substantially small and progressively decreasing visual area in the visual field test, and compromised levels of clarity measured during the visual acuity test. Additionally, optical tomography such as fundus and retinal (optical coherence) imagery provide further diagnostic tools when determining an RP diagnosis. Photographing the back of the dilated eye allows the confirmation of bone spicule accumulation in the fundus, which presents during the later stages of RP retinal degeneration. Combined with cross-sectional imagery of optical coherence tomography, which provides clues into photoreceptor thickness, retinal layer morphology, and retinal pigment epithelium physiology, fundus imagery can help determine the state of RP progression.

While visual field and acuity test results combined with retinal imagery support the diagnosis of retinitis pigmentosa, additional testing is necessary to confirm other pathological features of this disease. Electroretinography (ERG) confirms the RP diagnosis by evaluating functional aspects associated with photoreceptor degeneration, and can detect physiological abnormalities before the initial manifestation of symptoms. An electrode lens is applied to the eye as photoreceptor response to varying degrees of quick light pulses is measured. Patients exhibiting the retinitis pigmentosa phenotype would show decreased or delayed electrical response in the rod photoreceptors, as well as possibly compromised cone photoreceptor cell response.

The patient's family history is also considered when determining a diagnosis due to the genetic mode of inheritance of retinitis pigmentosa. At least 35 different genes or loci are known to cause "nonsyndromic RP" (RP that is not the result of another disease or part of a wider syndrome). Indications of the RP mutation type can be determine through DNA testing, which is available on a clinical basis for:

- (autosomal recessive, Bothnia type RP)

- (autosomal dominant, RP1)

- (autosomal dominant, RP4)

- (autosomal dominant, RP7)

- (autosomal dominant, RP13)

- (autosomal dominant, RP18)

- CRB1 (autosomal recessive, RP12)

- (autosomal recessive, RP19)

- (autosomal recessive, RP20)

For all other genes (e.g. DHDDS), molecular genetic testing is available on a research basis only.

RP can be inherited in an autosomal dominant, autosomal recessive, or X-linked manner. X-linked RP can be either recessive, affecting primarily only males, or dominant, affecting both males and females, although males are usually more mildly affected. Some digenic (controlled by two genes) and mitochondrial forms have also been described.

Genetic counseling depends on an accurate diagnosis, determination of the mode of inheritance in each family, and results of molecular genetic testing.

Arts syndrome should be included in the differential diagnosis of infantile hypotonia and weakness aggravated by recurrent infection with a family history of X-linked inheritance. Sequence analysis of PRPS1, the only gene associated with Arts syndrome, has detected mutations in both kindreds reported to date. Arts syndrome patients were also found to have reduced levels of hypoxanthine levels in urine and uric acid levels in the serum. In vitro, PRS-1 activity was reduced in erythrocytes and fibroblasts.

The severity and prognosis vary with the type of mutation involved.

Although there has been extensive research in the past decades on this disease, there is still no evidence based therapies for this condition. This condition is often diagnosed at an early age; usually as a teenager or young adult.

To make a specific diagnosis, intraocular fluid samples may be taken and sent for analysis. In some cases, blood or cerebrospinal fluid (CSF) are also tested. Imaging may be done to help make the diagnosis.

Blood lactate and pyruvate levels usually are elevated as a result of increased anaerobic metabolism and a decreased ratio of ATP:ADP. CSF analysis shows an elevated protein level, usually >100 mg/dl, as well as an elevated lactate level.

A neuro-ophthalmologist is usually involved in the diagnosis and management of KSS. An individual should be suspected of having KSS based upon clinical exam findings. Suspicion for myopathies should be increased in patients whose ophthalmoplegia does not match a particular set of cranial nerve palsies (oculomotor nerve palsy, fourth nerve palsy, sixth nerve palsy). Initially, imaging studies are often performed to rule out more common pathologies. Diagnosis may be confirmed with muscle biopsy, and may be supplemented with PCR determination of mtDNA mutations.

While nothing currently can be done to stop or reverse the retinal degeneration, there are steps that can be taken to slow the rate of vision loss. UV-blocking sunglasses for outdoors, appropriate dietary intake of fresh fruit and leafy green vegetables, antioxidant vitamin supplements, and regular intake of dietary omega-3 very-long-chain fatty acids are all recommended.

One study found that a dietary supplement of lutein increases macular pigment levels in patients with choroideremia. Over a long period of time, these elevated levels of pigmentation could slow retinal degeneration. Additional interventions that may be needed include surgical correction of retinal detachment and cataracts, low vision services, and counseling to help cope with depression, loss of independence, and anxiety over job loss.

There are two types of retinitis: Retinitis pigmentosa (RP) and cytomegalovirus (CMV) retinitis. Both conditions result in the swelling and damage to the retinitis. However, the key difference in both these conditions is that Retinitis pigmentosa is a genetic eye disease that you inherit from one or both of your parents. On the other hand, CMV retinitis develops from a viral infection in the retina. Although there is no cure for this disease, there are steps you can take to protect your eyes from worsening. Supplements can slow the progression of the disease and alleviate symptoms temporarily. Research also shows that vitamin A, lutein, and omega-3 fatty acids also help alleviate symptoms.

Retinitis pigmentosa is the leading cause of inherited blindness, with approximately 1/4,000 individuals experiencing the non-syndromic form of their disease within their lifetime. It is estimated that 1.5 million people worldwide are currently affected. Early onset RP occurs within the first few years of life and is typically associated with syndromic disease forms, while late onset RP emerges from early to mid-adulthood.

Autosomal dominant and recessive forms of retinitis pigmentosa affect both male and female populations equally; however, the less frequent X-linked form of the disease affects male recipients of the X-linked mutation, while females usually remain unaffected carriers of the RP trait. The X-linked forms of the disease are considered severe, and typically lead to complete blindness during later stages. In rare occasions, a dominant form of the X-linked gene mutation will affect both males and females equally.

Due to the genetic inheritance patterns of RP, many isolate populations exhibit higher disease frequencies or increased prevalence of a specific RP mutation. Pre-existing or emerging mutations that contribute to rod photoreceptor degeneration in retinitis pigmentosa are passed down through familial lines; thus, allowing certain RP cases to be concentrated to specific geographical regions with an ancestral history of the disease. Several hereditary studies have been performed to determine the varying prevalence rates in Maine (USA), Birmingham (England), Switzerland (affects 1/7000), Denmark (affects 1/2500), and Norway. Navajo Indians display an elevated rate of RP inheritance as well, which is estimated as affecting 1 in 1878 individuals. Despite the increased frequency of RP within specific familial lines, the disease is considered non-discriminatory and tends to equally affect all world populations.

Oguchi's disease is unique in its electroretinographic responses in the light- and dark-adapted conditions. The A- and b-waves on single flash electroretinograms (ERG) are decreased or absent under lighted conditions but increase after prolonged dark adaptation. There are nearly undetectable rod b waves in the scotopic 0.01 ERG and nearly negative scotopic 3.0 ERGs.

Dark-adaptation studies have shown that highly elevated rod thresholds decrease several hours later and eventually result in a recovery to the normal or nearly normal level.

The S, M and L cone systems are normal.

Currently, purine replacement via S-adenosylmethionine (SAM) supplementation in people with Arts syndrome appears to improve their condition. This suggests that SAM supplementation can alleviate symptoms of PRPS1 deficient patients by replacing purine nucleotides and open new avenues of therapeutic intervention. Other non-clinical treatment options include educational programs tailored to their individual needs. Sensorineural hearing loss has been treated with cochlear implantation with good results. Ataxia and visual impairment from optic atrophy are treated in a routine manner. Routine immunizations against common childhood infections and annual influenza immunization can also help prevent any secondary infections from occurring.

Regular neuropsychological, audiologic, and ophthalmologic examinations are also recommended.

Carrier testing for at-risk relatives and prenatal testing for pregnancies at increased risk are possible if the disease-causing mutation in the family is known.

Progressive vision loss in any dog in the absence of canine glaucoma or cataracts can be an indication of PRA. It usually starts with decreased vision at night, or nyctalopia. Other symptoms include dilated pupils and decreased pupillary light reflex. Fundoscopy to examine the retina will show shrinking of the blood vessels, decreased pigmentation of the nontapetal fundus, increased reflection from the tapetum due to thinning of the retina, and later in the disease a darkened, atrophied optic disc. Secondary cataract formation in the posterior portion of the lens can occur late in the disease. In these cases diagnosis of PRA may require electroretinography (ERG). For many breeds there are specific genetic tests of blood or buccal mucosa for PRA.

Absent a genetic test, animals of breeds susceptible to PRA can be cleared of the disease only by the passage of time—that is, by living past the age at which PRA symptoms are typically apparent in their breed. Breeds in which the PRA gene is recessive may still be carriers of the gene and pass it on to their offspring, however, even if they lack symptoms, and it is also possible for onset of the disease to be later than expected, making this an imperfect test at best.

Other conditions with similar appearing fundi include

- Cone dystrophy

- X-linked retinitis pigmentosa

- Juvenile macular dystrophy

These conditions do not show the Mizuo-Nakamura phenomenon.

The diagnosis of CMV retinitis can be done via the following:

- Ophthalmic screening frequency is based on CD4 count,(CD4 < 50 cells/mL, 0- 35% possibility of CMV retinitis)

- BUN

- CD8+ T-lymphocyte count

- CMV DNA capture ( polymerase chain reaction (PCR) test)

- DNA PCR ( ocular fluids)

- Viral load

- Complete blood count

Until recently, doctors have diagnosed patients with FHS based on clinical observations and how well they fit the disease description, usually occurring in early childhood. Molecular genetic testing is also used now to test for genetic mutations. By performing a sequence analysis test of select exons, mutations can be detected in exon 34 of the SRCAP gene. This mutation has been observed in 19 patients to date.

In most cases, if the patient shows classic facial features of FHS, the molecular testing will show a mutation on the SRCAP gene.

FHS shares some common features with Rubinstein–Taybi (due to overlapping effects of mutations on SRCAP), however cranial and hand anomalies are distinctive: broad thumbs, narrow palate, and microcephaly are absent in Floating-Harbor Syndrome. One child in the UK has a diagnosis of microcephaly alongside Floating–Harbor syndrome.

Since Usher syndrome is incurable at present, it is helpful to diagnose children well before they develop the characteristic night blindness. Some preliminary studies have suggested as many as 10% of congenitally deaf children may have Usher syndrome. However, a misdiagnosis can have bad consequences.

The simplest approach to diagnosing Usher syndrome is to test for the characteristic chromosomal mutations. An alternative approach is electroretinography, although this is often disfavored for children, since its discomfort can also make the results unreliable. Parental consanguinity is a significant factor in diagnosis. Usher syndrome I may be indicated if the child is profoundly deaf from birth and especially slow in walking.

Thirteen other syndromes may exhibit signs similar to Usher syndrome, including Alport syndrome, Alstrom syndrome, Bardet-Biedl syndrome, Cockayne syndrome, spondyloepiphyseal dysplasia congenita, Flynn-Aird syndrome, Friedreich ataxia, Hurler syndrome (MPS-1), Kearns-Sayre syndrome (CPEO), Norrie syndrome, osteopetrosis (Albers-Schonberg disease), Refsum's disease (phytanic acid storage disease), and Zellweger syndrome (cerebrohepatorenal syndrome).

Diagnosis requires a neurological examination and neuroimaging can be helpful.

BVVL can be differentially diagnosed from similar conditions like Fazio-Londe syndrome and amyotrophic lateral sclerosis, in that those two conditions don't involve sensorineural hearing loss, while BVVL, Madras motor neuron disease, Nathalie syndrome, and Boltshauser syndrome do. Nathalie syndrome does not involve lower cranial nerve symptoms, so it can be excluded if those are present. If there is evidence of lower motor neuron involvement, Boltshauser syndrome can be excluded. Finally, if there is a family history of the condition, then BVVL is more likely than MMND, as MMND tends to be sporadic.

Genetic testing is able to identify genetic mutations underying BVVL.

Since Usher syndrome results from the loss of a gene, gene therapy that adds the proper protein back ("gene replacement") may alleviate it, provided the added protein becomes functional. Recent studies of mouse models have shown one form of the disease—that associated with a mutation in myosin VIIa—can be alleviated by replacing the mutant gene using a lentivirus. However, some of the mutated genes associated with Usher syndrome encode very large proteins—most notably, the "USH2A" and "GPR98" proteins, which have roughly 6000 amino-acid residues. Gene replacement therapy for such large proteins may be difficult.

Though it is only definitively diagnosed by a genetic test, autosomal dominant porencephaly type I can be suspected if the disease is known to run in the family or if someone shows symptoms. CT scanning or MRI may be useful in indicating a diagnosis. COL4A1 may be mutated in other diseases that need to be distinguished, including brain small vessel disease with hemorrhage and HANAC syndrome. CADASIL syndrome is caused by a mutation in a different gene, but may cause similar symptoms. Sporadic porencephaly is another disorder that can appear similar.

If the nematode can be seen by an ophthalmologist, which occurs in less than half of cases, it should be treated with photocoagulation for extramacular location and surgical removal in case the larva is lying in the macula. After the worm is killed, visual acuity loss usually does not progress. Alternatively, Antihelminthic treatment such as high dose oral Albendazole and prednisolone may be used.

The center of the cornea shows normal thickness, with an intact central epithelium, but the inferior cornea exhibits a peripheral band of thinning, to about 1–2 mm. The portion of the cornea that is immediately adjacent to the limbus is spared, usually a strip of about 1–2 mm. In PMD we can see high against the rule astigmatism along with horizontal bow ties. The inferior peripheral thinning is seen between the 4 o'clock and 8 o'clock positions.

PMD lacks apical corneal scarring, Rizutti's phenomenon, Munson's sign, and the central corneal thickness is usually normal.

The gold standard diagnostic test for PMD is corneal topography. However, it may not as specific as corneal pachymetry, because corneal topography only evaluates the degree and distribution of surface irregularities on the cornea, not the thickness of the cornea. Corneal topography may show a "crab claw-like" appearance, a finding that is seen in both keratoconus and in pellucid marginal degeneration. Thus, if corneal topography is used for diagnosis, it should be in conjunction with clinical findings of peripheral, inferior corneal thinning.

The incidence and prevalence of PMD are unknown, and no studies have yet investigated its prevalence or incidence. However, it is generally agreed that PMD is a very rare condition. Some uncertainty regarding the incidence of PMD may be attributed to its confusion with keratoconus. PMD is not linked to race or age, although most cases present early in life, between 20 and 40 years of age. While PMD is usually considered to affect men and women equally, some studies suggest that it may affect men more frequently.

Several diseases have been observed in patients with PMD. However, no causal relationships have been established between any of the associated diseases and the pathogenesis of PMD. Such diseases include: chronic open-angle glaucoma, retinitis pigmentosa, retinal lattice degeneration, scleroderma, kerato-conjunctivitis, eczema, and hyperthyroidism.